Analog television
Encyclopedia
Analog television is the analog transmission
that involves the broadcasting
of encoded analog audio and analog video
signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency
of the signal. All broadcast television system
s preceding digital transmission of digital television
(DTV) were systems utilizing analog signal
s. Analog television may be wireless
or can require copper wire used by cable converters.
systems used spinning disks with patterns of holes punched into the disc to "scan" an image. A similar disk reconstructed the image at the receiver. Synchronization of the receiver disc rotation was handled through sync pulses broadcast with the image information. However these mechanical systems were slow, the images were dim and flickered severely, and the image resolution very low. Camera systems used similar spinning discs and required intensely bright illumination of the subject for the light detector to work.
Analog television did not really begin as an industry until the development of the cathode-ray tube (CRT), which uses a steered electron beam to "write" lines of electrons across a phosphor
coated surface. The electron beam could be swept across the screen much faster than any mechanical disc system, allowing for more closely spaced scan lines and much higher image resolution, while slow-fade phosphors removed image flicker effects. Also far less maintenance was required of an all-electronic system compared to a spinning disc system.
, PAL
or SECAM
analog encoding and then use RF modulation to modulate this signal onto a Very high frequency
(VHF) or Ultra high frequency
(UHF) carrier. Each frame of a television image is composed of lines drawn on the screen. The lines are of varying brightness; the whole set of lines is drawn quickly enough that the human eye perceives it as one image. The next sequential frame is displayed, allowing the depiction of motion. The analog television signal contains timing and synchronization information so that the receiver can reconstruct a two-dimensional moving image from a one-dimensional time-varying signal.
In many countries, over-the-air broadcast television of analog audio and analog video
signals is being discontinued, to allow the re-use of the television broadcast radio spectrum
for other services such as datacasting
and subchannels.
The first commercial television
systems were black-and-white
; The beginning of color television
was in the 1950s.
A practical television system needs to take luminance
, chrominance
(in a color system), synchronization (horizontal and vertical), and audio
signals, and broadcast
them over a radio
transmission. The transmission system must include a means of television channel
selection.
Analog broadcast television systems come in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. The monochrome combinations still existing in the 1950s are standardized by the International Telecommunication Union
(ITU) as capital letters A through N. When color television was introduced, the hue and saturation information was added to the monochrome signals in a way that black & white televisions ignore. This way backwards compatibility was achieved. That concept is true for all analog television standards.
However there are three standards for the way the additional color information can be encoded and transmitted. The first was the American NTSC
(National Television Systems Committee) color television
system. The European/Australian PAL
(Phase Alternation Line rate) and the French-Former Soviet Union SECAM
(Séquentiel Couleur Avec Mémoire) standard were developed later and attempt to cure certain defects of the NTSC system. PAL's color encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC.
In principle all three color encoding systems can be combined with any scan line/frame rate combination. Therefore, in order to describe a given signal completely, it's necessary to quote the color system and the broadcast standard as capital letter. For example the United States uses NTSC-M, the UK uses PAL-I, France uses SECAM-L, much of Western Europe and Australia uses PAL-B/G, most of Eastern Europe uses PAL-D/K or SECAM-D/K and so on.
However not all of these possible combinations actually exist. NTSC is currently only used with system M, even though there were experiments with NTSC-A (405 line) and NTSC-I (625 line) in the UK. PAL is used with a variety of 625-line standards (B,G,D,K,I,N) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.
For this reason many people refer to any 625/25 type signal as "PAL" and to any 525/30 signal as "NTSC", even when referring to digital signals, for example, on DVD-Video
which don't contain any analog color encoding, thus no PAL or NTSC signals at all. Even though this usage is common, it is misleading as that is not the original meaning of the terms PAL/SECAM/NTSC.
Although a number of different broadcast television systems were in use worldwide, the same principles of operation apply.
. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the luminance
of that point. A color television
system is identical except that an additional signal known as chrominance
controls the color of the spot.
Raster scanning is shown in a slightly simplified form below.
When analog television was developed, no affordable technology for storing any video signals existed; the luminance signal has to be generated and transmitted at the same time at which it is displayed on the CRT. It is therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact synchronization
with the scanning in the television.
The physics of the CRT require that a finite time interval is allowed for the spot to move back to the start of the next line (horizontal retrace) or the start of the screen (vertical retrace). The timing of the luminance signal must allow for this.
The human eye has a characteristic called Persistence of vision
. Quickly displaying successive scan images will allow the apparent illusion
of smooth motion. Flickering of the image can be partially solved using a long persistence
phosphor coating on the CRT, so that successive images fade slowly. However, slow phosphor has the negative side-effect of causing image smearing and blurring when there is a large amount of rapid on-screen motion occurring.
The maximum frame rate
depends on the bandwidth of the electronics and the transmission system, and the number of horizontal scan lines in the image. A frame rate of 25 or 30 hertz
is a satisfactory compromise, while the process of interlacing two video fields of the picture per frame
is used to build the image. This process doubles the apparent number of video fields per second and further reduces flicker and other defects in transmission.
s. These types of display screens use lower voltages than older CRT displays. Many dual system television receivers, equipped to receive both analog transmission
s and digital transmissions have analog tuner (television) receiving capability and must use a television antenna
.
s within the UHF or VHF frequency ranges. A channel actually consists of two signals: the picture information is transmitted using amplitude modulation
on one frequency, and the sound is transmitted with frequency modulation
at a frequency at a fixed offset (typically 4.5 to 6 MHz) from the picture signal.
The channel frequencies chosen represent a compromise between allowing enough bandwidth for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called vestigial sideband is used to reduce the channel spacing, which would be at least twice the video bandwidth if pure AM was used.
Signal reception is invariably done via a superheterodyne receiver
: the first stage is a tuner which selects a television channel
and frequency-shifts it to a fixed intermediate frequency
(IF). The signal amplifier
(from the microvolt range to fractions of a volt) performs amplification to the IF stages.
at one frequency and the sound carrier at a fixed offset. A demodulator recovers the video signal and sound as an FM signal at the offset frequency (this is known as intercarrier sound).
The FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of the NICAM
and MTS
systems, TV sound transmissions were invariably monophonic.
signal; this contains luminance, chrominance and synchronization signals; this is identical to the video signal format used by analog video
devices such as VCRs or CCTV cameras
. Note that the RF signal modulation is inverted compared to the conventional AM: the minimum video signal level corresponds to maximum carrier amplitude, and vice versa. The carrier is never shut off altogether; this is to ensure that intercarrier sound demodulation can still occur.
Each line of the displayed image is transmitted using a signal as shown above. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for PAL
, NTSC
and SECAM
television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the color burst, and the chrominance signal) are not present.
The front porch is a brief (about 1.5 microsecond
) period inserted between the end of each transmitted line of picture and the leading edge of the next line sync pulse. Its purpose was to allow voltage
levels to stabilise in older televisions, preventing interference between picture lines. The front porch is the first component of the horizontal blanking interval which also contains the horizontal sync pulse and the back porch.
The back porch is the portion of each scan line between the end (rising edge) of the horizontal sync pulse and the start of active video. It is used to restore the black level (300 mV.) reference in analog video. In signal processing terms, it compensates for the fall time
and settling time
following the sync pulse.
In color TV systems such as PAL
and NTSC
, this period also includes the colorburst
signal. In the SECAM
system it contains the reference subcarrier for each consecutive color difference signal in order to set the zero-color reference.
In some professional systems, particularly satellite link
s between locations, the audio is embedded within the back porch of the video signal, to save the cost of renting a second channel.
In a monochrome receiver the luminance signal is amplified to drive the control grid
in the electron gun
of the CRT. This changes the intensity of the electron beam and therefore the brightness of the spot being scanned. Brightness and contrast controls determine the DC shift and amplification, respectively.
A color signal conveys picture information for each of the red, green, and blue components of an image (see the article on Color space
for more information). However, these are not simply transmitted as three separate signals, because:
Instead, the RGB signals are converted into YUV
form, where the Y signal represents the overall brightness, and can be transmitted as the luminance signal. This ensures a monochrome receiver will display a correct picture. The U and V signals are the difference between the Y signal and the B and R signals respectively. The U signal then represents how "blue" the color is, and the V signal how "red" it is. The advantage of this scheme is that the U and V signals are zero when the picture has no color content. Since the human eye is more sensitive to errors in luminance than in color, the U and V signals can be transmitted in a relatively lossy (specifically: bandwidth-limited) way with acceptable results. The G signal is not transmitted in the YUV system, but rather it is recovered electronically at the receiving end.
In the NTSC
and PAL
color systems, U and V are transmitted by adding a color subcarrier to the composite video signal, and using quadrature amplitude modulation
on it. For NTSC, the subcarrier is usually at about 3.58 MHz, but for the PAL system it is at about 4.43 MHz. These frequencies are within the luminance signal band, but their exact frequencies were chosen such that they are midway between two harmonics of the horizontal line repetition rate, thus ensuring that the majority of the power of the luminance signal does not overlap with the power of the chrominance signal.
In the British
PAL (D) system, the actual chrominance center frequency is 4.43361875 MHz, a direct multiple of the scan rate frequency. This frequency was chosen to minimize the chrominance beat interference pattern that would be visible in areas of high color saturation in the transmitted picture.
The two signals (U and V) modulate both the amplitude
and phase
of the color carrier, so to demodulate them it is necessary to have a reference signal against which to compare it. For this reason, a short burst of reference signal known as the color burst is transmitted during the back porch (re-trace period) of each scan line. A reference oscillator in the receiver locks onto this signal (see phase-locked loop
) to achieve a phase reference, and uses its amplitude to set an AGC
system to achieve an amplitude reference.
The U and V signals are then demodulated by band-pass filtering to retrieve the color subcarrier, mixing it with the in-phase and quadrature signals from the reference oscillator, and low-pass filtering the results.
NTSC uses this process unmodified. Unfortunately, this often results in poor color reproduction due to phase errors in the received signal. The PAL D (delay) system corrects this by reversing the phase of the signal on each successive line, and the averaging the results over pairs of lines. This process is achieved by the use of a 1H (where H = horizontal scan frequency) duration delay line. (A typical circuit used with this device converts the low frequency color signal to ultrasonic sound and back again). Phase shift errors between successive lines are therefore cancelled out and the wanted signal amplitude is increased when the two in-phase (coincident
) signals are re-combined.
In the SECAM
television system, U and V are transmitted on alternate lines, using simple frequency modulation
of two different color subcarriers.
In analog color CRT displays, the brightness control signal (luminance
) is fed to the cathode
connections of the electron guns, and the color difference signals (chrominance
signals) are fed to the control grids connections. This simple matrix mixing technique was replaced in later solid state
designs of signal processing.
and video frame ensure that the sweep oscillators in the receiver remain locked in step with the transmitted signal, so that the image can be reconstructed on the receiver screen.
A sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. (see section below - Other technical information, for extra detail.)
s. The horizontal sync signal is a single short pulse which indicates the start of every line. The rest of the scan line
follows, with the signal ranging from 0.3 V (black) to 1 V (white), until the next horizontal or vertical synchronization pulse.
The format of the horizontal sync pulse varies. In the 525-line NTSC
system it is a 4.85 µs
-long pulse at 0 V
. In the 625-line PAL
system the pulse is 4.7 µs synchronization pulse at 0 V
. This is lower than the amplitude of any video signal (blacker than black) so it can be detected by the level-sensitive "sync stripper" circuit of the receiver.
. The vertical sync pulses are made by prolonging the length of HSYNC pulses through almost the entire length of the scan line.
The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or mid-way through).
The format of such a signal in 525-line NTSC
is:
Each pre- or post- equalizing pulse consists in half a scan line
of black signal: 2 µs at 0 V, followed by 30 µs at 0.3 V.
Each long sync pulse consists in an equalizing pulse with timings inverted: 30 µs at 0 V, followed by 2 µs at 0.3 V.
In video production and computer graphics, changes to the image are often kept in step with the vertical synchronization pulse to avoid visible discontinuity of the image. Since the frame buffer of a computer graphics
display imitates the dynamics of a cathode-ray display, if it is updated with a new image while the image is being transmitted to the display, the display shows a mishmash of both frames, producing a page tearing
artifact
partway down the image.
Vertical synchronization eliminates this by timing frame buffer fills to coincide with the vertical blanking interval
, thus ensuring that only whole frames are seen on-screen. Software such as computer games and Computer aided design (CAD) packages often allow vertical synchronization as an option, because it delays the image update until the vertical blanking interval. This produces a small penalty in latency, because the program has to wait until the video controller has finished transmitting the image to the display before continuing. Triple buffering reduces this latency significantly.
Two timing intervals are defined - the front porch between the end of displayed video and the start of the sync pulse, and the back porch after the sync pulse and before displayed video. These and the sync pulse itself are called the horizontal blanking (or retrace) interval and represent the time that the electron beam in the CRT is returning to the start of the next display line.
. Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen.
to digital over-the-air (terrestrial television
) broadcasting was Luxembourg
in 2006, followed later in 2006 by the Netherlands
; in 2007 by Finland
, Andorra
, Sweden
, Norway
, and Switzerland
; in 2008 by Belgium (Flanders)
and Germany
; in 2009 by the United States
(high power stations -- the important ones), southern Canada
, the Isle of Man
, Norway
, and Denmark
. In 2010, Belgium (Wallonia), Spain
, Wales
, Latvia
, Estonia
, the Channel Islands
, and Slovenia
; in 2011 Israel
, Austria
, Monaco
, Scotland
, Cyprus
, Japan
(excluding Miyagi
, Iwate
, and Fukushima
Prefectures), Malta
and France
completed the transition.
In the United States, high-power over-the-air broadcasts are solely in the ATSC
digital format since June 12, 2009, the date that the Federal Communications Commission
(FCC) set for the end of all high-power analog TV transmissions. As a result, almost two million households could no longer watch TV because they were not prepared for the transition. The switchover was originally scheduled for February 17, 2009, until the U.S. Congress passed the DTV Delay Act. By special dispensation, some analog TV signals ceased on the original date. While the majority of the viewers of over-the-air broadcast television in the U.S. watch full-power stations (which number about 1800), there are three other categories of TV stations in the U.S.: low-power broadcasting
stations, Class A stations
, and TV translator stations
. There is presently no deadline for these stations, about 7100 in number, to convert to digital broadcasting.
It is necessary to be cognizant of the fact that in broadcasting, whatever happens in the United States also happens simultaneously in southern Canada and in northern Mexico because those areas are covered by TV stations in the U.S. Furthermore, the major cities of southern Canada made their transitions to digital TV broadcasts simultaneously with the U.S.: Toronto
, Montreal
, Vancouver
, Ottawa
, Winnipeg
, Sault Ste. Marie
, Quebec City
, Charlottetown
, Halifax
, and so forth.
In Japan, the switch to digital occurred on the 24th of July, 2011 (with the exception of Fukushima, Iwate, and Miyagi prefectures, where conversion was delayed one year due to complications from the 2011 Tōhoku earthquake and tsunami
). In Canada, it is scheduled to happen August 31, 2011. China is scheduled to switch in 2015. In the United Kingdom, the digital switchover has different times for each part of the country. However, the entire U.K. should be on digital TV by 2012.
Brazil
switched to digital TV on December 2, 2007, in its major cities, and now it is estimated that it will take about seven years for complete conversion over all of Brazil -- but understand that large parts of Brazil are unpopulated by people who have electricity and TV. Australia will turn off analog TV in steps, TV network by network, between 2010 and 2013, region by region.
In Malaysia, the Malaysian Communications & Multimedia Commission (MCMC) advertised for tender bids to be submitted in the third quarter of 2009 for the 470 through 742 MHz UHF allocation, to enable Malaysia's broadcast system to move into DTV. The new broadcast band
allocation would result in Malaysia's having to build an infrastructure
for all broadcasters, using a single digital terrestrial transmission/TV broadcast (DTTB) channel.
People also need to understand that large portions of Malaysia are covered by TV broadcasts from Singapore
, Thailand
, Brunei
, and/or Indonesia
(from Borneo
).
Users may then encode
and transmit their television program
s on this channels` digital data stream. The winner was to be announced at the end of 2009 or early 2010. A condition of the award is that digital transmission must start as soon as possible, and analog switch-off was proposed for 2015. The scheme may not go ahead as the Government successor, Najib Tun Razak
deferred the transition indefinitely in favor of his own 1Malaysia
concept, which means that analog television will continue for longer than originally planned.
Image synchronization is achieved by transmitting negative-going pulses; in a composite video signal of 1 volt amplitude, these are approximately 0.3 V below the "black level"
. The horizontal sync signal is a single short pulse which indicates the start of every line. Two timing intervals are defined - the front porch between the end of displayed video and the start of the sync pulse, and the back porch after the sync pulse and before displayed video. These and the sync pulse itself are called the horizontal blanking (or retrace) interval and represent the time that the electron beam in the CRT is returning to the start of the next display line.
The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or mid-way through).
In the TV receiver, a sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync.
Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen.
display sync pulses are fed to horizontal and vertical timebase amplifier circuits. These generate modified sawtooth
and parabola
current waveforms to scan the electron beam in a linear
way. The waveform shapes are necessary to make up for the distance variations from the electron beam source and the screen surface. Each beam direction switching circuit is reset by the appropriate sync timing pulse. These waveforms are fed to the horizontal and vertical scan coils wrapped around the CRT tube. These coils produce a magnetic field
proportional to the changing current, and this deflects the electron beam across the screen. In the 1950s, television receiver timebase supply was derived directly from the mains supply. A simple circuit consisted of a series
voltage dropper resistance
and a rectifier
valve
(tube) or semiconductor
diode
. This avoided the cost of a large high voltage mains supply (50 or 60 Hz) transformer
. This type of circuit was used for thermionic valve (tube) technology. It was inefficient and produced a lot of heat which led to premature failures in the circuitry. In the 1960s, semiconductor
technology was introdued into timebase circuits. During the late 1960s in the U.K., synchronous, (with the scan line rate), power generation was introduced into solid state
receiver designs. These had very complex circuits in which faults were difficult to trace, but had very efficient use of power. In the early 1970s AC
mains (50 Hz), and line timebase (15,625 Hz), thyristor
based switching circuits were introduced. In the U.K. use of the simple (50 Hz) types of power circuits were discontinued. The reason for design changes arose from the electricity supply contamination problems arising from EMI
, and supply loading issues due to energy being taken from only the positive half cycle of the mains supply waveform.
- or IC
-based designs) operates from a comparatively low-voltage DC power supply. However, the anode
connection for a cathode-ray tube requires a very high voltage (typically 10-30 kV) for correct operation.
This voltage is not directly produced by the main power supply
circuitry; instead the receiver makes use of the circuitry used for horizontal scanning. Direct current
(DC), is switched though the line output transformer, and alternating current
([AC]) is induced into the scan coils. At the end of each horizontal scan line the magnetic field
which has built up in both transformer and scan coils by the current, is a source of latent electromagnetic energy. This stored collapsing magnetic field energy can be captured. The reverse flow, short duration, (about 10% of the line scan time) current from both the line output transformer and the horizontal scan coil is discharged again into the primary winding
of the flyback transformer
by the use of a rectifier which blocks this negative reverse emf
. A small value capacitor
is connected across the scan switching device. This tunes the circuit inductance
s to resonate at a much higher frequency
. This slows down (lengthens) the flyback time from the extremely rapid decay rate that would result if they were electrically isolated during this short period. One of the secondary windings on the flyback transformer then feeds this brief high voltage pulse to a Cockcroft
design voltage multiplier
. This produces the required EHT
supply. A flyback converter
is a power supply circuit operating on similar principles.
Typical modern design incorporates the flyback transformer and rectifier circuitry into a single unit with a captive output lead, (known as a diode split line output transformer), so that all high-voltage parts are enclosed. Earlier designs used a separate line output transformer and a well insulated high voltage multiplier unit. The high frequency (15 kHz or so) of the horizontal scanning allows reasonably small components to be used.
Analog transmission
Analog transmission is a transmission method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable...
that involves the broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
of encoded analog audio and analog video
Analog video
Analog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the signal. All broadcast television system
Broadcast television system
Broadcast television systems are encoding or formatting standards for the transmission and reception of terrestrial television signals. There are three main analog television systems in current use around the world: NTSC, PAL, and SECAM...
s preceding digital transmission of digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
(DTV) were systems utilizing analog signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
s. Analog television may be wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
or can require copper wire used by cable converters.
Development
The earliest mechanical televisionMechanical television
Mechanical television was a broadcast television system that used mechanical or electromechanical devices to capture and display video images. However, the images themselves were usually transmitted electronically and via radio waves...
systems used spinning disks with patterns of holes punched into the disc to "scan" an image. A similar disk reconstructed the image at the receiver. Synchronization of the receiver disc rotation was handled through sync pulses broadcast with the image information. However these mechanical systems were slow, the images were dim and flickered severely, and the image resolution very low. Camera systems used similar spinning discs and required intensely bright illumination of the subject for the light detector to work.
Analog television did not really begin as an industry until the development of the cathode-ray tube (CRT), which uses a steered electron beam to "write" lines of electrons across a phosphor
Phosphor
A phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
coated surface. The electron beam could be swept across the screen much faster than any mechanical disc system, allowing for more closely spaced scan lines and much higher image resolution, while slow-fade phosphors removed image flicker effects. Also far less maintenance was required of an all-electronic system compared to a spinning disc system.
Standards
Broadcasters using analog television systems encode their signal using NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
, PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
or SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
analog encoding and then use RF modulation to modulate this signal onto a Very high frequency
Very high frequency
Very high frequency is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency , and the next higher frequencies are known as Ultra high frequency...
(VHF) or Ultra high frequency
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
(UHF) carrier. Each frame of a television image is composed of lines drawn on the screen. The lines are of varying brightness; the whole set of lines is drawn quickly enough that the human eye perceives it as one image. The next sequential frame is displayed, allowing the depiction of motion. The analog television signal contains timing and synchronization information so that the receiver can reconstruct a two-dimensional moving image from a one-dimensional time-varying signal.
In many countries, over-the-air broadcast television of analog audio and analog video
Analog video
Analog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
signals is being discontinued, to allow the re-use of the television broadcast radio spectrum
Radio spectrum
Radio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz ....
for other services such as datacasting
Datacasting
Datacasting is the broadcasting of data over a wide area via radio waves. It most often refers to supplemental information sent by television stations along with digital television, but may also be applied to digital signals on analog TV or radio...
and subchannels.
The first commercial television
Commercial Television
Commercial Television was the third free-to-air broadcast television station in Hong Kong. It first went on air in 1975, and ceased transmissions in 1978.-History:...
systems were black-and-white
Black-and-white
Black-and-white, often abbreviated B/W or B&W, is a term referring to a number of monochrome forms in visual arts.Black-and-white as a description is also something of a misnomer, for in addition to black and white, most of these media included varying shades of gray...
; The beginning of color television
Color television
Color television is part of the history of television, the technology of television and practices associated with television's transmission of moving images in color video....
was in the 1950s.
A practical television system needs to take luminance
Luma (video)
In video, luma, sometimes called luminance, represents the brightness in an image . Luma is typically paired with chrominance. Luma represents the achromatic image without any color, while the chroma components represent the color information...
, chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
(in a color system), synchronization (horizontal and vertical), and audio
Sound recording and reproduction
Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...
signals, and broadcast
Broadcast
Broadcast or Broadcasting may refer to:* Broadcasting, the transmission of audio and video signals* Broadcast, an individual television program or radio program* Broadcast , an English electronic music band...
them over a radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
transmission. The transmission system must include a means of television channel
Television channel
A television channel is a physical or virtual channel over which a television station or television network is distributed. For example, in North America, "channel 2" refers to the broadcast or cable band of 54 to 60 MHz, with carrier frequencies of 55.25 MHz for NTSC analog video and...
selection.
Analog broadcast television systems come in a variety of frame rates and resolutions. Further differences exist in the frequency and modulation of the audio carrier. The monochrome combinations still existing in the 1950s are standardized by the International Telecommunication Union
International Telecommunication Union
The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
(ITU) as capital letters A through N. When color television was introduced, the hue and saturation information was added to the monochrome signals in a way that black & white televisions ignore. This way backwards compatibility was achieved. That concept is true for all analog television standards.
However there are three standards for the way the additional color information can be encoded and transmitted. The first was the American NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
(National Television Systems Committee) color television
Color television
Color television is part of the history of television, the technology of television and practices associated with television's transmission of moving images in color video....
system. The European/Australian PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
(Phase Alternation Line rate) and the French-Former Soviet Union SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
(Séquentiel Couleur Avec Mémoire) standard were developed later and attempt to cure certain defects of the NTSC system. PAL's color encoding is similar to the NTSC systems. SECAM, though, uses a different modulation approach than PAL or NTSC.
In principle all three color encoding systems can be combined with any scan line/frame rate combination. Therefore, in order to describe a given signal completely, it's necessary to quote the color system and the broadcast standard as capital letter. For example the United States uses NTSC-M, the UK uses PAL-I, France uses SECAM-L, much of Western Europe and Australia uses PAL-B/G, most of Eastern Europe uses PAL-D/K or SECAM-D/K and so on.
However not all of these possible combinations actually exist. NTSC is currently only used with system M, even though there were experiments with NTSC-A (405 line) and NTSC-I (625 line) in the UK. PAL is used with a variety of 625-line standards (B,G,D,K,I,N) but also with the North American 525-line standard, accordingly named PAL-M. Likewise, SECAM is used with a variety of 625-line standards.
For this reason many people refer to any 625/25 type signal as "PAL" and to any 525/30 signal as "NTSC", even when referring to digital signals, for example, on DVD-Video
DVD-Video
DVD-Video is a consumer video format used to store digital video on DVD discs, and is currently the dominant consumer video format in Asia, North America, Europe, and Australia. Discs using the DVD-Video specification require a DVD drive and a MPEG-2 decoder...
which don't contain any analog color encoding, thus no PAL or NTSC signals at all. Even though this usage is common, it is misleading as that is not the original meaning of the terms PAL/SECAM/NTSC.
Although a number of different broadcast television systems were in use worldwide, the same principles of operation apply.
Displaying an image
A cathode-ray tube (CRT) television displays an image by scanning a beam of electrons across the screen in a pattern of horizontal lines known as a rasterRaster scan
A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems...
. At the end of each line the beam returns to the start of the next line; at the end of the last line it returns to the top of the screen. As it passes each point the intensity of the beam is varied, varying the luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
of that point. A color television
Color television
Color television is part of the history of television, the technology of television and practices associated with television's transmission of moving images in color video....
system is identical except that an additional signal known as chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
controls the color of the spot.
Raster scanning is shown in a slightly simplified form below.
When analog television was developed, no affordable technology for storing any video signals existed; the luminance signal has to be generated and transmitted at the same time at which it is displayed on the CRT. It is therefore essential to keep the raster scanning in the camera (or other device for producing the signal) in exact synchronization
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
with the scanning in the television.
The physics of the CRT require that a finite time interval is allowed for the spot to move back to the start of the next line (horizontal retrace) or the start of the screen (vertical retrace). The timing of the luminance signal must allow for this.
The human eye has a characteristic called Persistence of vision
Persistence of vision
Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina....
. Quickly displaying successive scan images will allow the apparent illusion
Illusion
An illusion is a distortion of the senses, revealing how the brain normally organizes and interprets sensory stimulation. While illusions distort reality, they are generally shared by most people....
of smooth motion. Flickering of the image can be partially solved using a long persistence
Persistence
Persistence may refer to:* Image persistence, in LCD monitors* Multidrug tolerance, a dormant, persistent state of a bacterial population* Persistence , the characteristic of data that outlives the execution of the program that created it...
phosphor coating on the CRT, so that successive images fade slowly. However, slow phosphor has the negative side-effect of causing image smearing and blurring when there is a large amount of rapid on-screen motion occurring.
The maximum frame rate
Frame rate
Frame rate is the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...
depends on the bandwidth of the electronics and the transmission system, and the number of horizontal scan lines in the image. A frame rate of 25 or 30 hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
is a satisfactory compromise, while the process of interlacing two video fields of the picture per frame
Film frame
In filmmaking, video production, animation, and related fields, a film frame or video frame is one of the many still images which compose the complete moving picture...
is used to build the image. This process doubles the apparent number of video fields per second and further reduces flicker and other defects in transmission.
Other types of display screens
Plasma screens and LCD screens have been used in analog television setTelevision set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
s. These types of display screens use lower voltages than older CRT displays. Many dual system television receivers, equipped to receive both analog transmission
Analog transmission
Analog transmission is a transmission method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable...
s and digital transmissions have analog tuner (television) receiving capability and must use a television antenna
Television antenna
A television antenna, or TV aerial, is an antenna specifically designed for the reception of over the air broadcast television signals, which are transmitted at frequencies from about 41 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries...
.
Receiving signals
The television system for each country will specify a number of television channelTelevision channel
A television channel is a physical or virtual channel over which a television station or television network is distributed. For example, in North America, "channel 2" refers to the broadcast or cable band of 54 to 60 MHz, with carrier frequencies of 55.25 MHz for NTSC analog video and...
s within the UHF or VHF frequency ranges. A channel actually consists of two signals: the picture information is transmitted using amplitude modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
on one frequency, and the sound is transmitted with frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
at a frequency at a fixed offset (typically 4.5 to 6 MHz) from the picture signal.
The channel frequencies chosen represent a compromise between allowing enough bandwidth for video (and hence satisfactory picture resolution), and allowing enough channels to be packed into the available frequency band. In practice a technique called vestigial sideband is used to reduce the channel spacing, which would be at least twice the video bandwidth if pure AM was used.
Signal reception is invariably done via a superheterodyne receiver
Superheterodyne receiver
In electronics, a superheterodyne receiver uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency...
: the first stage is a tuner which selects a television channel
Television channel
A television channel is a physical or virtual channel over which a television station or television network is distributed. For example, in North America, "channel 2" refers to the broadcast or cable band of 54 to 60 MHz, with carrier frequencies of 55.25 MHz for NTSC analog video and...
and frequency-shifts it to a fixed intermediate frequency
Intermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
(IF). The signal amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
(from the microvolt range to fractions of a volt) performs amplification to the IF stages.
Extracting the sound
At this point the IF signal consists of a video carrier waveCarrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
at one frequency and the sound carrier at a fixed offset. A demodulator recovers the video signal and sound as an FM signal at the offset frequency (this is known as intercarrier sound).
The FM sound carrier is then demodulated, amplified, and used to drive a loudspeaker. Until the advent of the NICAM
NICAM
Near Instantaneous Companded Audio Multiplex is an early form of lossy compression for digital audio. It was originally developed in the early 1970s for point-to-point links within broadcasting networks...
and MTS
Multichannel television sound
Multichannel television sound, better known as MTS , is the method of encoding three additional channels of audio into an NTSC-format audio carrier.- History :...
systems, TV sound transmissions were invariably monophonic.
Structure of a video signal
The video carrier is demodulated to give a composite videoComposite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
signal; this contains luminance, chrominance and synchronization signals; this is identical to the video signal format used by analog video
Analog video
Analog video is a video signal transferred by an analog signal. An analog color video signal contains luminance, brightness and chrominance of an analog television image...
devices such as VCRs or CCTV cameras
Closed-circuit television
Closed-circuit television is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors....
. Note that the RF signal modulation is inverted compared to the conventional AM: the minimum video signal level corresponds to maximum carrier amplitude, and vice versa. The carrier is never shut off altogether; this is to ensure that intercarrier sound demodulation can still occur.
Each line of the displayed image is transmitted using a signal as shown above. The same basic format (with minor differences mainly related to timing and the encoding of color) is used for PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
, NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
and SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
television systems. A monochrome signal is identical to a color one, with the exception that the elements shown in color in the diagram (the color burst, and the chrominance signal) are not present.
The front porch is a brief (about 1.5 microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
) period inserted between the end of each transmitted line of picture and the leading edge of the next line sync pulse. Its purpose was to allow voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
levels to stabilise in older televisions, preventing interference between picture lines. The front porch is the first component of the horizontal blanking interval which also contains the horizontal sync pulse and the back porch.
The back porch is the portion of each scan line between the end (rising edge) of the horizontal sync pulse and the start of active video. It is used to restore the black level (300 mV.) reference in analog video. In signal processing terms, it compensates for the fall time
Fall time
In electronics, fall time \scriptstyle t_f\, is the time required for the amplitude of a pulse to decrease from a specified value to another specified value...
and settling time
Settling time
The settling time of an amplifier or other output device is the time elapsed from the application of an ideal instantaneous step input to the time at which the amplifier output has entered and remained within a specified error band, usually symmetrical about the final value.Settling time includes a...
following the sync pulse.
In color TV systems such as PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
and NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
, this period also includes the colorburst
Colorburst
Colorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...
signal. In the SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
system it contains the reference subcarrier for each consecutive color difference signal in order to set the zero-color reference.
In some professional systems, particularly satellite link
Satellite link
A satellite link is a communications subsystem that involves a link between a transmitting Earth station and a receiving Earth station via a communications satellite.-See also:*Earth terminal*Telecommunications link*Satellite truck*Ku band*C band...
s between locations, the audio is embedded within the back porch of the video signal, to save the cost of renting a second channel.
Monochrome video signal extraction
The luminance component of a composite video signal varies between 0 V and approximately 0.7 V above the 'black' level. In the NTSC system, there is a blanking signal level used during the front porch and back porch, and a black signal level 75 mV above it; in PAL and SECAM these are identical.In a monochrome receiver the luminance signal is amplified to drive the control grid
Control grid
The control grid is an electrode used in thermionic valves used to modulate the flow of electrons in the cathode to anode or plate circuit.- Operation :...
in the electron gun
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
of the CRT. This changes the intensity of the electron beam and therefore the brightness of the spot being scanned. Brightness and contrast controls determine the DC shift and amplification, respectively.
Color video signal extraction
Color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
for more information). However, these are not simply transmitted as three separate signals, because:
- such a signal would not be compatible with monochrome receivers (an important consideration when color broadcasting was first introduced)
- it would occupy three times the bandwidth of existing television, requiring a decrease in the number of TV channels available
- typical problems with signal transmission (such as differing received signal levels between different colors) would produce unpleasant side effects.
Instead, the RGB signals are converted into YUV
YUV
YUV is a color space typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently...
form, where the Y signal represents the overall brightness, and can be transmitted as the luminance signal. This ensures a monochrome receiver will display a correct picture. The U and V signals are the difference between the Y signal and the B and R signals respectively. The U signal then represents how "blue" the color is, and the V signal how "red" it is. The advantage of this scheme is that the U and V signals are zero when the picture has no color content. Since the human eye is more sensitive to errors in luminance than in color, the U and V signals can be transmitted in a relatively lossy (specifically: bandwidth-limited) way with acceptable results. The G signal is not transmitted in the YUV system, but rather it is recovered electronically at the receiving end.
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
and PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
color systems, U and V are transmitted by adding a color subcarrier to the composite video signal, and using quadrature amplitude modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
on it. For NTSC, the subcarrier is usually at about 3.58 MHz, but for the PAL system it is at about 4.43 MHz. These frequencies are within the luminance signal band, but their exact frequencies were chosen such that they are midway between two harmonics of the horizontal line repetition rate, thus ensuring that the majority of the power of the luminance signal does not overlap with the power of the chrominance signal.
In the British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
PAL (D) system, the actual chrominance center frequency is 4.43361875 MHz, a direct multiple of the scan rate frequency. This frequency was chosen to minimize the chrominance beat interference pattern that would be visible in areas of high color saturation in the transmitted picture.
The two signals (U and V) modulate both the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
of the color carrier, so to demodulate them it is necessary to have a reference signal against which to compare it. For this reason, a short burst of reference signal known as the color burst is transmitted during the back porch (re-trace period) of each scan line. A reference oscillator in the receiver locks onto this signal (see phase-locked loop
Phase-locked loop
A phase-locked loop or phase lock loop is a control system that generates an output signal whose phase is related to the phase of an input "reference" signal. It is an electronic circuit consisting of a variable frequency oscillator and a phase detector...
) to achieve a phase reference, and uses its amplitude to set an AGC
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
system to achieve an amplitude reference.
The U and V signals are then demodulated by band-pass filtering to retrieve the color subcarrier, mixing it with the in-phase and quadrature signals from the reference oscillator, and low-pass filtering the results.
NTSC uses this process unmodified. Unfortunately, this often results in poor color reproduction due to phase errors in the received signal. The PAL D (delay) system corrects this by reversing the phase of the signal on each successive line, and the averaging the results over pairs of lines. This process is achieved by the use of a 1H (where H = horizontal scan frequency) duration delay line. (A typical circuit used with this device converts the low frequency color signal to ultrasonic sound and back again). Phase shift errors between successive lines are therefore cancelled out and the wanted signal amplitude is increased when the two in-phase (coincident
Coincident
Coincident is a geometric term that pertains to the relationship between two vectors. Vectors consist of a "magnitude" and a "direction". Vectors can be said to be coincident when their direction is the same though the magnitude may be different; that is to say, they lie one on top of the other...
) signals are re-combined.
In the SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
television system, U and V are transmitted on alternate lines, using simple frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
of two different color subcarriers.
In analog color CRT displays, the brightness control signal (luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
) is fed to the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
connections of the electron guns, and the color difference signals (chrominance
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
signals) are fed to the control grids connections. This simple matrix mixing technique was replaced in later solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
designs of signal processing.
Synchronization
Synchronizing pulses added to the video signal at the end of every scan lineScan line
A scan line or scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube display of a television set or computer monitor....
and video frame ensure that the sweep oscillators in the receiver remain locked in step with the transmitted signal, so that the image can be reconstructed on the receiver screen.
A sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync. (see section below - Other technical information, for extra detail.)
Horizontal synchronization
The horizontal synchronization pulse (horizontal sync HSYNC), separates the scan lineScan line
A scan line or scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube display of a television set or computer monitor....
s. The horizontal sync signal is a single short pulse which indicates the start of every line. The rest of the scan line
Scan line
A scan line or scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube display of a television set or computer monitor....
follows, with the signal ranging from 0.3 V (black) to 1 V (white), until the next horizontal or vertical synchronization pulse.
The format of the horizontal sync pulse varies. In the 525-line NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
system it is a 4.85 µs
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
-long pulse at 0 V
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
. In the 625-line PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
system the pulse is 4.7 µs synchronization pulse at 0 V
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
. This is lower than the amplitude of any video signal (blacker than black) so it can be detected by the level-sensitive "sync stripper" circuit of the receiver.
Vertical synchronization
Vertical synchronization (Also vertical sync or V-SYNC) separates the video fields. In PAL and NTSC, the vertical sync pulse occurs within the vertical blanking intervalVertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
. The vertical sync pulses are made by prolonging the length of HSYNC pulses through almost the entire length of the scan line.
The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or mid-way through).
The format of such a signal in 525-line NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
is:
- pre-equalizing pulses (6 to start scanning odd lines, 5 to start scanning even lines)
- long-sync pulses (5 pulses)
- post-equalizing pulses (5 to start scanning odd lines, 4 to start scanning even lines)
Each pre- or post- equalizing pulse consists in half a scan line
Scan line
A scan line or scanline is one line, or row, in a raster scanning pattern, such as a line of video on a cathode ray tube display of a television set or computer monitor....
of black signal: 2 µs at 0 V, followed by 30 µs at 0.3 V.
Each long sync pulse consists in an equalizing pulse with timings inverted: 30 µs at 0 V, followed by 2 µs at 0.3 V.
In video production and computer graphics, changes to the image are often kept in step with the vertical synchronization pulse to avoid visible discontinuity of the image. Since the frame buffer of a computer graphics
Computer graphics
Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....
display imitates the dynamics of a cathode-ray display, if it is updated with a new image while the image is being transmitted to the display, the display shows a mishmash of both frames, producing a page tearing
Page tearing
Screen tearing is a visual artifact in video where information from two or more different frames is shown in a display device in a single screen draw....
artifact
Digital artifact
A digital artifact is any undesired alteration in data introduced in a digital process by an involved technique and/or technology.-Possible causes:...
partway down the image.
Vertical synchronization eliminates this by timing frame buffer fills to coincide with the vertical blanking interval
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
, thus ensuring that only whole frames are seen on-screen. Software such as computer games and Computer aided design (CAD) packages often allow vertical synchronization as an option, because it delays the image update until the vertical blanking interval. This produces a small penalty in latency, because the program has to wait until the video controller has finished transmitting the image to the display before continuing. Triple buffering reduces this latency significantly.
Two timing intervals are defined - the front porch between the end of displayed video and the start of the sync pulse, and the back porch after the sync pulse and before displayed video. These and the sync pulse itself are called the horizontal blanking (or retrace) interval and represent the time that the electron beam in the CRT is returning to the start of the next display line.
Horizontal hold and vertical hold
The lack of precision timing components available in early television receivers meant that the timebase circuits occasionally needed manual adjustment. The adjustment took the form of horizontal hold and vertical hold controls, usually on the rear of the television setTelevision set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
. Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen.
Transition to digital broadcasts
As of late 2009, ten countries had completed the process of turning off analog terrestrial broadcasting. Many other countries had plans to do so or were in the process of a staged conversion. The first country to make a wholesale switchDigital television transition
The digital television transition is the process in which analog television broadcasting is converted to and replaced by digital television. This primarily involves both TV stations and over-the-air viewers; however it also involves content providers like TV networks, and cable television...
to digital over-the-air (terrestrial television
Terrestrial television
Terrestrial television is a mode of television broadcasting which does not involve satellite transmission or cables — typically using radio waves through transmitting and receiving antennas or television antenna aerials...
) broadcasting was Luxembourg
Luxembourg
Luxembourg , officially the Grand Duchy of Luxembourg , is a landlocked country in western Europe, bordered by Belgium, France, and Germany. It has two principal regions: the Oesling in the North as part of the Ardennes massif, and the Gutland in the south...
in 2006, followed later in 2006 by the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
; in 2007 by Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
, Andorra
Andorra
Andorra , officially the Principality of Andorra , also called the Principality of the Valleys of Andorra, , is a small landlocked country in southwestern Europe, located in the eastern Pyrenees mountains and bordered by Spain and France. It is the sixth smallest nation in Europe having an area of...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, and Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
; in 2008 by Belgium (Flanders)
Flanders
Flanders is the community of the Flemings but also one of the institutions in Belgium, and a geographical region located in parts of present-day Belgium, France and the Netherlands. "Flanders" can also refer to the northern part of Belgium that contains Brussels, Bruges, Ghent and Antwerp...
and Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
; in 2009 by the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
(high power stations -- the important ones), southern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, the Isle of Man
Isle of Man
The Isle of Man , otherwise known simply as Mann , is a self-governing British Crown Dependency, located in the Irish Sea between the islands of Great Britain and Ireland, within the British Isles. The head of state is Queen Elizabeth II, who holds the title of Lord of Mann. The Lord of Mann is...
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
, and Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
. In 2010, Belgium (Wallonia), Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
, Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
, Latvia
Latvia
Latvia , officially the Republic of Latvia , is a country in the Baltic region of Northern Europe. It is bordered to the north by Estonia , to the south by Lithuania , to the east by the Russian Federation , to the southeast by Belarus and shares maritime borders to the west with Sweden...
, Estonia
Estonia
Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...
, the Channel Islands
Channel Islands
The Channel Islands are an archipelago of British Crown Dependencies in the English Channel, off the French coast of Normandy. They include two separate bailiwicks: the Bailiwick of Guernsey and the Bailiwick of Jersey...
, and Slovenia
Slovenia
Slovenia , officially the Republic of Slovenia , is a country in Central and Southeastern Europe touching the Alps and bordering the Mediterranean. Slovenia borders Italy to the west, Croatia to the south and east, Hungary to the northeast, and Austria to the north, and also has a small portion of...
; in 2011 Israel
Israel
The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...
, Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
, Monaco
Monaco
Monaco , officially the Principality of Monaco , is a sovereign city state on the French Riviera. It is bordered on three sides by its neighbour, France, and its centre is about from Italy. Its area is with a population of 35,986 as of 2011 and is the most densely populated country in the...
, Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, Cyprus
Cyprus
Cyprus , officially the Republic of Cyprus , is a Eurasian island country, member of the European Union, in the Eastern Mediterranean, east of Greece, south of Turkey, west of Syria and north of Egypt. It is the third largest island in the Mediterranean Sea.The earliest known human activity on the...
, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
(excluding Miyagi
Miyagi Prefecture
is a prefecture of Japan in the Tōhoku Region on Honshu island. The capital is Sendai.- History :Miyagi Prefecture was formerly part of the province of Mutsu. Mutsu Province, on northern Honshu, was one of the last provinces to be formed as land was taken from the indigenous Emishi, and became the...
, Iwate
Iwate Prefecture
is the second largest prefecture of Japan after Hokkaido. It is located in the Tōhoku region of Honshū island and contains the island's easternmost point. The capital is Morioka. Iwate has the lowest population density of any prefecture outside Hokkaido...
, and Fukushima
Fukushima Prefecture
is a prefecture of Japan located in the Tōhoku region on the island of Honshu. The capital is the city of Fukushima.-History:Until the Meiji Restoration, the area of Fukushima prefecture was known as Mutsu Province....
Prefectures), Malta
Malta
Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...
and France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
completed the transition.
In the United States, high-power over-the-air broadcasts are solely in the ATSC
ATSC
ATSC standards are a set of standards developed by the Advanced Television Systems Committee for digital television transmission over terrestrial, cable, and satellite networks....
digital format since June 12, 2009, the date that the Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(FCC) set for the end of all high-power analog TV transmissions. As a result, almost two million households could no longer watch TV because they were not prepared for the transition. The switchover was originally scheduled for February 17, 2009, until the U.S. Congress passed the DTV Delay Act. By special dispensation, some analog TV signals ceased on the original date. While the majority of the viewers of over-the-air broadcast television in the U.S. watch full-power stations (which number about 1800), there are three other categories of TV stations in the U.S.: low-power broadcasting
Low-power broadcasting
Low-power broadcasting is electronic broadcasting at very low power and low cost, to a small community area.The terms "low-power broadcasting" and "micropower broadcasting" should not be used interchangeably, because the markets are not the same...
stations, Class A stations
Class A television service
The class A television service is a system for regulating some low-power television stations in the United States. Class A stations are denoted by the broadcast callsign suffix "-CA" or "-CD" , although very many analog -CA stations have a digital companion channel that was assigned the -LD...
, and TV translator stations
Broadcast relay station
A broadcast relay station, relay transmitter, broadcast translator , rebroadcaster , or repeater is a broadcast transmitter which relays, repeats, or reflects the signal of another radio station or television station, usually to an area not covered by the signal of the originating station...
. There is presently no deadline for these stations, about 7100 in number, to convert to digital broadcasting.
It is necessary to be cognizant of the fact that in broadcasting, whatever happens in the United States also happens simultaneously in southern Canada and in northern Mexico because those areas are covered by TV stations in the U.S. Furthermore, the major cities of southern Canada made their transitions to digital TV broadcasts simultaneously with the U.S.: Toronto
Toronto
Toronto is the provincial capital of Ontario and the largest city in Canada. It is located in Southern Ontario on the northwestern shore of Lake Ontario. A relatively modern city, Toronto's history dates back to the late-18th century, when its land was first purchased by the British monarchy from...
, Montreal
Montreal
Montreal is a city in Canada. It is the largest city in the province of Quebec, the second-largest city in Canada and the seventh largest in North America...
, Vancouver
Vancouver
Vancouver is a coastal seaport city on the mainland of British Columbia, Canada. It is the hub of Greater Vancouver, which, with over 2.3 million residents, is the third most populous metropolitan area in the country,...
, Ottawa
Ottawa
Ottawa is the capital of Canada, the second largest city in the Province of Ontario, and the fourth largest city in the country. The city is located on the south bank of the Ottawa River in the eastern portion of Southern Ontario...
, Winnipeg
Winnipeg
Winnipeg is the capital and largest city of Manitoba, Canada, and is the primary municipality of the Winnipeg Capital Region, with more than half of Manitoba's population. It is located near the longitudinal centre of North America, at the confluence of the Red and Assiniboine Rivers .The name...
, Sault Ste. Marie
Sault Ste. Marie, Ontario
Sault Ste. Marie is a city on the St. Marys River in Algoma District, Ontario, Canada. It is the third largest city in Northern Ontario, after Sudbury and Thunder Bay, with a population of 74,948. The community was founded as a French religious mission: Sault either means "jump" or "rapids" in...
, Quebec City
Quebec City
Quebec , also Québec, Quebec City or Québec City is the capital of the Canadian province of Quebec and is located within the Capitale-Nationale region. It is the second most populous city in Quebec after Montreal, which is about to the southwest...
, Charlottetown
Charlottetown
Charlottetown is a Canadian city. It is both the largest city on and the provincial capital of Prince Edward Island, and the county seat of Queens County. Named after Queen Charlotte, the wife of George III, Charlottetown was first incorporated as a town in 1855 and designated as a city in 1885...
, Halifax
Halifax County, Nova Scotia
Halifax County is a county in the Canadian province of Nova Scotia.The Municipality of the County of Halifax was the municipal government of Halifax County, apart from the separately incorporated towns and cities therein...
, and so forth.
In Japan, the switch to digital occurred on the 24th of July, 2011 (with the exception of Fukushima, Iwate, and Miyagi prefectures, where conversion was delayed one year due to complications from the 2011 Tōhoku earthquake and tsunami
2011 Tōhoku earthquake and tsunami
The 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...
). In Canada, it is scheduled to happen August 31, 2011. China is scheduled to switch in 2015. In the United Kingdom, the digital switchover has different times for each part of the country. However, the entire U.K. should be on digital TV by 2012.
Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
switched to digital TV on December 2, 2007, in its major cities, and now it is estimated that it will take about seven years for complete conversion over all of Brazil -- but understand that large parts of Brazil are unpopulated by people who have electricity and TV. Australia will turn off analog TV in steps, TV network by network, between 2010 and 2013, region by region.
In Malaysia, the Malaysian Communications & Multimedia Commission (MCMC) advertised for tender bids to be submitted in the third quarter of 2009 for the 470 through 742 MHz UHF allocation, to enable Malaysia's broadcast system to move into DTV. The new broadcast band
Broadcast band
A Broadcast band is a segment of the radio spectrum used for broadcasting.-See also:* North American broadcast television frequencies* Dead air* Internet radio* radio networks* Music radio* Old-time radio* Radio astronomy* Radio programming...
allocation would result in Malaysia's having to build an infrastructure
Infrastructure
Infrastructure is basic physical and organizational structures needed for the operation of a society or enterprise, or the services and facilities necessary for an economy to function...
for all broadcasters, using a single digital terrestrial transmission/TV broadcast (DTTB) channel.
People also need to understand that large portions of Malaysia are covered by TV broadcasts from Singapore
Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...
, Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...
, Brunei
Brunei
Brunei , officially the State of Brunei Darussalam or the Nation of Brunei, the Abode of Peace , is a sovereign state located on the north coast of the island of Borneo, in Southeast Asia...
, and/or Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
(from Borneo
Borneo
Borneo is the third largest island in the world and is located north of Java Island, Indonesia, at the geographic centre of Maritime Southeast Asia....
).
Users may then encode
Encode
Encode may refer to:* Can be related to "Code"* Encode ApS, a Danish software company* Encode SA, a Greek information security company* ENCODE, the ENCyclopedia Of DNA Elements...
and transmit their television program
Television program
A television program , also called television show, is a segment of content which is intended to be broadcast on television. It may be a one-time production or part of a periodically recurring series...
s on this channels` digital data stream. The winner was to be announced at the end of 2009 or early 2010. A condition of the award is that digital transmission must start as soon as possible, and analog switch-off was proposed for 2015. The scheme may not go ahead as the Government successor, Najib Tun Razak
Najib Tun Razak
Dato' Sri Haji Mohd Najib bin Tun Haji Abdul Razak is the sixth, and since 2009, Prime Minister of Malaysia. He previously held the post of Deputy Prime Minister from 7 January 2004 until he succeeded Tun Abdullah Ahmad Badawi as Prime Minister on 3 April 2009. Najib is President of the United...
deferred the transition indefinitely in favor of his own 1Malaysia
1Malaysia
1Malaysia is an on-going programme designed by Malaysian Prime Minister Najib Tun Razak on September 16, 2010, calling for the cabinet, government agencies, and civil servants to more strongly emphasize ethnic harmony, national unity, and efficient governance....
concept, which means that analog television will continue for longer than originally planned.
Components of a television system
A typical analog television receiver is based around the block diagram shown below:Sync Separator
Image synchronization is achieved by transmitting negative-going pulses; in a composite video signal of 1 volt amplitude, these are approximately 0.3 V below the "black level"
Black level
Video black level is defined as the level of brightness at the darkest part of a visual image or the level of brightness at which no light is emitted from a screen, resulting in a pure black screen....
. The horizontal sync signal is a single short pulse which indicates the start of every line. Two timing intervals are defined - the front porch between the end of displayed video and the start of the sync pulse, and the back porch after the sync pulse and before displayed video. These and the sync pulse itself are called the horizontal blanking (or retrace) interval and represent the time that the electron beam in the CRT is returning to the start of the next display line.
The vertical sync signal is a series of much longer pulses, indicating the start of a new field. The sync pulses occupy the whole of line interval of a number of lines at the beginning and end of a scan; no picture information is transmitted during vertical retrace. The pulse sequence is designed to allow horizontal sync to continue during vertical retrace; it also indicates whether each field represents even or odd lines in interlaced systems (depending on whether it begins at the start of a horizontal line, or mid-way through).
In the TV receiver, a sync separator circuit detects the sync voltage levels and sorts the pulses into horizontal and vertical sync.
Loss of horizontal synchronization usually resulted in an unwatchable picture; loss of vertical synchronization would produce an image rolling up or down the screen.
Timebase circuits
In an analog receiver with a CRTCathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
display sync pulses are fed to horizontal and vertical timebase amplifier circuits. These generate modified sawtooth
Sawtooth wave
The sawtooth wave is a kind of non-sinusoidal waveform. It is named a sawtooth based on its resemblance to the teeth on the blade of a saw....
and parabola
Parabola
In mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...
current waveforms to scan the electron beam in a linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
way. The waveform shapes are necessary to make up for the distance variations from the electron beam source and the screen surface. Each beam direction switching circuit is reset by the appropriate sync timing pulse. These waveforms are fed to the horizontal and vertical scan coils wrapped around the CRT tube. These coils produce a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
proportional to the changing current, and this deflects the electron beam across the screen. In the 1950s, television receiver timebase supply was derived directly from the mains supply. A simple circuit consisted of a series
Series and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...
voltage dropper resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
and a rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
(tube) or semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
. This avoided the cost of a large high voltage mains supply (50 or 60 Hz) transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
. This type of circuit was used for thermionic valve (tube) technology. It was inefficient and produced a lot of heat which led to premature failures in the circuitry. In the 1960s, semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
technology was introdued into timebase circuits. During the late 1960s in the U.K., synchronous, (with the scan line rate), power generation was introduced into solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
receiver designs. These had very complex circuits in which faults were difficult to trace, but had very efficient use of power. In the early 1970s AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
mains (50 Hz), and line timebase (15,625 Hz), thyristor
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
based switching circuits were introduced. In the U.K. use of the simple (50 Hz) types of power circuits were discontinued. The reason for design changes arose from the electricity supply contamination problems arising from EMI
EMI
The EMI Group, also known as EMI Music or simply EMI, is a multinational music company headquartered in London, United Kingdom. It is the fourth-largest business group and family of record labels in the recording industry and one of the "big four" record companies. EMI Group also has a major...
, and supply loading issues due to energy being taken from only the positive half cycle of the mains supply waveform.
CRT flyback power supply design and operation principles
Most of the receiver's circuitry (at least in transistorTransistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
- or IC
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
-based designs) operates from a comparatively low-voltage DC power supply. However, the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
connection for a cathode-ray tube requires a very high voltage (typically 10-30 kV) for correct operation.
This voltage is not directly produced by the main power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
circuitry; instead the receiver makes use of the circuitry used for horizontal scanning. Direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC), is switched though the line output transformer, and alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
([AC]) is induced into the scan coils. At the end of each horizontal scan line the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
which has built up in both transformer and scan coils by the current, is a source of latent electromagnetic energy. This stored collapsing magnetic field energy can be captured. The reverse flow, short duration, (about 10% of the line scan time) current from both the line output transformer and the horizontal scan coil is discharged again into the primary winding
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
of the flyback transformer
Flyback transformer
A flyback transformer , also called a line output transformer , is a special transformer, which is used for conversion of energy in electronic circuits. It was initially designed to generate high current sawtooth signals at a relatively high frequency...
by the use of a rectifier which blocks this negative reverse emf
Electromotive force
In physics, electromotive force, emf , or electromotance refers to voltage generated by a battery or by the magnetic force according to Faraday's Law, which states that a time varying magnetic field will induce an electric current.It is important to note that the electromotive "force" is not a...
. A small value capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
is connected across the scan switching device. This tunes the circuit inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
s to resonate at a much higher frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
. This slows down (lengthens) the flyback time from the extremely rapid decay rate that would result if they were electrically isolated during this short period. One of the secondary windings on the flyback transformer then feeds this brief high voltage pulse to a Cockcroft
Cockcroft
Cockcroft may refer to:* Sir John Douglas Cockcroft , British nuclear physicist* Colin Cockcroft , South African military commander* Barry Cockcroft , British television documentary director...
design voltage multiplier
Voltage multiplier
thumb|right|280px|Villard cascade voltage multiplier.A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically by means of a network of capacitors and diodes....
. This produces the required EHT
Extra high tension
The definition of extra high tension or extra high voltage depends on the context of the discussion.In electric power transmission engineering this refers to equipment designed for more than 235,000 volts between conductors....
supply. A flyback converter
Flyback converter
The flyback converter is used in both AC/DC and DC/DC conversion with galvanic isolation between the input and any outputs. More precisely, the flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional...
is a power supply circuit operating on similar principles.
Typical modern design incorporates the flyback transformer and rectifier circuitry into a single unit with a captive output lead, (known as a diode split line output transformer), so that all high-voltage parts are enclosed. Earlier designs used a separate line output transformer and a well insulated high voltage multiplier unit. The high frequency (15 kHz or so) of the horizontal scanning allows reasonably small components to be used.
See also
- Amateur televisionAmateur televisionAmateur television is the transmission of Broadcast quality video and audio over the wide range of frequencies of allocated for Radio amateur use. ATV is used for non-commercial experimentation, pleasure and public service events...
- Analog transmissionAnalog transmissionAnalog transmission is a transmission method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable...
- Broadcast television systemBroadcast television systemBroadcast television systems are encoding or formatting standards for the transmission and reception of terrestrial television signals. There are three main analog television systems in current use around the world: NTSC, PAL, and SECAM...
- Color burst
- Digital televisionDigital televisionDigital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
- Narrow-bandwidth televisionNarrow-bandwidth televisionNarrow-bandwidth television is a type of television designed to fit into a low-bandwidth channel, in the extreme case using amateur radio voice frequency channels that only range up to a few kilohertz...
- OverscanOverscanOverscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube .-Origins of overscan:Early televisions...
- Slow-scan televisionSlow-scan televisionSlow-scan television is a picture transmission method used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or color.A technical term for SSTV is narrowband television...
- TelevisionTelevisionTelevision is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
- Terrestrial televisionTerrestrial televisionTerrestrial television is a mode of television broadcasting which does not involve satellite transmission or cables — typically using radio waves through transmitting and receiving antennas or television antenna aerials...
- TV transmitter
- Vertical blanking intervalVertical blanking intervalThe vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
- Video frame