Video signal generator
Encyclopedia
A video signal generator is a type of signal generator
which outputs predetermined video
and/or television
oscillation
waveforms, and other signals used in the synchronization
of television devices and to stimulate faults in, or aid in parametric measurements of, television and video systems. There are several different types of video signal generators in widespread use. Regardless of the specific type, the output of a video generator will generally contain synchronization signals appropriate for television, including horizontal and vertical sync pulses (in analog) or sync words (in digital). Generators of composite video signals (such as NTSC
and PAL
) will also include a colorburst
signal as part of the output.
, PAL
, or both), another may generate CCIR 601
, and a third may generate MPEG streams over an ASI
.
Many manufacturers sell signal generation platforms, which can be populated with multiple modules providing the above capabilities (and supporting different formats). Many such platforms also include audio generation capability (as television includes audio as well as video), supporting either embedded audio or standalone audio formats.
, and other useful test signals, for troubleshooting and analyzing television systems. These devices are generally intended for off-line use (test patterns are seldom broadcast, unless a station is not operating properly or is off the air at the time), as they output complete television signals.
Examples of signals output by such a device include:
A few specialized signals are used in digital
environments:
In addition, sophisticated signal generators may allow modification of the video timing, adjustment of the gains of the various components (including out of range), the introduction of jitter
or bit errors (into digital signals), the introduction of motion, or other effects.
Signal generator
Signal generators, also known variously as function generators, RF and microwave signal generators, pitch generators, arbitrary waveform generators, digital pattern generators or frequency generators are electronic devices that generate repeating or non-repeating electronic signals...
which outputs predetermined video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
and/or television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
waveforms, and other signals used in the 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....
of television devices and to stimulate faults in, or aid in parametric measurements of, television and video systems. There are several different types of video signal generators in widespread use. Regardless of the specific type, the output of a video generator will generally contain synchronization signals appropriate for television, including horizontal and vertical sync pulses (in analog) or sync words (in digital). Generators of composite video signals (such as 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 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...
) will also include a 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 as part of the output.
Types of video signal generators
Video signal generators are primarily classed according to their function. In addition, they may be classified according to the video formats and interface standard they support—one generator may generate composite analog signals (typically 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 both), another may generate CCIR 601
CCIR 601
ITU-R Recommendation BT.601, more commonly known by the abbreviations Rec. 601 or BT.601 is a standard published in 1982 by International Telecommunication Union - Radiocommunications sector for encoding interlaced analog video signals in digital video form...
, and a third may generate MPEG streams over an ASI
Asynchronous serial interface
Asynchronous Serial Interface, or ASI, is a streaming data format which often carries an MPEG Transport Stream .An ASI signal can carry one or multiple SD, HD or audio programs that are already compressed, not like an uncompressed SD-SDI or HD-SDI . An ASI signal can be at varying transmission...
.
Many manufacturers sell signal generation platforms, which can be populated with multiple modules providing the above capabilities (and supporting different formats). Many such platforms also include audio generation capability (as television includes audio as well as video), supporting either embedded audio or standalone audio formats.
Test signal generators
A test signal generator generates test patternsTest card
A test card, also known as a test pattern in North America and Australia, is a television test signal, typically broadcast at times when the transmitter is active but no program is being broadcast...
, and other useful test signals, for troubleshooting and analyzing television systems. These devices are generally intended for off-line use (test patterns are seldom broadcast, unless a station is not operating properly or is off the air at the time), as they output complete television signals.
Examples of signals output by such a device include:
- Color bars, one of several test signals used to verify the proper reproduction of a TV system's color gamut, and/or that a television signal or plant is compliant with the appropriate analog transmission standards
- Flat fields, a signal consisting of nothing but a specific color (typically white, black, a shade of gray, or one of the primary colorPrimary colorPrimary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....
s (red, green, and blue) at maximum saturation). A red field is especially important in PALPALPAL, 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...
applications, as it is the "red difference" portion of the chroma signal whose phase alternates every line; the red field should appear as a solid block of color, with no visible "bands" going across the screen. - Multibursts, sweeps, and pulse signals, used to test the frequency response of a television system
- Ramp signals and staircase signals, used to check the voltage linearity of a television system
- Convergence patterns, used to check the alignment and linearity of a receiver
- The bowtie signal, used to check the relative (inter-channel) timing of a component video signal.
A few specialized signals are used in digital
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
environments:
- The PLL test signal is a pathological test signal used to stress the phase-locked loopPhase-locked loopA 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...
of a serial digital receiver; this is done by outputting a bit pattern which, after passing through the linear feedback shift registerLinear feedback shift registerA linear feedback shift register is a shift register whose input bit is a linear function of its previous state.The most commonly used linear function of single bits is XOR...
used to scramble serial digital signals, resulting (with a high degree of probability) in a long strings of zeroes or ones, followed by a long string of the opposite polarity, on the digital (NRZI) signal; an issue which can cause poorly-designed PLLs to unlock. - The equalizer test signal is another pathological signal, consisting of a long string of zeroes or ones, followed by a single bit of the opposite polarity. It can cause poorly-designed cable equalizers to malfunction.
- The SDI Checkfield signal, standardized by SMPTE RP178 (for SD) and RP198 (for HD), is a test signal which contains one of the above signals in the upper portion of the video, and the other in the lower portion of the video.
In addition, sophisticated signal generators may allow modification of the video timing, adjustment of the gains of the various components (including out of range), the introduction of jitter
Jitter
Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
or bit errors (into digital signals), the introduction of motion, or other effects.
VITS inserters
A vertical interval test signal inserter, or VITS inserter inserts test patterns into the vertical interval of a television signal. Unlike test signal generators; a VITS inserter is used to insert the test signal into live programming, so that inline measurements of a transmission chain can be made while the chain is operational. (As the vertical interval is typically not visible on end-user televisions, this can be done without producing any artifacts noticeable to viewers). Since VITS signals can often be transmitted, it is also possible for a television station to receive its own on-air feed, and use the VITS to detect and troubleshoot problems in on-air transmission.Sync pulse generators (SPG)
A sync pulse generator is a special type of generator which produces synchronization signals, with a high level of stability and accuracy. These devices are used to provide a master timing source for a video facility. The output of an SPG will typically be in one of several forms, depending on the needs of the facility:- A continuous waveContinuous waveA continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
signal - In standard-definition applications, a bi-level sync signal, often with a colorburstColorburstColorburst 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 facilities that have analog equipment. Typically, this is either in NTSCNTSCNTSC, 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...
or PALPALPAL, 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...
format. As the resulting signal is usually indistinguishable from an all-black television signal of the same format, this sort of reference is commonly known as black or black burst. - In some high-definition applications, a tri-level sync signal is used instead. This signal is virtually identical to the synchronization signal used in component analogue videoComponent videoComponent video is a video signal that has been split into two or more component channels. In popular use, it refers to a type of component analog video information that is transmitted or stored as three separate signals...
(CAV); and is similar to the synchronization signals used in VGA (the main difference being, in VGA the horizontal and vertical syncs are carried on different wires; whereas TLS signals include both H and V syncs).
Logo inserters
Logo inserters are devices used to insert a television station's logo, or other fixed graphics, into a live television signal. Often called a "Bug Generator."External links
- Tektronix, Inc.A Guide to Digital Television Systems and Measurements
- Tektronix, Inc. http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/App_Notes/25_7049/eng/%3Fwt%3D480%26wtwi%3D2219%26wtla%3DEN%26wtty%3DTI%26wtsty%3DPrimer%26wtpt%3DDETAILS%26wtbu%3DInstruments+Business%26wtpl%3DVideo+PL%26wtlit%3D25W-7049-4%26wtcat%3Dsignal+generators%26wtmd%3DTG2000%252CVM700T%252CVITS200%26wtti%3DTelevision+Measurements%253A+NTSC+Systems&FrameSet=televisionNTSC Systems Television Measurements]
- Tektronix, Inc. PAL Systems Television Measurements