Cable television headend
Encyclopedia
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
signals for processing and distribution over a cable television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
system. The headend facility is normally unstaffed and surrounded by some type of security fencing and is typically a building or large shed housing electronic equipment used to receive and re-transmit video over the local cable infrastructure. One can also find head end
Head end
Head end may refer to:* A cable television headend, a master facility for receiving television signals for processing and distribution over a cable television system* gateway...
s in power line communication
Power line communication
Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...
(PLC) substation
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...
s and Internet communications networks.
Reception
The cable TV headend will normally have several large C-Band FSSFixed Service Satellite
Fixed Service Satellite , is the official classification for geostationary communications satellites used for broadcast feeds for television stations and radio stations and broadcast networks, as well as for telephony, telecommunications and data communications.FSS satellites have also been used...
-type television receive-only
Television receive-only
Television receive-only, TVRO, or Big ugly dish , is a term used in North America to refer to the reception of satellite television from FSS-type satellites, generally on C-band analog; free-to-air and unconnected to a commercial DBS provider...
satellite television
Satellite television
Satellite television is television programming delivered by the means of communications satellite and received by an outdoor antenna, usually a parabolic mirror generally referred to as a satellite dish, and as far as household usage is concerned, a satellite receiver either in the form of an...
dishes for reception of cable/satellite TV networks. A dedicated, non-movable dish
Satellite dish
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.-Principle of operation:...
is required for each satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
that the cable TV utility wishes to receive cable channels from for distribution over its system. For reception of signals from several adjacent satellites, a larger non-parabolic
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...
multi-satellite dish (such as the Simulsat
Simulsat
Simulsat is a quasi-parabolic antenna product line designed and manufactured by Antenna Technology Communications. Whereas satellite dishes with one low-noise block converter are able to receive a satellite television broadcast from one communications satellite at a time, Simulsat antennas are...
) that can see up to 3 or more satellites is often used. Many digital cable
Digital cable
Digital cable is a generic term for any type of cable television distribution using digital video compression or distribution. The technology was originally developed by Motorola.-Background:...
systems use services like HITS
Headend in the Sky
Headend in the Sky is Comcast's satellite multiplex service that provides cable channels to cable television operations.At a traditional cable television headend, multitudes of satellite dishes and antennas are used to grab cable stations from dozens of communication satellites...
("Headend in the Sky
Headend in the Sky
Headend in the Sky is Comcast's satellite multiplex service that provides cable channels to cable television operations.At a traditional cable television headend, multitudes of satellite dishes and antennas are used to grab cable stations from dozens of communication satellites...
"), a unit of Comcast
Comcast
Comcast Corporation is the largest cable operator, home Internet service provider, and fourth largest home telephone service provider in the United States, providing cable television, broadband Internet, and telephone service to both residential and commercial customers in 39 states and the...
, which carry hundreds of channels on just a few satellites; this is commonly used by small systems to expand service without adding expensive new dishes or other equipment.
Most cable TV systems also carry local over-the-air
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...
television networks for distribution. Since each terrestrial channel represents a defined 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...
, a dedicated commercial-grade receiving antenna is needed for each channel that the cable company wishes to receive and distribute. Smaller systems may use a broadband antenna to share several channels. These antenna are often built into a single tower structure called a master antenna television structure. Commercial TV pre-amplifiers
Low-noise amplifier
Low-noise amplifier is an electronic amplifier used to amplify possibly very weak signals . It is usually located very close to the detection device to reduce losses in the feedline. This active antenna arrangement is frequently used in microwave systems like GPS, because coaxial cable feedline is...
strengthen the weakened terrestrial TV signals for distribution.
Some cable TV systems receive the local television stations' programming by dedicated coaxial
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
, microwave link or fiber-optic line, installed between the local station and the headend. A device called a modulator at the local station's facilities feed their programming over this line to the cable TV headend, which in turn receives it with another device called a demodulator. It is then distributed through the cable TV headend to subscribers. This is usually more reliable than receiving the local stations' broadcasts over the air with an antenna. However, off-air reception is used as a backup by the headend in case of failure. In some cases systems receive local channels by satellite.
Other sources of programming include those delivered via fiber optics, telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
wires, the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
towers and local Public-access television
Public-access television
Public-access television is a form of non-commercial mass media where ordinary people can create content television programming which is cablecast through cable TV specialty channels...
channels that are sent to the cable headend on an upstream frequency over the cable system itself (known in the industry as "T"-channels
North American cable television frequencies
In North American cable TV networks, the radio frequencies used to carry signals to the customer are allocated to standardarized channel numbers listed in the CEA standard 542. Cable channel frequencies are generally different from off-air broadcast frequencies...
), or via a dedicated line set up by the cable company, as mentioned earlier for reception of local television stations' programming by the headend.
Signal processing
Once a television signal is received, it must be processed. For digital satellite TV signals, a dedicated commercial satellite receiver such as a GI DSR4400X or a Scientific Atlanta/PowerVu satellite receiver is needed for each channel that is to be distributed by the cable system; these are usually rack-mountable receivers that are designed to take up less space than consumer receivers. They output video and stereo audio signals as well as a digital signal for digital plants.Analog terrestrial TV signals require a processor which is a RF receiver that outputs video and audio. In some cases the processor will include a built-in modulator.
Digital terrestrial TV signals require a special digital processor.
Digital channels are usually received on an L band
L band
L band refers to four different bands of the electromagnetic spectrum: 40 to 60 GHz , 1 to 2 GHz , 1565 nm to 1625 nm , and around 3.5 micrometres .-NATO L band:...
QAM stream from a satellite, which uses multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
. Using special receivers such as the Motorola MPS, the signal can be demultiplexed or "Demuxed" to extract specific channels from the multiplexed signal. At this point, local insertion
Local insertion
In broadcasting, local insertion is the act or capability of a broadcast television station, radio station, or cable TV system to insert or replace part of a broadcast network feed with content unique to the local station or system...
may be performed to add content specifically targeted to the local geographic area.
Modulation
Cable television signals are then mixed in accordance with the cable system's channel numbering scheme using a series of cable modulators (one for each channel), which is in turn fed into a frequency multiplexerMultiplexer
In electronics, a multiplexer is a device that selects one of several analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are used to select which input line to send to the output...
or signal combiner. The mixed signals are sent into a broadband amplifier, then sent into the cable system by the trunk line and continuously re-amplified as needed.
Modulators essentially take an input signal and attach it to a specific frequency. For example in North America, 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...
standards dictate that CH2 is a 6 MHz wide channel with its luminance carrier at 55.25 MHz, so the modulator for channel 2 will impose the appropriate input signal on to the 55.25 MHz frequency to be received by any TV tuned to Channel 2.
Digital channels are modulated as well; however, instead of each channel being modulated on to a specific frequency, multiple digital channels are modulated on to one specific NTSC frequency. Using QAM (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...
), a CATV operator can place usually up to eight channels on one specific frequency so channel 2 may actually be carrying channels 2-10 in your city. STBs (Set top boxes) or CableCard
CableCARD
CableCARD is a special-use PCMCIA card that allows consumers in the United States to view and record digital cable television channels on digital video recorders, personal computers and television sets without the use of other equipment such as a set top box provided by a cable television company...
s are required to receive these digital signals and are provided by the cable operator themselves.
Distribution plant
The plant normally consists of trunk lines that run from one distribution amplifierDistribution amplifier
In electronics, a distribution amplifier or simply distribution amp, is a device that accepts a single input signal and provides this same signal to multiple isolated outputs....
to the next. Feed lines run from a bridge amplifier inside the distribution amplifier to the drops
Drop (telecommunication)
Drop, in telecommunication, has several meanings.* In a communications network, a drop is the portion of a device directly connected to the internal station facilities, such as toward a telephone switchboard, toward a switching center, or toward a telephone exchange. A drop can also be a wire or...
that are placed in front of each house. From the drop a coax runs to each home. Unused taps on the drop are normally terminated with anti-theft terminators. To extend the feed lines even further line extenders are used which are small amplifiers. Some small systems have been built without trunk lines using only line extenders every few hundred feet.
See also
- Cable modem termination systemCable modem termination systemA cable modem termination system or CMTS is a piece of equipment typically located in a cable company's headend or hubsite, and used to provide high speed data services, such as cable Internet or voice over Internet Protocol, to cable subscribers...
- Head endHead endHead end may refer to:* A cable television headend, a master facility for receiving television signals for processing and distribution over a cable television system* gateway...
- North American broadcast television frequenciesNorth American broadcast television frequenciesThe North American broadcast television frequencies are on designated television channels numbered 2 through 69, approximately between 54 and 806 MHz. Traditionally, the frequencies are divided into two sections, the very high frequency band and the ultra high frequency band. The VHF band is...
- North American cable television frequenciesNorth American cable television frequenciesIn North American cable TV networks, the radio frequencies used to carry signals to the customer are allocated to standardarized channel numbers listed in the CEA standard 542. Cable channel frequencies are generally different from off-air broadcast frequencies...