Cable television headend
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Cable television headend is a master facility for receiving television signals for processing and distribution over a cable television 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 headends in power line communication (PLC) substations.
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[edit] Reception
The cable TV headend will normally have several large FSS-type television receive-only satellite television dishes for reception of cable/satellite TV networks such as ESPN, CNN, or HBO; a dedicated, non-movable dish is required for each satellite 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 multi-satellite dish (such as the Torus or Simulsat) that can see up to 3 or more satellites is often used. Many digital cable systems use services like HITS ("Headend in the Sky", a unit of Comcast distributed by the Comcast Media Center), 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.[citation needed]
Cable TV systems usually also carry local over-the-air television networks for distribution. Since each terrestrial channel represents a defined frequency, a dedicated commercial-grade receiving antenna is needed for each channel that the cable company wishes to receive and distribute. These antennas are often built into a single tower structure called a master antenna television structure. Often, commercial TV pre-amplifiers are used to strengthen weakened terrestrial TV signals as much as possible before distribution.
Some cable TV systems receive the local television stations' programming by instead having a dedicated coaxial or fiber-optic line installed between the local station and the headend. A device called a modulator is installed at the local station's facilities to feed their programming over this line to the cable TV headend, which in turn receives it with another device called a de-modulator. It is then distributed through the cable TV headend to the subscriber. This is usually a more reliable method than receiving the local stations' broadcasts over the air with an antenna, however, off-air reception is used as a backup by the headend if the dedicated line, modulator or demodulator were to fail.
Other sources of programming include those delivered via fiber optics, telephone wires, the Internet, microwave towers and local community access channels that are sent to the cable headend on an outbound frequency over the cable system itself (known in the industry as "T"-channels), or via a dedicated line set up by the cable company, as mentioned earlier for reception of local television stations' programming by the headend.
[edit] Signal processing
Once a television signal is received, it must be processed. For satellite TV signals, a dedicated commercial satellite receiver such as a GI DSR4400X 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. Other signal types such as Analog terrestrial TV signals do not need a special receiver for reception; instead, these channels can be received with a commercial RF antenna.
Digital channels are usually received on an L band QAM stream from a satellite usually called a Multiplex or MUX. Using special receivers such as the Motorola MPS, the signal can be demultiplexed or "Demuxed" to extract specific channels from the ones in the original MUX. After the signal is demux'd, the cable TV operator can then modify the signal in anyway that they see fit. For example, a CATV operator may combine local programming in to the received MUX and then recombine these signals in to a new MUX using equipment such as a Terayon Cherrypicker or an RGB Networks BNP and then redistribute the new MUX out on to their cable plant.
[edit] Modulation
Once received, 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 multiplexer. Once processed, the television signals are sent over the cable system's coaxial cables 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 standards dictate that CH2 is a 6MHz wide channel with its luminance carrier at 55.25MHz, so the modulator for channel 2 will impose the appropriate input signal on to the 55.25MHz 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), 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. STB's (Set top boxes) or CableCards are required to receive these digital signals and are provided by the cable operator themselves.
[edit] See also
- Cable modem termination system
- North American broadcast television frequencies
- North American cable television frequencies
