Cable television headend
A cable television headend is a master facility for receiving television signals for processing and distribution over a cable television system. A headend facility may be staffed or unstaffed and is typically surrounded by some type of security fencing. The building is typically sturdy and purpose-built to provide security, cooling, and easy access for the electronic equipment used to receive and re-transmit video over the local cable infrastructure. One can also find head ends in power-line communication substations and Internet communications networks.
Reception
Nearly all cable TV systems carry subscription content that is relayed from a satellite in geosynchronous orbit. Encrypted to prevent unauthorized use, this content is uplinked from one or more earth stations operated by various content delivery companies. The content is then analog or digitally modulated and transmitted through the cable network to subscriber homes by means of coaxial or fiber-optic cables buried underground or strung from utility poles.Most cable TV systems also carry local over-the-air television stations for distribution. While each terrestrial channel represents a defined frequency, one or more commercial-grade receiving television antennas are used to receive the multiple channels that the cable company wishes to distribute. These antennas are often built into a single tower structure called a master antenna television structure. Commercial TV pre-amplifiers strengthen the weakened terrestrial TV signals for distribution, usually after re-modulation using a cable-specific analog or digital scheme.
Some cable TV systems receive the local television stations' programming by dedicated coaxial cable, 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 wires, the Internet, microwave towers and local public-access television channels that are sent to the cable headend on an upstream frequency over the cable system itself, 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 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 QAM stream from a satellite, which uses multiplexing. 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 may be performed to add content specifically targeted to the local geographic area.
Analog Modulation
Cable television signals are then mixed in accordance with the cable system's channel numbering scheme using a series of cable modulators, which is in turn fed into a frequency multiplexer 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 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 Modulation
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 ATSC frequency. Using QAM, a CATV operator can place usually up to eight subchannels on each channel so channel 2 may actually be carrying channels 1–8 in a viewer's city. Set-top boxes or CableCards are required to receive these digital signals and are provided by the cable operator themselves.Many modern cable systems are now "all digital" meaning analog video signals have been discontinued in order to reuse spectrum. The RF channels analog used to occupy are now open for a cable system to reuse most commonly as High Speed Data channels to increase subscriber download/upload internet speeds. Analog video removal also essentially eliminates cable theft since analog signals were transmitted unencrypted. Most digital video signals are compressed to MPEG-2 and MPEG-4 formats in order to combine multiple video streams into a QAM making the most efficient use of spectrum which a customer cable set top box receives, demodulates, de-encrypts and displays as a virtual channel number that the viewer recognizes. In many cases the same TV network may appear multiple times in a local channel lineup as a different channel the viewer sees this is due to previous generations of channel lineups kept in service and intended to not confuse viewers who are familiar with the network appearing on a number they are used to. Although a channel may be in a line up multiple times the RF QAM it is combined or "muxed" into is modulated and compressed just once. A set top box tunes to that same QAM when any instance of that network is called by the viewer. Virtual channeling also allows the cable operator to change the physical frequency a QAM is on without the viewer noticing the channel number changing in their lineup.
Most digital cable systems encrypt their signals to eliminate unauthorized reception.