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 (one for each channel), 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 (
Quadrature Amplitude Modulation), 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 (STBs) 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 (commonly referred to in the industry as "HSD") channels to increase subscriber download/upload internet speeds. (see
DOCSIS) 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 (I.E. CNN as 34, 334, 1034) 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 (both data and video) to eliminate unauthorized reception. ==National Video Transporting==