Channels
Digital cable technology can allow many TV channels to occupy the frequency space that would normally be occupied by a single analog cable TV channel. The number of channels placed on a single analog frequency depends on the compression used. Many cable providers are able to fit about 10 digital SD channels or 2 digital HD channels on a single analog channel frequency. Some providers are able to squeeze more channels onto a single frequency with higher compression, but often this can cause the video quality of the channel to degrade.
The addition of this capability complicates the notion of a "channel" in digital cable (as well as in over-the-air ATSC digital broadcasts). The formal names for the two numbers that now identify a channel are the physical channel and the subchannel.
The physical channel is a number corresponding to a specific 6 MHz frequency range. See: North American cable television frequencies.
The subchannel is a logical channel of data within the physical channel. Technically there can be up to 1024 subchannels in a physical channel, though in practice only a few are used (as the bandwidth must be divided among all the subchannels).
There are two ways providers try to make this easier for consumers. The first, accomplished through PSIP, is where program and channel information is broadcast along with the video, allowing the consumer's decoder (set-top box or display) to automatically identify the many channels and subchannels.
The second (also accomplished through PSIP) is where, in an effort to hide subchannels entirely, many cable companies map virtual channel numbers to underlying physical and sub-channels. For example, a cable company might call channel 5-1 "channel 732" and channel 5-2 "channel 733". This also allows the cable company to change the frequency of a channel without changing what the customer sees as a channel number. In such arrangements, the physical/sub-channel numbers are called the "QAM channel", and the alternative channel designation is called the "mapped channel", "virtual channel", or simply "channel".
In theory, a set-top box can decode the PSIP information from every channel it receives and use that information to build the mapping between QAM channel and virtual channel. However, cable companies do not always reliably transmit PSIP information. Alternatively, CableCards receive the channel mapping and can communicate that to the set-top box.
Read more about this topic: Digital Cable
Famous quotes containing the word channels:
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