Digital cable
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Digital cable is a type of cable television distribution using digital video compression. The technology was developed by Motorola.[citation needed]
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[edit] Uses
Digital cable is used by cable distributors to increase the variety of programming available on their networks, using video compression to transfer more channels through their cable networks already in place. The use of the digital technology also adds the functionality of the cable box to have two-way communication, allowing for the purchasing of pay-per-view programming without use of a phone line, in addition to video on demand services.
[edit] Channels
The addition complicates the notion of a "channel" in digital cable (as well as in over-the-air ATSC digital broadcasts). For example, what was "channel 5" may now have "sub-channel 2", which would be simply called "channel 5-2". The formal names for these two numbers are the "major channel" number and the "minor channel" number.
The major channel number specifies the 6 MHz frequency band, and has the same mapping as analog channel frequencies (both in OTA and cable). For example, cel 5 is 77.25 MHz over the air and 79.25 MHz over cable. The minor channel is a logical channel of data within that frequency band. Over-the-air ATSC specifies that minor channel 0 is the analog NTSC signal. Thus, "channel 5-0" is defined to be analog "channel 5". Technically there can be up to 1024 minor channels in a major channel, though in practice only a few are used (since the bandwidth must be divided among the minor channels).
There are two ways providers try to make this easier for consumers. The first is PSIP, in which 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.
Second, in an effort to hide subchannels entirely, many cable companies map virtual channel numbers to underlying major and minor 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 major/minor channel number 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.
[edit] Technical information
The standard for HDTV signal transmission over digital cable television systems in the United States is now fixed as both 64-QAM and 256-QAM (Quadrature Amplitude Modulation), which is specified in SCTE 07, and is part of the DVB standard (but not ATSC). This method carries 38.4 Mbit/s using 256-QAM on a 6 MHz channel, which can carry nearly two full ATSC 19.39 Mbit/s transport streams. Each 6-MHz channel is typically used to carry 7–12 digital SDTV channels (256-QAM, MPEG2 MP/ML streams of 3–5 Mbit/s). On many boxes with QAM tuners (most notably the DVR boxes), High Definition versions of local channels and some cable channels are available, and can be picked up even on the older analog TVs, however, the signal is downgraded to an analog signal.
Digital Cable allows for the broadcast of EDTV (480p) as well as HDTV (720p, 1080i, and eventually 1080p). By contrast, analog cable transmits programs solely in the 480i format (the lowest television definition).
The ATSC standards include a provision for 16-VSB transmission over cable at 38.4 Mbit/s, but the encoding has not yet gained wide acceptance. Some MATV systems may carry 8-VSB and QAM signals, mostly in apartment buildings and similar facilities that use a combination of terrestrial antennas and cable distribution sources (such as HITS or "Headend In The Sky", a unit of Comcast that delivers digital channels by satellite to small cable systems).
Digital cable channels typically are allocated above 552 MHz, the upper frequency of cable channel 78. (Cable channels above channel 13 are at lower frequencies than UHF broadcast channels with the same number.) Between 552 and 750 MHz, there is space for 33 6-MHz channels (231–396 SDTV channels); when going all the way to 864 MHz, there is space for 52 6-MHz channels (364–624 SDTV channels).
In the U.S., digital cable systems with 750 MHz or greater activated channel capacity are required to comply with a set of SCTE and CEA standards, and to provide CableCARDs to customers that request them.
