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Digital cable

From Academic Kids

Digital cable is a term for a type of cable digital television that delivers more channels than possible with analog cable by using digital video compression. Digital cable also enables two-way communication, enabling services such the ability to purchase pay-per-view programming without the use of a phone line. Recently, some companies have also added video on demand services.

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).

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 (e.g., HITS) sources.

Digital cable channels typically are allocated above 552 MHz, the upper frequency of cable channel 78. (N.B.: over-the-air channels are at higher frequencies than cable channels.) 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 more than 750 MHz activated channel capacity are required to comply with a set of SCTE and CEA standards, and to provide CableCARDs to customers that request them.

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