Technical Information
The standard for 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 38.47 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.
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 in use today).
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, as seen in North American cable television frequencies.) 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.
Read more about this topic: Digital Cable
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