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.[1]
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[edit] Uses
Cable distributors use digital video compression to transfer more channels through cable networks already in place. Digital technology also gives two-way communication capability to the cable box, so users can purchase pay-per-view programming without use of a telephone line, in addition to video on demand services and a secure signal. Some providers also display caller ID information to users which have that provider's cable telephone service.
[edit] Channels
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 these three numbers are the "major channel" number, "minor channel" number, and "physical channel".
The major number is also known as a virtual channel number. This is a number that the broadcaster chooses that can display on your television (if their cable box doesn't reassign it) that masks the actual "physical channel."
The minor channel is a logical channel of data within the major/physical channel. 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).
The physical channel is a number corresponding to a specific frequency range. See: North American cable television frequencies.
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 converted 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 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.
[edit] See also
[edit] References
- ^ Decades of proven experience. motorola.com. Retrieved on 2008-03-05.
[edit] External links
- National Cable and Telecommunications Association - Digital Cable
- Digital cable & Digital house
- US CATV Channels and Frequencies table (up to channel 78)
- US CATV Channels and Frequencies table (comprehensive)
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