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Digital cable is a generic term for any type of cable television distribution using digital video compression or distribution. The technology was originally developed by Motorola.

Background

In 1989, General Instrument (which was later acquired by Motorola) demonstrated that it was possible to convert an analog cable signal to digital and transmit it in a standard 6-MHz television channel. In the 1990s cable providers began to invest heavily in new digital based distribution systems. Increased competition and programming choices from Direct-broadcast satellite services such as DirecTV, Dish Network, and PrimeStar caused cable providers to seek new ways to provide more progamming. Customers were increasingly interested in more channels, pay-per-view programming, digital music services, and high speed internet services. By 2000, most cable providers in the US were offering some form of digital services to their customers.

Digital cable technology has allowed cable providers to compress video channels so that they take up less frequency space and to offer various two-way communication capabilities. This has enabled digital cable providers to offer more channels, video on demand services (without use of a telephone line), telephone services, high speed internet services, and interactive television services. In addition, digital cable technology allows for error correction to ensure the quality of the received signal and uses a secure digital distribution system (i.e. a secure encrypted signal to prevent eavesdropping and "stealing" service).

Most digital cable providers use QAM for video services and DOCSIS standards for data services. Some providers have also begun to roll out video services using IPTV or Switched video technologies.

Technical information

The standard for signal transmission over digital cable television systems in the United Statesmarker 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.

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