Digital cable is the distribution
of cable television
using digital video compression
for distribution. The technology was developed by General Instrument, which was succeeded by Motorola
and subsequently by ARRIS Group
. Cable companies converted to digital cable during the 2000s, during the period that broadcast television converted to the digital HDTV
standard, which was incompatible with existing analog cable systems. In addition to providing higher resolution HD
video, digital cable systems provide more services such as pay-per-view
programming, cable internet access
and cable telephone services
. Most digital cable signals are encrypted
, which reduced the incidence of cable television piracy
which occurred in analog systems.
In 1990, General Instrument
(now owned by ARRIS Group
) demonstrated that it was possible to use digital compression to deliver high quality HDTV in a standard 6 MHz television channel. Using the same technology General Instrument (GI) demonstrated the digital transmission of multiple high quality standard definition programs in a 6 MHz cable channel.
In the 1990s, cable providers began to invest heavily in this new multi-channel digital TV technology to expand the number of channels and services available to subscribers. 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 programming. 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 cable TV to their customers.
Digital cable technology has allowed cable providers to compress video channels so that they take up less bandwidth
and to offer two-way communication
capabilities. This has enabled providers to offer more channels, video-on-demand
services that don't require a separate telephone line
, telephone services, high speed internet services, and interactive television services. Digital cable implements error correction
to ensure the integrity of the received signal and uses a secure digital distribution system (i.e. a secure encrypted signal to prevent eavesdropping and theft of 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
Digital cable technology can allow many TV channels to occupy the frequency space that would normally be occupied by a single analog cable TV channel. The number of channels placed on a single analog frequency depends on the compression used. Many cable providers are able to fit about 10 digital SD channels or 2 digital HD channels on a single analog channel frequency. Some providers are able to squeeze more channels onto a single frequency with higher compression, but often this can cause the video quality of the channel to degrade.
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 the two numbers that now identify a channel are the physical channel and the subchannel.
The physical channel is a number corresponding to a specific 6 MHz frequency range. See: North American cable television frequencies
is a logical channel of data within the physical channel. Technically there can be up to 1024 subchannels in a physical channel, though in practice only a few are used (as the bandwidth must be divided among all the subchannels).
There are two ways providers try to make this easier for consumers. The first, accomplished through PSIP
, is where 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.
The second (also accomplished through PSIP) is where, in an effort to hide subchannels entirely, many cable companies map virtual channel
numbers to underlying physical and sub-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 physical/sub-channel numbers 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.
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 tuner
s (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 1080p). By contrast, analog cable transmits programs solely in the 480i format (the lowest television definition in use today).
The Advanced Television Systems Committee 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 SMATV
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 CableCARD
s to customers that request them.
*Cable television in the United States
* Significantly viewed
External linksNational Cable and Telecommunications Association - Digital CableDigital cable & Digital house