Digital cable
Encyclopedia
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
.
(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 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 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 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 witched video]
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
.
The subchannel
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 (since 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.
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 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 CableCARD
s to customers that request them.
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
distribution
Distribution (business)
Product distribution is one of the four elements of the marketing mix. An organization or set of organizations involved in the process of making a product or service available for use or consumption by a consumer or business user.The other three parts of the marketing mix are product, pricing,...
using digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
video compression or distribution. The technology was originally developed by Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
.
Background
In 1989, General InstrumentGeneral Instrument
General Instrument was an electronics manufacturer based in Horsham, PA specializing in semiconductors and cable television equipment. The company was active until 1997, when it split into which was later acquired by Vishay Intertechnology in 2001, CommScope and NextLevel Systems General...
(which was later acquired by Motorola
Motorola
Motorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
) 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
DirecTV
DirecTV is an American direct broadcast satellite service provider and broadcaster based in El Segundo, California. Its satellite service, launched on June 17, 1994, transmits digital satellite television and audio to households in the United States, Latin America, and the Anglophone Caribbean. ...
, Dish Network
Dish Network
Dish Network Corporation is the second largest pay TV provider in the United States, providing direct broadcast satellite service—including satellite television, audio programming, and interactive television services—to 14.337 million commercial and residential customers in the United States. Dish...
, and PrimeStar
PrimeStar
PrimeStar was a U.S. direct broadcast satellite broadcasting company formed in 1991 by a consortium of cable television system operators. PrimeStar was the first medium-powered DBS system in the United States but slowly declined in popularity with the arrival of DirecTV in 1994 and Dish Network in...
caused cable providers to seek new ways to provide more programming. Customers were increasingly interested in more channels, pay-per-view
Pay-per-view
Pay-per-view provides a service by which a television audience can purchase events to view via private telecast. The broadcaster shows the event at the same time to everyone ordering it...
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
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
capabilities. This has enabled digital cable providers to offer more channels, video on demand services (without use of a telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
), 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 theft of service)
Most digital cable providers use QAM for video services and DOCSIS
DOCSIS
Data Over Cable Service Interface Specification is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV system...
standards for data services. Some providers have also begun to roll out video services using IPTV
IPTV
Internet Protocol television is a system through which television services are delivered using the Internet protocol suite over a packet-switched network such as the Internet, instead of being delivered through traditional terrestrial, satellite signal, and cable television formats.IPTV services...
or witched video]
Channels
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
North American cable television frequencies
In North American cable TV networks, the radio frequencies used to carry signals to the customer are allocated to standardarized channel numbers listed in the CEA standard 542. Cable channel frequencies are generally different from off-air broadcast frequencies...
.
The subchannel
Digital subchannel
In broadcasting, digital subchannels are a means to transmit more than one independent program at the same time from the same digital radio or digital television station on the same radio frequency channel. This is done by using data compression techniques to reduce the size of each individual...
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 (since 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
Virtual channel
In telecommunications, a logical channel number , also known as virtual channel, is a channel designation which differs from that of the actual radio channel on which the signal travels....
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.
Preserving Bandwidth
Digital cable has allowed for out of market television stations to be removed from the basic cable lineup from Channels 2 through 13. They would be moved over to digital cable channels 100 and above for bandwidth preservation. Currently, Comcast Cable outlets in New Jersey and Virginia are adapting this in significantly viewed counties that are bordering nearby television markets.Technical information
The standard for signal transmission over digital cable television systems in the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
is now fixed as both 64-QAM and 256-QAM (Quadrature Amplitude Modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
), which is specified in SCTE
SCTE
The Society of Cable Telecommunications Engineers or SCTE is a non-profit professional association for the advancement of technology related to cable telecommunications engineering. Founded in 1969, SCTE has a current membership of over 12,000 individuals.- Publications :SCTE offers several...
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
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio 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
QAM tuner
QAM stands for quadrature amplitude modulation, the format by which digital cable channels are encoded and transmitted via cable television providers...
s (most notably the DVR
Digital video recorder
A digital video recorder , sometimes referred to by the merchandising term personal video recorder , is a consumer electronics device or application software that records video in a digital format to a disk drive, USB flash drive, SD memory card or other local or networked mass storage device...
boxes), High Definition versions of local channels and some cable channels are available.
Digital Cable allows for the broadcast of EDTV
EdTV
EDtv is a 1999 American comedy film directed by Ron Howard. An adaptation of the Quebec film Louis 19, le roi des ondes , it stars Matthew McConaughey, Jenna Elfman, Woody Harrelson, Ellen DeGeneres, Martin Landau, Rob Reiner, Sally Kirkland, Elizabeth Hurley, Clint Howard, and Dennis Hopper.The...
(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
16VSB
16VSB is an abbreviation for 16-level vestigial sideband modulation, capable of transmitting four bits at a time.-How it works:Other slower but more rugged forms of VSB include 2VSB, 4VSB, and 8VSB...
transmission over cable at 38.4 Mbit/s, but the encoding
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...
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
Headend in the Sky
Headend in the Sky is Comcast's satellite multiplex service that provides cable channels to cable television operations.At a traditional cable television headend, multitudes of satellite dishes and antennas are used to grab cable stations from dozens of communication satellites...
", a unit of Comcast
Comcast
Comcast Corporation is the largest cable operator, home Internet service provider, and fourth largest home telephone service provider in the United States, providing cable television, broadband Internet, and telephone service to both residential and commercial customers in 39 states and the...
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
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
broadcast channels with the same number, as seen in North American cable television frequencies
North American cable television frequencies
In North American cable TV networks, the radio frequencies used to carry signals to the customer are allocated to standardarized channel numbers listed in the CEA standard 542. Cable channel frequencies are generally different from off-air broadcast 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
CableCARD
CableCARD is a special-use PCMCIA card that allows consumers in the United States to view and record digital cable television channels on digital video recorders, personal computers and television sets without the use of other equipment such as a set top box provided by a cable television company...
s to customers that request them.
See also
- Digital televisionDigital televisionDigital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
- QAM TunerQAM tunerQAM stands for quadrature amplitude modulation, the format by which digital cable channels are encoded and transmitted via cable television providers...
- CableCARDCableCARDCableCARD is a special-use PCMCIA card that allows consumers in the United States to view and record digital cable television channels on digital video recorders, personal computers and television sets without the use of other equipment such as a set top box provided by a cable television company...
- Tru2wayTru2WayTru2way is a brand name for interactive digital cable services delivered over the cable video network, for example interactive program guides, interactive ads, games, chat, web browsing, and t-commerce. The brand also appears as “'” and is used to market cable services, applications, and devices...
- Cable television in the United StatesCable television in the United StatesCable television in the United States is a common form of television delivery, generally by subscription. Cable television first became available in the United States in 1948, with subscription services in 1949. Data by SNL Kagan shows that as of 2006 about 58.4% of all American homes subscribe to...
- Direct-broadcast satellite
- Significantly viewed