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
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.
See also
References
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