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The Advanced Television Systems Committee (ATSC) is the group that helped to develop the new digital television standard for the United States, also adopted by Canada, Mexico and South Korea and being considered by other countries. It is a competitor to the more widely-used DVB standards.
The ATSC standard uses AC-3 (Dolby Digital) audio. It allows for the transmission of multiple audio and video channels of information per channel assignment. One standard VHF or UHF channel six megahertz wide might carry up to six standard-definition TV programs, plus PSIP (virtual channel) and other auxiliary data.
HDTV
ATSC also allows for the transmission of HDTV (high-definition television) pictures which provide higher resolution and better color representation when compared to standard-definition or enhanced-definition signals.
EDTV
The US digital television system also allows lower, non HD resolutions to be encoded, such as Enhanced Digital Television (EDTV), which is a standard size of 720 × 480 (PAL: 720 × 576) TV picture, only in progressive format, allowing 60 (PAL: 50) full frames per second. Also included is a system for broadcasting Standard Definition Television (SDTV) with the interlacing.
AC-3
Dolby Digital AC-3 is used as the audio codec, allowing the transport of up to 5 channels of sound with a 6th channel for low frequency effects (the so-called "5.1" configuration). Japanese HDTV broadcasts use MPEG's Advanced Audio Coding (AAC) as the audio codec, which also allows 5.1 audio output. DVB allows both.
Transmission
Digital HDTV transmission is designed to occupy the same 6 MHz terrestrial band now used in the US for analog NTSC broadcasts. A single NTSC 6 MHz channel can carry 19.2 Mbit/s of information using ATSC's standard 8-VSB (8-level Vestigial Side Band) modulation method. This is sufficient to carry up to 6 standard definition TV channels, or a single HDTV channel. As a side note, the standard for HD signal transmission over digital cable television systems in the US is now fixed as QAM 256 (Quadrature Amplitude Modulation), which is technically part of the DVB standard (but not ATSC) and is a de facto cable industry standard. This method has higher bandwidth than 8-VSB, allowing two 19.2 Mbit/s channels in a 6 MHz bandwidth, due to its lower tolerance for errors which are generally less of a concern in a wired environment. The ATSC standards included a provision for 16-VSB transmission over cable at 38.4 Mbit/s, but the encoding never gained wide acceptance.
Resolution
Due to technical reasons having to do with the video equipment, recording technologies, and the 19.2 Mbit/s-limited ATSC channel, some HDTV signals will not reach their nominal resolution. Most notably, 1080i60 is impossible to broadcast without artifacts at this bandwidth using ATSC. Most 1080i broadcast signals actually are filtered to 1440 horizontal samples to allow adequate compression, and most current consumer HDTVs based on CRTs cannot resolve even 1440 horizontal samples (most rear-projection CRTs will resolve 1200-1300 at best, unless based on 9" guns). Despite this, HDTV viewed even on existing sets is still far superior to PAL or NTSC in resolution, and future sets are likely to offer superior resolution at the same or lower prices.
Formats
See HDTV
See also
External link
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