Science Fair Project Encyclopedia
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 intended to replace the NTSC system and produce wide screen 16:9 images up to 1920×1080 pixels in size—more than six times the display resolution of the earlier standard. However, a host of different image sizes are supported, so up to six standard-definition "virtual channels" can be carried in a single broadcast. ATSC also boasts "theater quality" audio because it uses the Dolby Digital (AC-3) format to provide "5.1" surround sound. Numerous auxiliary data services can also be provided.
ATSC is a competitor to the more widely-used DVB standards, and ISDB being developed in Japan. The system includes the capability to carry PAL- and SECAM-format video (576 displayable lines, 50 fields per second) along with NTSC (480 displayable lines, 60 fields per second) and film (24 frames per second). Broadcasters who use ATSC and must retain an analog signal have to broadcast on two separate channels, as the ATSC system requires use of an entire six megahertz channel. This flurry of standards and abbreviations all makes the entire system very complicated and expensive to implement and use, which have been significant criticisms.
The ATSC system supports a host of different display resolutions and frame rates. The formats below list lines of resolution and frame/field rates:
The different resolutions can operate in progressive scan or interlaced mode, although the highest 1080-line system is more limited and cannot display progressive images at the rate of 60 frames per second. Such technology was seen as too advanced at the time, plus the image quality was deemed to be too poor considering the amount of data that can be transmitted. A terrestrial (over-the-air) transmission carries 19.39 megabits of data per second, compared to a DVD which typically has an upper limit of 9 or 10 Mbit/s.
"EDTV" is largely a marketing term created to sell low-resolution televisions with minor enhancements. Such TVs can display progressive scan content and frequently have a 16:9 wide screen format. Such resolutions are 720×480 or 720×576 in PAL, allowing 60 progressive frames per second or 50 in PAL.
Brushing aside marketing-speak, there are three basic display sizes for ATSC. Basic and enhanced NTSC and PAL image sizes are at the bottom level at 480 or 576 lines. Medium-sized images have 720 lines of resolution 960 or 1280 pixels wide. The top tier has 1080 lines either 1440 or 1920 pixels wide. All of these image sizes come in wide screen and traditional 4:3 versions.
ATSC is based on the MPEG-2 system. It should be noted that MPEG-2 defines an entire system of encoding and encapsulating information (a "transport"), and is not merely a video compression algorithm. ATSC uses 188-byte MPEG-TS packets to carry data. This is the "raw" data that a decoder interprets, following demodulation and error correction of the data stream.
1080-line video is actually encoded with 1920×1088 pixel frames, but the last 8 lines are discarded prior to display. This is due to a restriction of the MPEG-2 video format.
Dolby Digital AC-3 is used as the audio codec, though it was officially standardized as A/52 by the ATSC. It allows the transport of up to 5 channels of sound with a 6th channel for low frequency effects (the so-called "5.1" configuration). In contrast, Japanese ISDB HDTV broadcasts use MPEG's Advanced Audio Coding (AAC) as the audio codec, which also allows 5.1 audio output. DVB allows both.
Modulation and Transmission
ATSC signals are designed to use the same 6 MHz bandwidth as NTSC television channels. Once the video signals have been compressed and the data stream can be modulated in a variety of manners, depending on the method of transmission.
Terrestrial (local) broadcasters use a 8-VSB modulation that can transfer at a maximum rate of 19.39 Mb/s, sufficient to carry several video channels and metadata depending on conditions. Cable television operators generally have a lower signal-to-noise ratio and can use 16-VSB or 256-QAM to achieve a throughput of 38.78 Mb/s, using the same 6 MHz.
In recent years cable operators have become accustomed to compressing standard resolution video for digital cable systems, making it harder to find duplicate 6 MHz channels for local broadcasters on uncompressed "basic" cable. Cable operators lobbied the Federal Communications Commission to allow 256-QAM in addition to the 16-VSB standard originally mandated. Though successful, cable operators have still been slow to add ATSC channels to their lineups.
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