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Established in 1982, the Advanced Television Systems Committee is the group that developed the ATSC digital television standard for the United States, also adopted by Canada, Mexico, Taiwan, and South Korea, and being considered by other countries.

It is intended to replace the NTSC system used mostly in North America 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 broadcast on a single TV station using the existing 6MHz channel. ATSC also boasts "theater quality" audio because it uses the Dolby Digital AC-3 format to provide 5.1-channel surround sound. Numerous auxiliary datacasting services can also be provided.

ATSC coexists with the earlier and more widely-used DVB standard, and ISDB being implemented in Japan and soon in Brazil. A similar standard called ADTB was developed for use as part of China's new DMB-T/H dual standard. Because of potential use outside of existing NTSC areas, the ATSC system includes the capability to carry PAL- and SECAM-format video (576 displayable lines, 50 fields or 25 frames per second) along with NTSC (480 displayable lines, 60 fields or 30 frames 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 (in-band adjacent-channel), as the ATSC system (like DVB) requires use of an entire channel. Virtual channels allow channel numbers to be remapped from their physical RF channel to any other number 1 to 99, so that ATSC stations can either appear on the numbers they always have, or all stations on a network can use the same number. There is also a standard for distributed transmission (DTx) which allows for booster stations.

The system has been criticized as being complicated and expensive to implement and use. Many aspects of ATSC are patented, including the AC-3 audio coding, and the VSB modulation. The standards ATSC depends on are often ambiguous, one example would be the EIA-708 standard for closed captioning.

Contents

Resolution

The ATSC system supports a host of different display resolutions and frame rates. The formats below are listed by resolution, form of scanning (progressive or interlaced), and frames per second (for more informations and links, see also the TV resolution overview at the end of this article):

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 59.94 or 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 the maximum possible bitrate of 10.08Mbit/s allowed in the DVD standard.

"EDTV" is largely a marketing term created to sell standard-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 in NTSC or 720×576 in PAL, allowing 60 progressive frames per second in NTSC 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 and are 960 or 1280 pixels wide (for [[4:3]], traditional version, and [[16:9]], wide screen version, aspect ratio respectively). The top tier has 1080 lines either 1440 or 1920 pixels wide (here, too, for [[4:3]] and [[16:9]] aspect ratio respectively). 1080-Line video is actually encoded with 1920×1088 pixel frames, but the last eight lines are discarded prior to display. This is due to a restriction of the MPEG-2 video format, which requires the number of coded luma samples (i.e., pixels) to be divisible by 16.

Codecs

For transport, ATSC uses the MPEG-2 Systems specification, known as Transport stream, to encapsulate data. ATSC uses 188-byte MPEG transport stream packets to carry data. Before decoding of audio and video takes place, the receiver must demodulate and apply error correction to the signal. Then, the transport stream may be demultiplexed into its constituent streams.

MPEG-2 video is used as the video codec.

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 five channels of sound with a sixth 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

Main articles: 8VSB, 256QAM

ATSC signals are designed to use the same 6 MHz bandwidth as NTSC television channels. Once the video signals have been compressed, 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 Mbit/s, sufficient to carry several video channels and metadata depending on conditions. Cable television operators generally have a higher signal-to-noise ratio and can use 16-VSB or 256-QAM to achieve a throughput of 38.78 Mbit/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.

There is also a standard for transmitting ATSC via satellite, however this is only used by TV networks. It is not used for direct broadcast satellite systems, which even in North America have long used DVB-S.

Shortcomings

The ATSC signal cannot be adapted to changes in radio propagation conditions, very unlike DVB-T and ISDB-T. If ATSC were able to dynamically change its error correction modes, code rates, interleaver mode, and randomizer, the signal could be more robust even if the modulation itself does not change. It also lacks hierarchical modulation, which allows the SDTV part of an HDTV signal to be received even in fringe areas where signal strength is low.

In spite of ATSC's fixed transmission mode, it is still a robust waveform under normal conditions. 8VSB was chosen over COFDM in part because many areas of North America are rural and have a much lower population density, thereby requiring larger transmitters and resulting in large fringe areas. In these areas, 8VSB was shown to perform better, although in metropolitan areas where the great and increasting majority of North Americans live, COFDM is much better at handling multipath. COFDM is used in both DVB-T and ISDB-T, and for ISDB-H, as well as DVB-H and HD Radio in the United States.

Standards

Below are the ATSC-published standards for digital television. Prior to the group's DTV work, it published A/49, a ghosting-canceling reference signal for NTSC.

See also

External links

Digital video resolutions
Designation Usage examples Definition (lines) Rate (Hz)
Interlaced (fields) Progressive (frames)
MP@LL LDTV, VCD SIF) 24, 30; 25
Standard; MP@ML SDTV, SVCD, DVD, DV 480 (NTSC, PAL-M); 60; 24, 30;
576 (PAL, SECAM) 50 25
Enhanced EDTV 480; 576 60; 50
High; MP@HL HDTV, HD DVD, BD, HDV 720 24, 25, 30, 50, 60
1080 50, 60 24, 25, 30

This table illustrates total horizontal and vertical detail via box size. It does not accurately reflect the screen shape (aspect ratio) of these formats, which is always stretched or squeezed to 4:3 or 16:9. The table assumes an average vertical detail loss of .75x due to interlace. The actual loss is variable due to content, motion, opinion on acceptable levels of flicker, and possible success of deinterlacing. 1920 × 1080i is not included because all common use of 1080i is filtered to 1440 or less.

 

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