ATSC Standards document a digital television format intended to replace (in the United States) the analog NTSC television system (NTSC is used mostly in North America and Japan). It was developed by the Advanced Television Systems Committee.
The high definition television standards defined by the ATSC 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 also supported, so that up to six standard-definition "virtual channels" can be broadcast on a single TV station using the existing 6 MHz 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.
Broadcasters who use ATSC and want to retain an analog signal must broadcast on two separate channels, as the ATSC system requires the 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 be associated with the related NTSC channel numbers, 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.
ATSC standards are marked A/x (x is the standard number) and can be downloaded freely from ATSC website (see External Links).
Many aspects of ATSC are patented, including elements of the MPEG video coding, the AC-3 audio coding, and the 8VSB modulation. As with other systems, ATSC depends on numerous interwoven standards, e.g. the EIA-708 standard for digital closed captioning, leading to variations in implementation.
The ATSC system supports a number of different display resolutions, aspect ratios, and frame rates. The formats are listed here by resolution, form of scanning (progressive or interlaced), and number of frames (or fields) per second (see also the TV resolution overview below):
The different resolutions can operate in progressive scan or interlaced mode, although the highest 1080-line system 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 maximum possible bitrate of 10.08 Mbit/s allowed in the DVD standard.
"EDTV" displays can reproduce 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.
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.
For transport, ATSC uses the MPEG-2 systems specification, known as transport stream, to encapsulate data, subject to certain constraints. 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, also with certain constraints.
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 (see below) allows both.
Main articles: 8VSB, QAM_tuner
ATSC signals are designed to use the same 6 MHz bandwidth as NTSC television channels (the interference requirements of A/53 DTV standards with adjacent NTSC or other DTV channels are very strict). Once the video and audio signals have been compressed and multiplexed, the transport stream can be modulated in different ways depending on the method of transmission.
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.
Currently, the Federal Communications Commission requires cable operators in the United States to carry the analog or digital transmission of a terrestrial broadcaster (but not both), when so requested by the broadcaster (the "must-carry rule"). The Canadian Radio-television and Telecommunications Commission in Canada has similar rules in force with respect to carrying ATSC signals.
However, cable operators in the US (and to a lesser extent Canada) can determine their own method of modulation for their plants.
There is also a standard for transmitting ATSC via satellite; however, this is only used by TV networks. Very few teleports outside the US support the ATSC satellite transmission standard, but teleport support for the standard is improving.
Terrestrial (local) broadcasters use 8VSB modulation that can transfer at a maximum rate of 19.39 Mbit/s, sufficient to carry several video and audio programs and metadata.
Cable television stations can generally operate at a higher signal-to-noise ratio and can use 16VSB or 256-QAM to achieve a throughput of 38.78 Mbit/s, using the same 6 MHz channel.
Consequently, most North American cable operators have added 256-QAM to the 16VSB standard originally used.
Cable operators have still been slow to add ATSC channels to their lineups for legal, regulatory, and plant & equipment related reasons.
256 QAM is a cable standard, not an ATSC standard; however, over time it is expected to be included in the ATSC standard
The ATSC satellite transmission system is not used for direct broadcast satellite systems, which in North America have long used a system similar to DVB-S. Modulation and transmission
A majority of the world's nations have chosen to adopt the DVB standard, as can be seen on the status list on the DVB Project website.
ATSC coexists with the DVB-T standard, and with ISDB-T being implemented in Japan. (ISDB modulation also serves as a basis of the SBTVD-T standard in Brazil.) A similar standard called ADTB was developed for use as part of China's new DMB-T/H dual standard. While China has officially chosen a dual standard, there is no requirement that a receiver work with both standards and there is no support for the ADTB modulation from broadcasters or equipment and receiver manufacturers. Taiwan (Republic of China) has chosen DVB-T COFDM as its official modulation. This was a direct result of broadcaster dissatisfaction with 8-VSB.
Because of potential use outside of existing NTSC areas, the ATSC system includes the capability to carry PAL and SECAM formatted video (576 displayable lines, 50 fields or 25 frames per second) along with NTSC (486 displayable lines, 60 x 1000/1001 fields or 30 x 1000/1001 frames per second) and film (24 frames per second).
While the ATSC system has been criticized as being complicated and expensive to implement and use, both broadcasting and receiving equipment are now comparable in cost with that of DVB.
The ATSC signal is definitely more susceptible to changes in radio propagation conditions than 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 did not change. It also lacks true hierarchical modulation, which allows the SDTV part of an HDTV signal to be received even in fringe areas where signal strength is low. For this reason, an additional modulation mode, enhanced-VSB (E-VSB) has been introduced, allowing for a similar benefit.
In spite of ATSC's fixed transmission mode, it is still a robust signal under various 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 than other systems.
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. In metropolitan areas, where the great and increasing majority of North Americans live, COFDM is said to be better at handling multipath. While ATSC is also incapable of true single-frequency network (SFN) operation, the distributed transmission mode, using on-channel repeaters, has been shown to improve reception under similar conditions. Thus, it may not require more spectrum allocation than DVB-T using SFNs.
Because the FCC forced broadcasters to use 8VSB modulation instead of COFDM, mobile reception of digital stations has (up till now) been difficult to impossible, especially when moving at vehicle speeds. To overcome this, there are now at least three standards which claim to improve mobile reception: Samsung's A-VSB, Harris and LG's MPH, and now the ATSC's own ATSC-M/H. This is in addition to other proprietary standards like MediaFLO, and worldwide open standards like DVB-H and DMB-T. Like DVB-H and ISDB 1seg, the proposed ATSC mobile standards are backward-compatible with existing tuners, despite being added to the standard well after the original standard was in wide use. Mobile reception of some stations will still be more difficult because the FCC sold-off the rights to 18 of the UHF channels, forcing several broadcasters to stay on VHF. This band requires larger antennas for reception, and is more prone to electromagnetic interference from engines and rapidly-changing multipath conditions, areas where ATSC's 8VSB is inferior to the COFDM of other standards.
Argentina did reconsider its choice of 8VSB, but has been sitting on the fence for a number of years. On November 17, 2006, the three standards (DVB, ATSC and ISDB) were presented to Argentinian Government officials, but no decision to change the standard has been made. Brazil has now chosen ISDB-T and this decision may influence other Central and South American countries to follow their lead.
Countries and territories using ATSC
- Puerto Rico
U.S. Virgin Islands Asia/Pacific
Advanced Television Systems Committee
List of ATSC standards
DVB - European digital television standard