ATSC
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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, South Korea, and recently Honduras and is being considered by other countries.
The ATSC standards are intended to replace the NTSC system used mostly in North America. The high definition television standards under the ATSC regime 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.
Many aspects of ATSC are patented, including elements of the MPEG video coding, the AC-3 audio coding, and the 8-VSB modulation[1]. 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.
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[edit] Resolution
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):
- 640x480 (4:3 Standard Definition; square pixel aspect ratio)
- interlaced
- 29.97 (59.94 fields/s)
- 30 (60 fields/s)
- progressive
- 23.976
- 24
- 29.97
- 30
- 59.94
- 60
- interlaced
- 704x480 (4:3 or 16:9 Standard Definition; non-square pixel aspect ratio)
- interlaced
- 29.97 (59.94 fields/s)
- 30 (60 fields/s)
- progressive
- 23.976
- 24
- 29.97
- 30
- 59.94
- 60
- interlaced
- 1280x720 (16:9 High Definition; square pixel aspect ratio)
- progressive
- 23.976
- 24
- 29.97
- 30
- 59.94
- 60
- progressive
- 1920x1080 (16:9 High Definition; square pixel aspect ratio)
- interlaced
- 29.97 (59.94 fields/s)
- 30 (60 fields/s)
- progressive
- 23.976
- 24
- 29.97
- 30
- interlaced
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 10.08Mbit/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.
[edit] Codecs
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 allows both.
[edit] Modulation and transmission
ATSC signals are designed to use the same 6 MHz bandwidth as NTSC television channels. Once the video and audio signals have been compressed and mutiplexed, the transport stream can be modulated in different ways depending on the method of transmission.
Terrestrial (local) broadcasters use 8-VSB 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 plants generally operate at 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 channel.
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 the cable operators 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 CRTC in Canada has similar rules in force with respect to ATSC signal carrage.
However, cable operators in the US (and to a lesser extent Canada) can determine their own method of modulation for their plants.
- Consequently, most North American cable operators have added 256-QAM to the 16-VSB 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
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.
- 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.
[edit] Other Systems
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 rejected 8-VSB (a method adopted for terrestrial broadcasting under the ATSC digital television standard in the United States and Canada) and has chosen DVB-T COFDM as its official modulation. This was a direct result of broadcaster dissatisfaction with the 8-VSB.[1]
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 (486 displayable lines, 60 x 1000/1001 fields or 30 x 1000/1001 frames per second) and film (24 frames per second).
[edit] Comparison
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 cannot be adapted to changes in radio propagation conditions, 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 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.
[edit] Standards
Below are the published standards for ATSC digital television service.
[edit] TV Resolution Overview
| Designation | Usage examples | Definition (lines) | Rate (Hz) | |
|---|---|---|---|---|
| Interlaced (fields) | Progressive (frames) | |||
| Low; MP@LL | LDTV, VCD | 240; 288 (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, Blu-ray Disc, HDV | 720 | 24, 30, 60; 25, 50 | |
| 1080 | 50, 60 | 24, 30; 25 | ||
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| This table illustrates total horizontal and vertical pixel resolution via box size. It does not accurately reflect the screen shape (aspect ratio) of these formats, which is either 4:3 or 16:9. | ||||
[edit] Other uses for the ATSC waveform
The ATSC waveform is excellent for autoranging on Earth Moon Earth (Moonbounce or EME) telecommunications circuits.
However, the amateur radio and astronomy community has not developed a slower speed version of the ATSC waveform for experimentation. The ATSC standards keeping organization has not worked to develop slower data rate versions of the signal for other uses.
- slowing the datarate of ATSC to 2 mbs would be enough to allow for very robust amateur television
- at even slower datarates like 512 kbs, ATSC becomes very optimal for EME circuits, but would still be capable of transmitting video
[edit] Notes
- ^ Argentina officially chose ATSC in 1998, and has been conducting experimental ATSC broadcasts since 1999. The governments of Argentina and Brazil had decided independently which digital TV standard each nation would deploy, but have recently agreed to work together to implement a single standard for the Mercosur customs union. The current government in Argentina appears to be reconsidering its earlier decision. ATSC and DVB are apparently both being considered, but there appears to be no interest in ISDB.
Later 10/26/06
Argentina did reconsider its choice of 8-VSB, 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.
