- ATSC Standards
ATSC Standards document a
digital televisionformat that will have replaced the analog NTSCtelevision system [ [http://home.businesswire.com/portal/site/home/index.jsp?ndmViewId=news_view&ndmConfigId=1000001&newsId=20071017005469&newsLang=en Major retailer stops selling analog sets] ] by February 17, 2009in the United States, [ [http://www.dtvtransition.org/ DTVTransition.org - A New Era in Television Broadcasting ] ] and August 31, 2011in Canada. [ [http://www.crtc.gc.ca/eng/NEWS/RELEASES/2007/r070517.htm The Commission establishes a new approach for Canadian conventional television ] ] It was developed by the Advanced Television Systems Committee.
The high definition television standards defined by the ATSC produce
wide screen 16:9images up to 1920×1080 pixels in size — more than six times the display resolutionof the earlier standard. However, many different image sizes are also supported, so that up to six standard-definition " virtual channels" can be broadcast on a single 6 MHz TV channel.
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 below).
Many aspects of ATSC are
patented, including elements of the MPEGvideo coding, the AC-3 audio coding, and the 8VSBmodulation. [ [http://www.chinadaily.com.cn/bizchina/2006-12/27/content_768603.htm TV makers to fight royalties ] ] As with other systems, ATSC depends on numerous interwoven standards, e.g. the EIA-708standard 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 at the end of this article). The table includes formats from both A/53 Part 4 (MPEG-2 Video System Characteristics) and A/63 (Standard for Coding 25/50 Hz Video).
The different resolutions can operate in
progressive scanor 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 DVDstandard.
"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 scanlines and are 1280 pixels wide. The top tier has 1080 lines 1920 pixels wide. 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-2systems 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 correctionto the signal. Then, the transport stream may be demultiplexed into its constituent streams.
Since July 2008, ATSC supports the MPEG4/H.264 video codec. The standard is split in two parts:
*A/72 part 1: Video System Characteristics of AVC in the ATSC Digital Television System [http://www.atsc.org/standards/a_72_part_1.pdf]
*A/72 part 2 : AVC Video Transport Subsystem Characteristics [http://www.atsc.org/standards/a_72_part_2.pdf]
The earlier specification also supports MPEG-2 video as the
video codec, with certain constraints. Concern has been expressed that this section is missing information about the technical constraints of the codec.
These constraints are currently addressed in
MPEG-2#ATSCbut should probably be addressed in this article instead, or in addition. Dolby DigitalAC-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 ISDBHDTV broadcasts use MPEG's Advanced Audio Coding(AAC) as the audio codec, which also allows 5.1 audio output. DVB (see below) allows both.
Modulation and transmission
ATSC signals are designed to use the same 6 MHz bandwidth as
NTSCtelevision 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.
* Terrestrial (local) broadcasters use
8VSBmodulation that can transfer at a maximum rate of 19.39 Mbit/s, sufficient to carry several video and audio programs and metadata.
Cable televisionstations can generally operate at a higher signal-to-noise ratioand can use 16VSBor 256-QAMto 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 cablesystems, making it harder to find duplicate 6 MHz channels for local broadcasters on uncompressed "basic" cable.
Federal Communications Commissionrequires 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-carryrule"). The Canadian Radio-television and Telecommunications Commissionin 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.
* Consequently, most North American cable operators have added
256-QAMto the 16VSBstandard 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 satellitesystems, which in North America have long used a system similar to DVB-S.
A majority of the world's nations have chosen to adopt the DVB standard, as can be seen on the [http://www.dvb.org/about_dvb/dvb_worldwide/index.xml status list on the DVB Project website] .
ATSC coexists with the
DVB-Tstandard, and with ISDB-Tbeing implemented in Japan. (ISDB modulation also serves as a basis of the SBTVD-Tstandard in Brazil.) A similar standard called ADTBwas developed for use as part of China's new DMB-T/Hdual 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 has chosen DVB-T COFDMas its official modulation.
Because of potential use outside of existing NTSC areas, the ATSC system includes the capability to carry
PALand SECAMformatted 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 more susceptible to changes in
radio propagationconditions than DVB-Tand 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 strengthis 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
COFDMin part because many areas of North America are ruraland 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
1seg, as well as DVB-Hand HD Radioin 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 spectrumallocation than DVB-Tusing SFNs.
Mobile reception of digital stations using ATSC has (until now) been difficult to impossible, especially when moving at vehicular speeds. To overcome this, there are several proposed systems that report improved mobile reception:
Samsung/ Rohde & Schwarz's A-VSB, Harris/ LG's MPH, and a recent proposal from Thomson/ Micronas; all of these systems are expected to be submitted as candidates for a new ATSC standard, ATSC-M/H. This is in addition to other proprietary standards like MediaFLO, and worldwide open standards such as DVB-Hand T-DMB. Like DVB-H and ISDB 1seg, the proposed ATSC mobile standards are backward-compatiblewith 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 18 UHF channels in the U.S. have been removed from TV service, forcing some broadcasters to stay on VHF. This band requires larger antennas for reception, and is more prone to
electromagnetic interferencefrom engines and rapidly-changing multipathconditions.Facts|date=January 2008
Countries and territories using ATSC
* (experimental) [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. Brasil has now chosen ISDB-Tand this decision may influence other Central and South American countries to follow their lead.Fact|date=December 2007]
* (To be completely transformed by 31 August 2011)
* (Selected channels)
* (experimental only in Lima)
* (To to be completely transformed by 17 February 2009)
** flagcountry|Puerto Rico
** flagcountry|U.S. Virgin Islands
Advanced Television Systems Committee
List of ATSC standards
Digital terrestrial television
Redesign project, project set up by cable operators, equipment manufacturers, and research organisations
T-DMBKorean terrestrial digital broadcasting system
DMB-T/HChinese terrestrial digital broadcasting system
Digital Video Broadcasting
ISDB- Integrated Services Digital Broadcasting
* [http://www.atsc.org ATSC website]
* [http://www.atsc.org/guide_default.html ATSC standards download page]
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