ATSC standards are a set of standards developed by the Advanced Television Systems Committee for digital television transmission over terrestrial, cable, and satellite networks.

The ATSC standards were developed in the early 1990s by the Grand Alliance, a consortium of electronics and telecommunications companies that assembled to develop a specification for what is now known as HDTV. ATSC formats also include standard-definition formats, although initially only HDTV services were launched in the digital format.

Background[link]

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, many different image sizes are also supported. The reduced bandwidth requirements of lower-resolution images allow up to six standard-definition "subchannels" to be broadcast on a single 6 MHz TV channel.

ATSC standards are marked A/x (x is the standard number) and can be downloaded freely from the ATSC's website at ATSC.org. ATSC Standard A/53, which implemented the system developed by the Grand Alliance, was published in 1995; the standard was adopted by the Federal Communications Commission in the United States in 1996. It was revised in 2009. ATSC Standard A/72 was approved in 2008 and introduces H.264/AVC video coding to the ATSC system.

ATSC also boasts 5.1-channel surround sound using the Dolby Digital AC-3 format. Numerous auxiliary datacasting services can also be provided.

Many aspects of ATSC are patented, including elements of the MPEG video coding, the AC-3 audio coding, and the 8VSB modulation.^[1] The cost of patent licensing, estimated at up to 50 US$ per digital TV receiver,^[2] has prompted complaints by manufacturers.^[3]

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.

Digital switchover[link]

ATSC replaced much of the analog NTSC television system ^[4] in the United States^[5][6] on June 12, 2009, replaced NTSC on August 31, 2011 in Canada,^[7] and will replace NTSC by December 31, 2015 in Mexico^[8], January 1, 2019 in El Salvador.^[9], and by end of 2012 in South Korea.

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 systems (DTx), a form of single-frequency network which allows for the synchronised operation of multiple on-channel booster stations.

Audio[link]

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.

MPEG-2 audio was a contender for the ATSC standard during the DTV "Grand Alliance" shootout, but lost out to Dolby AC-3. The Grand Alliance issued a statement finding the MPEG-2 system to be "essentially equivalent" to Dolby, but only after the Dolby selection had been made. Later, a story emerged that MIT had entered into an agreement with Dolby whereupon the university would be awarded a large sum if the MPEG-2 system was rejected.^[10] Dolby also offered an incentive for Zenith to switch their vote (which they did); however, it is unknown whether they accepted the offer.^{[citation needed]}

Video[link]

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).

For transport, ATSC uses the MPEG systems specification, known as an MPEG 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[link]

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 HDTV 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.

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 50, 59.94 or 60 frames per second, because such technology was seen as too advanced at the time and the image quality was deemed to be too poor considering the amount of data that needs to be transmitted.

A terrestrial (over-the-air) transmission carries 19.39 megabits of data per second (a fluctuating bandwidth of about 18.3 Mbit/s left after overhead such as error correction, program guide, closed captioning, etc.), compared to a maximum possible MPEG-2 bitrate of 10.08 Mbit/s (7 Mbit/s typical) allowed in the DVD standard and 48 Mbit/s (36 Mbit/s typical) allowed in the Blu-ray disc standard.

Although the ATSC A/53 standard limits MPEG-2 transmission to the formats listed below (with integer frame rates paired with 1000/1001-rate versions), the U.S. Federal Communications Commission declined to mandate that television stations obey this part of the ATSC's standard. In theory, television stations in the U.S. are free to choose any resolution, aspect ratio, and frame/field rate, within the limits of Main Profile @ High Level. Many stations do go outside the bounds of the ATSC specification by using other resolutions – for example, 720 × 480.

"EDTV" displays can reproduce progressive scan content and frequently have a 16:9 wide screen format. Such resolutions are 704×480 or 720×480 in NTSC and 720×576 in PAL, allowing 60 progressive frames per second in NTSC or 50 in PAL.

ATSC also supports PAL frame rates and resolutions which are defined in ATSC A/63 standard.

The ATC A/53 specification imposes certain constraints on MPEG-2 video stream:

• The maximum bitrate of the MPEG-2 video stream is exactly 19.4 Mbit/s for broadcast television, and exactly 38.8 Mbit/s for the "high-data-rate" mode (e.g., cable television). (The practical limit is somewhat lower, since the MPEG-2 video stream must fit inside a transport stream, with overhead, sent out at 19.3927... Mbit/s for broadcast.)
• The amount of MPEG-2 stream buffer required at the decoder (the vbv_buffer_size_value) must be less than or equal to 999,424 bytes.
• In most cases, the transmitter can't start sending a coded image until within a half-second of when it's to be decoded (vbv_delay less than or equal to 45000 90-kHz clock increments).
• The stream must include colorimetry information (gamma curve, the precise RGB colors used, and the relationship between RGB and the coded YCbCr).
• The video must be 4:2:0 (chrominance resolution must be 1/2 of luma horizontal resolution and 1/2 of luma vertical resolution).

The ATSC specification and MPEG-2 allow the use of progressive frames coded within an interlaced video sequence. For example, NBC stations transmit a 1080i60 video sequence, meaning the formal output of the MPEG-2 decoding process is sixty 540-line fields per second. However for prime-time television shows, those 60 fields can be coded using 24 progressive frames as a base - actually, an 1080p24 video stream (a sequence of 24 progressive frames per second) is transmitted, and MPEG-2 metadata instructs the decoder to interlace these fields and perform 3:2 pulldown before display, as in soft telecine.

The ATSC specification also allows 1080p30 and 1080p24 MPEG-2 sequences, however they are not used in practice, because broadcasters want to be able to switch between 60 Hz (news, soap operas) and 24 Hz (prime-time) content without ending the MPEG-2 sequence.

The 1080-line formats are encoded with 1920 × 1088 pixel luma matrices and 960 × 540 chroma matrices, but the last 8 lines are discarded by the MPEG-2 decoding and display process.

H.264/MPEG-4 AVC[link]

In July 2008, ATSC was updated to support the ITU-T H.264 video codec. The new standard is split in two parts:

• A/72 part 1: Video System Characteristics of AVC in the ATSC Digital Television System^[11]
• A/72 part 2 : AVC Video Transport Subsystem Characteristics^[12]

The new standards supports 1080p at 50, 59.94 and 60 frames per second; such frame rates require H.264/AVC High Profile Level 4.2, while standard HDTV frame rates only require Levels 3.2 and 4, and SDTV frame rates require Levels 3 and 3.1.

Transport Stream (TS)[link]

The file extension ".TS" is a video file. Specifically, it's an MPEG-2 file captured from a TV. This means that it has been taken from a television program using special software that converts the television's signal and records it.^[13]

Modulation and transmission[link]

ATSC signals are designed to use the same 6 MHz bandwidth as analog NTSC television channels (the interference requirements of A/53 DTV standards with adjacent NTSC or other DTV channels are very strict). Once the digital 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 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 either the 16VSB as defined in ATSC or the 256-QAM defined in SCTE, to achieve a throughput of 38.78 Mbit/s, using the same 6 MHz channel.

The proposals for modulation schemes for digital television were developed when cable operators carried standard-resolution video as uncompressed analog signals. 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 have still been slow to add ATSC channels to their lineups for legal, regulatory, and plant & equipment related reasons. One key technical and regulatory issue is the modulation scheme used on the cable: cable operators in the US (and to a lesser extent Canada) can determine their own method of modulation for their plants. Multiple standards bodies exist in the industry: the SCTE defined 256-QAM as a modulation scheme for cable in a cable industry standard, ANSI/SCTE 07 2006: Digital Transmission Standard For Cable Television. Consequently, most North American cable operators seeking additional capacity on the cable system have moved to 256-QAM from the 64-QAM modulation used in their plant, in preference to the 16VSB standard originally proposed by ATSC. Over time 256-QAM 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^{[citation needed]}. 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; in North America these have long used either DVB-S (in standard or modified form) or a proprietary system such as DSS or DigiCipher 2.

Other systems[link]

ATSC coexists with the DVB-T standard, and with ISDB-T. A similar standard called ADTB-T 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.

For compatibility with material from various regions and sources, ATSC supports the 480i video format used in the NSTC analog system (480 lines, approximately 60 fields or 30 frames per second), 576i formats used in most PAL regions (576 lines, 50 fields or 25 frames per second), and 24 frames-per-second formats used in film.

This unreferenced section requires citations to ensure verifiability.

While the ATSC system has been criticized as being complicated and expensive to implement and use,^[14] both broadcasting and receiving equipment are now comparable in cost with that of DVB.

The ATSC signal is more susceptible to changes in radio propagation conditions than DVB-T and ISDB-T. It also lacks true hierarchical modulation, which would allow the SDTV part of an HDTV signal (or the audio portion of a television program) to be received uninterrupted 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 1seg, 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 propagation. While ATSC is also incapable of true single-frequency network (SFN) operation, the distributed transmission mode, using multiple synchronised on-channel transmitters, has been shown to improve reception under similar conditions. Thus, it may not require more spectrum allocation than DVB-T using SFNs.

Mobile TV[link]

Mobile reception of digital stations using ATSC has, until 2008, been difficult to impossible, especially when moving at vehicular speeds. To overcome this, there are several proposed systems that report improved mobile reception: Samsung/Rhode & Schwarz's A-VSB, Harris/LG's MPH, and a recent proposal from Thomson/Micronas; all of these systems have been submitted as candidates for a new ATSC standard, ATSC-M/H. After one year of standardization, the solution based on LGE technology has been adopted and would have been deployed in 2009. This is in addition to other standards like MediaFLO, and worldwide open standards such as DVB-H and T-DMB. 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 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 interference from engines and rapidly-changing multipath conditions.^{[citation needed]}

Future standards[link]

By 2012, ATSC 2.0 may be a standard. It is intended to be used with the current standard, but will allow interactive services, including video on demand services, targeted advertising, better programming guides, and the ability to store information on new receivers.^[15]

Countries and territories using ATSC[link]

Americas[link]

•  Bahamas
•  Bermuda
•  Canada (converted on August 31, 2011 in provincial/territorial capitals and locations with 300,000 or more people)
•  Dominican Republic (decided on August 10, 2010; to be completely transitioned by September 24, 2015^[16])
•  El Salvador (decided on April 22, 2009,^[17] to be completely transitioned by January 1, 2019^[9])
•  Honduras (decided in January 2007,^[18] to be completely transitioned by December 2020)
•  Mexico (to be completely transitioned by December 31, 2015^[8])
•  United States (converted on June 12, 2009, excluding LPTV stations)
  •  Puerto Rico
  •  U.S. Virgin Islands

Asia/Pacific[link]

•  South Korea (to be completely transitioned by December 31, 2012)
• United States territories (converted on June 12, 2009, excluding LPTV stations)
  •  American Samoa
  •  Guam
  •  Northern Mariana Islands

See also[link]

• Advanced Television Systems Committee
• ATSC tuner
• List of ATSC standards
• Broadcast flag
• Broadcast safe
• Digital terrestrial television (DTT)
• Digital Video Broadcasting (DVB)
• EIA-708
• ISDB - Integrated Services Digital Broadcasting
• OpenCable
• Redesign project, project set up by cable operators, equipment manufacturers, and research organisations
• T-DMB Korean terrestrial mobile digital broadcasting system
• DMB-T/H Chinese terrestrial digital broadcasting system

References[link]

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2009)

External links[link]

• ATSC website
• ATSC standards download page
• ATSC-MH Platform for testing and deploying Mobile TV in the US
• ATSC Mobile DTV (ATSC-MH) Analysis, Monitoring, Measurement
• ATSC Mobile DTV (ATSC-MH) Mobile DTV Viewer

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