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, Taiwan, and South Korea, and being considered by other countries. Taiwan has since rejected 8-VSB and chosen DVB-T COFDM as its official modulation. This was a direct result of broadcaster disatisfaction with the 8-VSB.^[1]

It is intended to replace the NTSC system used mostly in North America and produces 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 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. 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 want to retain an analog signal must broadcast on two separate channels, as the ATSC system (like DVB) 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.

While the 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. As with DVB, many aspects of ATSC are patented, including elements of the MPEG video coding, the AC-3 audio coding, and the 8-VSB 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.

Contents 1 Resolution 2 Codecs 3 Modulation and transmission 4 Comparison with Other Systems 5 Standards 6 TV Resolution Overview 7 Notes 8 See also 9 External links

[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
• 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
• 1280x720 (16:9 High Definition; square pixel aspect ratio)
  • progressive
    • 23.976
    • 24
    • 29.97
    • 30
    • 59.94
    • 60
• 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

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 contraints. 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

Main articles: 8VSB, 256QAM

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"). However, cable operators can determine their own method of modulation for their plants. Consequently, 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.

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 in North America have long used a system similar to DVB-S.

[edit] Comparison with Other Systems

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

• A/52B: audio data compression (Dolby AC-3 and E-AC-3)
• A/53E: "ATSC Digital Television Standard" (the primary document governeing the standard)
• A/55: "Program Guide for Digital Television" (now deprecated in favor of A/65 PSIP)
• A/57A: "Content Identification and Labeling for ATSC Transport" (for assigning a unique digital number to each episode of each TV show, to assist DVRs)
• A/63: "Standard for Coding 25/50 Hz Video" (for use with PAL and SECAM-originated programming)
• A/64A "Transmission Measurement and Compliance for Digital Television"
• A/65C: "Program and System Information Protocol for Terrestrial Broadcast and Cable" (PSIP includes virtual channels, program guides, and content ratings)
• A/68: "PSIP Standard for Taiwan" (defines use of Chinese alphabet via Unicode 3.0)
• A/69: recommended practices for implementing PSIP at a TV station
• A/70A: "Conditional Access System for Terrestrial Broadcast"
• A/76: "Programming Metadata Communication Protocol" (XML-based PMCP maintains PSIP metadata though a Tv station's airchain)
• A/80: "Modulation and Coding Requirements for Digital TV (DTV) Applications Over Satellite"
• A/81: "Direct-to-Home Satellite Broadcast Standard" (not yet implemented by any services)
• A/90: "Data Broadcast Standard" (for datacasting)
• A/92: "Delivery of IP Multicast Sessions over Data Broadcast Standard" (for IP multicasting)
• A/93: "Synchronized/Asynchronous Trigger Standard"
• A/94: "ATSC Data Application Reference Model"
• A/95: "Transport Stream File System Standard" (TSFS is a special file system for downloading computer files)
• A/96: "ATSC Interaction Channel Protocols" (interactive TV)
• A/97: "Software Data Download Service" (used by UpdateTV for upgrades and software patches in ATSC tuners)
• A/100: "DTV Application Software Environment - Level 1" (DASE-1)
• A/101: "Advanced Common Application Platform" (ACAP)
• A/110A: "Synchronization Standard for Distributed Transmission" (single-frequency networks)

[edit] TV Resolution Overview

[edit] Notes

1. ^ 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, has been setting on the fence for a number of years now. Brazil has now chosen ISDB-T and it is expected that most other Central and South American countries will follow their lead.

On November 17, 2006, the three standards (DVB, ATSC and ISDB) were presented to Argentinean Government officials, but not standard has been chosen until that date.

[edit] See also

• ATSC tuner
• broadcast flag
• EIA-708
• OpenCable

[edit] External links

• ATSC website
• The common ATSC formats represented in different criteria