ATSC
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.
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 earlier and more widely-used
DVB standard, and
ISDB being implemented in
Japan and soon in
Brazil. A similar standard called
ADTB was developed for use as part of
China's new
DMB-T/H dual standard. 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 must retain an analog signal have to broadcast on two separate channels (
in-band adjacent-channel), as the ATSC system (like DVB) requires 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 appear on the numbers they always have, 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.
The system has been criticized as being complicated and expensive to implement and use. Many aspects of ATSC are
patented, including the AC-3 audio coding, and the VSB modulation. The standards ATSC depends on are often ambiguous, one example would be the
EIA-708 standard for
closed captioning.
The ATSC system supports a host of different display resolutions and
frame rates. The formats below are listed by resolution, form of scanning (
progressive or
interlaced), and frames per second (for more informations and links, see also the TV resolution overview at the end of this article):
*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 (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 is more limited and 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 the maximum possible bitrate of 10.08Mbit/s allowed in the
DVD standard.
"EDTV" is largely a
marketing term created to sell standard-resolution televisions with minor enhancements. Such TVs can display 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.
Brushing aside marketing-speak, 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. 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.
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 (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.
Main articles: 8VSB, 256QAMATSC signals are designed to use the same 6 MHz
bandwidth as
NTSC television channels. Once the video signals have been compressed, the data stream can be
modulated in a variety of manners depending on the method of transmission.
Terrestrial (local) broadcasters use a
8-VSB modulation that can transfer at a maximum rate of 19.39
Mbit/s, sufficient to carry several video channels and metadata depending on conditions.
Cable television operators generally have 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.
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. Cable operators lobbied the
Federal Communications Commission to allow
256-QAM in addition to the
16-VSB standard originally mandated. Though successful, 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 even in North America have long used
DVB-S.
Shortcomings
The ATSC signal cannot be adapted to changes in
radio propagation conditions, very 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
hierarchical modulation, which allows the SDTV part of an HDTV signal to be received even in fringe areas where
signal strength is low.
In spite of ATSC's fixed transmission mode, it is still a robust
waveform under normal 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, although in
metropolitan areas where the great and increasing majority of North Americans live, COFDM is much better at handling
multipath. 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. ATSC will likely never be able to cover these metropolitan areas. ATSC is also incapable of
SFN operation, which means it will always require more
spectrum than a COFDM-based standard like
DVB-T.
Below are the ATSC-published standards for digital television. Prior to the group's DTV work, it published A/49, a
ghosting-
canceling reference signal for NTSC.
*A/52B:
audio data compression (Dolby
AC-3] and
E-AC3)
*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)
*
broadcast flag*
EIA-708*
OpenCable*
ATSC website*
The common ATSC formats represented in different criteria
