ATSC standards
Advanced Television Systems Committee 'standards' are an international set of standards for broadcast and digital television transmission over terrestrial, cable and satellite networks. It is largely a replacement for the analog NTSC standard. Like NTSC, ATSC is used mostly in the United States, Mexico, Canada, South Korea, and Trinidad and Tobago. Several former NTSC users like Japan have not used ATSC during their digital television transition, because they adopted other systems like ISDB developed by Japan and DVB developed in Europe, for example.
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. The standard is now administered by the Advanced Television Systems Committee. It includes a number of patented elements, and licensing is required for devices that use these parts of the standard. Key among these is the 8VSB modulation system used for over-the-air broadcasts. ATSC 1.0 technology was primarily developed with patent contributions from LG Electronics, which held most of the patents for the ATSC standard.
ATSC includes two primary high definition video formats, 1080i and 720p. It also includes standard-definition formats, although initially only HDTV services were launched in the digital format. ATSC can carry multiple channels of information on a single stream, and it is common for there to be a single high-definition signal and several standard-definition signals carried on a single 6 MHz channel allocation.
Background
The high-definition television standards defined by the ATSC produce widescreen 16:9 images up to 1920×1080 pixels in sizemore 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 and can be downloaded for free 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 supports 5.1-channel surround sound using Dolby Digital's AC-3 format. Numerous auxiliary datacasting services can also be provided.
Many aspects of ATSC were patented, including elements of the MPEG video coding, the AC-3 audio coding, and the 8VSB modulation. The cost of patent licensing, estimated at up to per digital TV receiver, had prompted complaints by manufacturers.
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
ATSC replaced much of the analog NTSC television system in the United States on June 12, 2009, on August 31, 2011 in Canada, on December 31, 2012 in South Korea, and on December 31, 2015 in Mexico.Broadcasters who used ATSC and wanted to retain an analog signal were temporarily forced to broadcast on two separate channels, as the ATSC system requires the use of an entire separate channel. Channel numbers in ATSC do not correspond to RF frequency ranges, as they did with analog television. Instead, virtual channels, sent as part of the metadata along with the program, 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, a form of single-frequency network which allows for the synchronised operation of multiple on-channel booster stations.
Audio
AC-3 is used as the audio codec, though it was 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. In contrast, Japanese ISDB HDTV broadcasts use MPEG's Advanced Audio Coding as the audio codec, which also allows 5.1 audio output. DVB 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 of money if the MPEG-2 system was rejected. Dolby also offered an incentive for Zenith to switch their vote ; however, it is unknown whether they accepted the offer.
Video
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, and number of frames per second.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
There are four basic display sizes for ATSC, generally known by referring to the number of lines of the picture height. NTSC and PAL image sizes are smallest, with a width of 720 and a height of 480 or 576 lines. The third size is HDTV images that have 720 scan lines in height and are 1280 pixels wide. The largest size has 1080 lines high and 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 height of the picture in luma samples to be divisible by 16.The lower resolutions can operate either in progressive scan or interlaced mode, but not the largest picture sizes. The 1080-line system does not support progressive images at the highest frame rates of 50, 59.94 or 60 frames per second, because such technology was seen as too advanced at the time. The standard also requires 720-line video be progressive scan, since that provides better picture quality than interlaced scan at a given frame rate, and there was no legacy use of interlaced scan for that format. The result is that the combination of maximum frame rate and picture size results in approximately the same number of samples per second for both the 1080-line interlaced format and the 720-line format, as 1920*1080*30 is roughly equal to 1280*720*60. A similar equality relationship applies for 576 lines at 25 frame per second versus 480 lines at 30 frames per second.
A terrestrial transmission carries 19.39 megabits of data per second, compared to a maximum possible MPEG-2 bitrate of 10.08 Mbit/s allowed in the DVD standard and 48 Mbit/s allowed in the Blu-ray disc standard.
Although the ATSC A/53 standard limits MPEG-2 transmission to the formats listed [|below], 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, 352 x 480 or 720 x 480.
"EDTV" displays can reproduce progressive scan content and frequently have a 16:9 widescreen format, 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 ATSC A/53 specification imposes certain constraints on MPEG-2 video stream:
- The maximum bit rate value in the sequence header of the MPEG-2 video stream is 19.4 Mbit/s for broadcast television, and 38.8 Mbit/s for the "high data rate" mode. The actual MPEG-2 video bit rate will be lower, since the MPEG-2 video stream must fit inside a transport stream.
- The amount of MPEG-2 stream buffer required at the decoder 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.
- The stream must include colorimetry information.
- The video must be 4:2:0.
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 interlaced, 30 Hz progressive or PsF, and 24 Hz progressive content without ending the 1080i60 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
In July 2008, ATSC was updated to support the ITU-T H.264 video codec. This standard is split in two parts:- A/72 part 1: Video System Characteristics of AVC in the ATSC Digital Television System
- A/72 part 2 : AVC Video Transport Subsystem Characteristics