Стандарты цифрового телевидения ATSC в США

Standards for digital television in the US

Advanced Television Systems Committee (ATSC) 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 and, like that standard, is used mostly in the United States, Mexico, Canada, South Korea and Trinidad & Tobago. Several former NTSC users, such as Japan, have not used ATSC during their digital television transition, because they adopted other systems such as 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 technology was primarily developed with patent contributions from LG Electronics, which holds most of the patents for the ATSC standard. The cost of patent licensing, estimated at up to $50 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.

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(s), 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.

Dolby Digital 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 (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 of money if the MPEG 2 system was rejected. Dolby also offered an incentive for Zenith to switch their vote (which they did); however, it is unknown whether they accepted the offer.

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.

The file extension ". TS" stands for "transport stream", which is a media container format. It may contain a number of streams of audio or video content multiplexed within the transport stream. Transport streams are designed with synchronization and recovery in mind for potentially lossy distribution (such as over the air ATSC broadcast) in order to continue a media stream with minimal interruption in the face of data loss in transmission. When an over the air ATSC signal is captured to a file via hardware/software the resulting file is often in a TS file format.

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 does not have 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 U. S. (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 U. S. and Canadian 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. Very few teleports outside the U. S. 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 the U. S. and Canada these have long used either DVB S (in standard or modified form) or a proprietary system such as DSS or DigiCipher 2.

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 NTSC 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. 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 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 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 population density is highest, 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 synchronized 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. A comparison study found that ISDB T and DVB T performed similarly, and that both were outperformed by DVB T2.

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 merged between Samsung's AVSB and LGE's MPH technology has been adopted and would have been deployed in 2009. This is in addition to other standards like the now defunct 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.

ATSC 2.0 was a planned major new revision of the standard which would have been backward compatible with ATSC 1.0. The standard was to have allowed interactive and hybrid television technologies by connecting the TV with the Internet services and allowing interactive elements into the broadcast stream. Other features were to have included advanced video compression, audience measurement, targeted advertising, enhanced programming guides, video on demand services, and the ability to store information on new receivers, including Non realtime (NRT) content. However, ATSC 2.0 was never actually launched, as it was essentially outdated before it could be launched. All of the changes that were a part of the ATSC 2.0 revision were adopted into ATSC 3.0.

Better image quality. ATSC 3.0 allows 4K UHD transmission, including high dynamic range television (HDR TV), wide color gamut (WCG), and high frame rate (HFR). Reception upgrades. ATSC 3.0 allows the same aerial to receive more channels with better quality. Portable devices such as mobile phones, tablets, and car infotainment systems can receive TV signals. Enhanced emergency alerts. Emergency signals can be geographically oriented and inform only the specific areas where they are required. Audience measure. Telecommunication companies can easily take audience data gatherings. Targeted advertising with the assistance of local network Wi Fi. Content variety and diversification.