Standar ATSC

Standar ATSC ngadokumentasi hiji format televisi digital nu bakal ngaganti sistim televisi NTSC analog (di Amerika Sarikat) ti mimiti 17 Pebruari, 2009. Standar ieu dijieun ku Advanced Television Systems Committee.
Standar televisi definisi luhur nu ditetepkeun ku ATSC ngahasilkeun gambar layar lega 16:9 tepi ka ukuran 1920×1080 piksel — leuwih ti genep kalieun resolusi pidangan standar saméméhna. Ukuran gambar lianna ogé dirojong, antukna panglobana genep standard-definition "virtual channel" bisa disiarkeun dina hiji saluran TV tunggal 6 MHz.
ATSC ogé ngarojong audio "kualitas theater" sabab maké format AC-3 Dolby Digital keur nyadiakeun surround sound 5.1 kanal. Sababaraha jasa datacasting tambahan bisa disadiakeun oge.
Nu nyiarkeun maké ATSC, lamun hayang kénéh mertahankeun sinyal analog, kudu nyiarkeun dina dua saluran nu misah, sabab sistim ATSC kudu maké saluran sagemblengna. Sababaraha saluran bisa dipetakeun deui maké saluran virtual tina saluran RF fisik jadi nomer-nomer ti 1 nepi ka 99, antukna stasion ATSC bisa diasosiasikeun jeung nomer saluran NTSC nu patali, atawa bisa ogé sakabéh stasion dina hiji jaringan maké nomer nu sarua. Aya ogé hiji standar keur transmisi nu sumebar (DTx) nu ngidinan booster station.
Standar ATSC dicirian ku A/x (x nyaéta nomer standarna) tur bisa dicokot kalayan bébas tina ramatloka ATSC (tempo tumbu luar di handap).
Loba aspek ATSC nu dipatenkeun, kaasup unsur koding vidéo MPEG, koding audio AC-3, sarta modulasi 8VSB. Kawas sistim lianna, ATSC gumantung kana sababaraha standar nu patula-patali, contona standar EIA-708 keur closed captioning digital, geus bareda dina implementasina.
Sistim ATSC ngarojong sababaraha resolusi, rasio aspék, jeung frame rate pidangan nu béda. Formatna didaptar di dieu dumasar resolusi, wangun scanning (progresif atawa interlace), sarta jumlah frame (atawa field) per detikna (tempo ogé sawangan resolusi TV dina pungkasan artikel ieu):
Resolusi nu béda bisa jalan dina mode progressive scan atawa interlace, sanajan sistim 1080-gurat nu pangluhurna henteu bisa midangkeun gambar progresif dina laju 59.94 atawa 60 frame per detik oge. (Tehnologi kawas kieu katempona hadé teuing keur mangsa kiwari, katambah-tambah kualitas gambarna ogé goreng teuing lamun dibandingkeun jeung jumlah data nu bisa dikirimkeun.) Pangiriman terestrial (ngaliwatan udara) mawa 19.39 megabit data per detik, bandingkeun jeung bitrate maksimm nu mungkin, 10.08 Mbit/s kabawa dina standar DVD.
Pidangan "EDTV" bisa ngaréproduksi eusi progressive scan sarta sacara frekuentif boga format layar lega 16:9. Resolusi nu kawas kieu nyaéta 720×480 dina NTSC atawa 720×576 dina PAL, nu ngidinan 60 frame progresif per detik dina NTSC atawa 50 dina PAL.
Aya tilu ukuran pidangan dasar keur ATSC. Ukuran gambar NTSC jeung PAL basic jeung enhanced aya dina hambalan handap 480 atawa 576 gurat. Gambar ukuran sedeng mibanda resolusi 720 gurat sarta legana 960 atawa 1280 piksel (rasio aspek, patuturut keur 4:3, versi tradisional, jeung 16:9, versi layar lega). Pangluhurna boga 1080 gurat boh legana 1440 atawa 1920 piksel (di dieu oge, patuturut keur rasio aspek 4:3 jeung 16:9). Vidéo 1080 gurat sabenerna mah dienkode maké frame 1920×1088 piksel, tapi dalapan gurat pangpandeurina dipiceun dina pidangannana. Hl ieu alatan wawates format vidéo MPEG-2, nu nyaratkeun yén jumlah sampel luma nu dikodekeun (contona piksel) bisa dibagi ku 16.
Keur transpor, ATSC maké spesifikasi sistim MPEG-2, nu katelah transport stream, keur nga-enkapsulasi data, maké wawates tinangtu. ATSC maké paket trnsport stréam 188 bita keur mawa data. saméméh ngadekode audio jeung vidéo, panarima kudu ngademodulasi jeung nerapkeun koreksi kasalahan kana sinyal. Tuluy, transport stréam bisa didemultipleks jadi stréam poko.
Vidéo MPEG-2 dipaké minangka video codec, ogé kalayan wawates tinangtu.
Dolby Digital AC-3 dipaké minangka audio codec, sanajan resmina distandarkeun minangka A/52 ku ATSC. Hal ieu ngidinan transpor nepi ka lima saluran sora kalayan hiji saluran kagenep keur efek frekuensi handap (nu katelah konfigurasi "5.1"). Sabalikna, siaran HDTV ISDB Jepang mah maké Advanced Audio Coding (AAC) MPEG minangka audio codec, nu ogé bisa ngaluarkeun audio 5.1. DVB (tempo di handap) bisa duanana.
Sinyal ATSC dirancang pikeun maké lega pita 6 MHz nu sarua jeung saluran televisi NTSC (sarat interferensi standar DTV A/53 jeung NTSC nu padeudeukeut atawa jeung saluran DTV lianna kacida ketatna). Lamun sinyal vidéo jeung audio geus dikompresi sarta dimultipleks, transport stréamna bisa dimodulasi maké mangrupa-rupa cara gumantung kana cara pangirimannana.
In recent yéars, cable operators have become accustomed to compressing standard-resolution vidéo 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 in the US (and to a lesser extent Canada) can determine their own method of modulation for their plants.
There is also a standard for transmitting ATSC via satellite; however, this is only used by TV networks. Very few teleports outside the US support the ATSC satellite transmission standard, but teleport support for the standard is improving.
Mayoritas nagara di dunya geus milih pikeun maké standar DVB, kawas nu bisa ditingali dina daptar status ngeunaan ramatloka Proyek DVB.
ATSC marengan ayana standar DVB-T, sarta ISDB-T nu diterapkeun di Jepang. (modulasi ISDB dipaké ogé minangka dasar standar SBTVD-T di Brazil.) Standar nu ampir sarua nuu katelah ADTB dikembangkeun pikeun dipaké minangka bagéantina standar ganda DMB-T/H anyar bogana China. Sanajan resmina milih standar ganda, taya persaratan yén hiji panarima bisa cocog jeung dua standar sarta taya rojongan ogé keur modulasi ADTB ti panyiar atawa pabrik alat jeung panarima. Taiwan (Républik China) milih DVB-T COFDM minangka modulasi resmina.
Sabab bisa dipaké di saluareun wewengkon NTSC nu aya kiwari, sistim ATSC mampuh mawa vidéo dina format PAL jeung SECAM (576 gurat nu bisa kapidangkeun, 50 field atwa 25 frame per detik) lian ti keur NTSC (486 gurat nu bisa kapidangkeun, 60 x 1000/1001 field atawa 30 x 1000/1001 frame per detik) jeung pilem (24 frame per detik).
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 definitely more susceptible to changes in radio propagation conditions than DVB-T and ISDB-T. If ATSC were able to dynamically change its error correction modes, code rates, interléaver mode, and randomizer, the signal could be more robust even if the modulation itself did not change. It also lacks true hierarchical modulation, which allows the SDTV part of an HDTV signal to be received even in fringe aréas where signal strength is low. For this réason, 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 aréas of North America are rural and have a much lower population density, thereby requiring larger transmitters and resulting in large fringe aréas. In these aréas, 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 gréat and incréasing 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 repéaters, has been shown to improve reception under similar conditions. Thus, it may not require more spectrum allocation than DVB-T using SFNs.
Because the FCC forced broadcasters to use 8VSB modulation instéad of COFDM, mobile reception of digital stations has (up till now) been difficult to impossible, especially when moving at vehicle speeds. To overcome this, there are now at léast three standards which claim to improve mobile reception: Samsung's A-VSB, Harris and LG's MPH, and now the ATSC's own ATSC-M/H. This is in addition to other proprietary standards like MediaFLO, and worldwide open standards like DVB-H and DMB-T. 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 the FCC sold-off the rights to 18 of the UHF channels, forcing several 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, aréas where ATSC's 8VSB is inferior to the COFDM of other standards.