Throughput[édit | sunting sumber]
Dina saluran 6 MHz (megahertz) nu dipake keur siaran ATSC, 8VSB mawa 19.39 Mbit/det data nu kapake, sok sanajan bit rate nu aktual dikirimna kacida leuwih gedena oge alatan ditambahkeunnana kode forward error correction. Dalapan hambalan sinyal dipilih make enkoder trellis. Aya oge modulasi 2VSB, 4VSB, jeung 16VSB nu sawanda. 16VSB dipake keur kabel digital ATSC, tapi quadrature amplitude modulation (QAM) geus jadi standar industri sacara de facto.
|Artikel ieu keur dikeureuyeuh, ditarjamahkeun tina basa Inggris.
Bantosanna diantos kanggo narjamahkeun.
Kauntungan nyimpen daya[édit | sunting sumber]
Kauntungan utama 8VSB keur nu siaran nyaeta alatan tehnik ieu ngabutuhkeun leuwih saeutik daya pikeun nedunan hiji wewengkon nu sabanding jeung sistim NTSC nu aya samemehna, tur dilaporkeun yen leuwih hade tinimbang sisim alternatif pangilaharna, COFDM. Sababaraha stasion bisa cover the same area while transmitting at an effective radiated power of approximately 10% of analog broadcast power. While NTSC and most other analog television systems also use a vestigial sideband technique, the unwanted sideband is filtered much more effectively in ATSC 8VSB transmissions. 8VSB uses a Nyquist filter to achieve this, though the filter also causes the transmission to be very "noisy". Receivers must sample the incoming signal at very precise intervals to properly interpret the 8-level signal. Improper timing results in receiving useless information, so this makes reception in moving vehicles nearly impossible due to the Doppler effect. Reed-Solomon error correction is the primary system used to retain data integrity.
Dina usum panas 2005, ATSC nerbitkeun standar keur Enhanced VSB, atawa E-VSB . Ku ayana ieu, bisa narima DTV dina panarima handheld nu dayana saeutik kalayan anteneu nu leuwih leutik kawas DVB-H di Eropa.
Disputes over ATSC's use[édit | sunting sumber]
For some period of time, there had been a continuing lobby for changing the modulation for ATSC to COFDM, the way DVB-T is transmitted in Europe, and ISDB-T in Japan. However, the FCC has always held that 8VSB is the better modulation for use in U.S. digital television broadcasting. In a 1999 report, the Commission found that 8VSB has better threshold or carrier-to-noise (C/N) performance, has a higher data rate capability, requires less transmitter power for equivalent coverage, and is more robust to impulse and phase noise. As a result, it denied in 2000 a petition for rulemaking from Sinclair Broadcast Group requesting the change. (Since then, further improvements to VSB reception technologies have also quelled this challenge.) Because of continued adoption of the 8VSB-based ATSC standard in the U.S., and a large growing ATSC receiver population, a switch to COFDM is now essentially impossible. Congress passed a law mandating that all analog terrestrial transmissions in the US will be turned off in February 2009, and 8VSB tuners are increasingly widespread in new TVs due to FCC tuner mandates, effectively ending this debate.
8VSB vs COFDM[édit | sunting sumber]
The previously cited FCC Report also found that COFDM has better performance in dynamic and high level static multipath situations, and offers advantages for single frequency networks and mobile reception. Nonetheless, in 2001, a technical report compiled by the COFDM Technical Group concluded that COFDM did not offer any significant advantages over 8VSB. The report recommended in conclusion that receivers be linked to outdoor antennas raised to roughly 30 feet (9 m) in height. Neither 8VSB nor COFDM performed acceptably in most indoor test installations. 
However, there were questions whether the COFDM receiver selected for these tests − a transmitter monitor lacking normal front end filtering − colored these results. Retests that were performed using the same COFDM receivers with the addition of a front end band pass filter gave much improved results for the DVB-T receiver, but further testing was not pursued.
The debate over 8VSB versus COFDM modulation is still ongoing. Proponents of COFDM argue that it resists multipath far better than 8VSB. Early 8VSB DTV (digital television) receivers often had difficulty receiving a signal in urban environments. However, newer 8VSB receivers are far better at dealing with multipath. Moreover, 8VSB modulation requires less power to transmit a signal the same distance. In less populated areas, 8VSB may outperform COFDM because of this. However, in some urban areas, as well as for mobile use, COFDM may offer better reception than 8VSB. In order to broaden the application of VSB, several "enhanced" VSB systems are now in development, most notably E-VSB, A-VSB, and MPH.
Bifurcation of digital transmission systems[édit | sunting sumber]
The United States is also notable for creating a separate transmission system for digital radio. An in-band on-channel (IBOC) system developed by iBiquity will be used instead of the Eureka 147 Digital Audio Broadcast system that has been selected in Europe. This is partially because the L band normally used for that technology is unavailable in the U.S. However, the American IBOC system uses COFDM, as does Eureka 147 and another standard known as Digital Radio Mondiale.
Rujukan[édit | sunting sumber]
- DTV REPORT ON COFDM AND 8-VSB PERFORMANCE, FCC Office of Engineering and Technology, diakses pada 4 Maret 2007 , September 30, 1999.
- 8VSB/COFDM Comparison Report