8VSB

From Academic Kids

8VSB is the 8-level vestigial sideband modulation method adopted for terrestrial broadcast of the ATSC digital television standard in the United States, which is also used in a few other countries. In the 6 megahertz channel used for broadcast ATSC, 8VSB carries 19.39 megabits of usable data per second, although the actual transmitted bitrate is significantly higher due to the addition of forward error correction codes. The eight signal levels are selected with the use of a trellis encoder. There are also the similar modulations 2VSB, 4VSB, and 16VSB. 16VSB was notably intended to be used for ATSC digital cable, but quadrature amplitude modulation (QAM) has become the industry standard instead.

A significant advantage of 8VSB for broadcasters is that it requires much less energy to cover an area comparable to that of the earlier NTSC system, and it is reportedly better at this than the most common alternative system, COFDM. Some stations can cover the same area while transmitting at an effective radiated power nearly ten times lower. 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 receving 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.

There has 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 continued to assert that 8VSB is the better modulation for use in U.S. digital television broadcasting. In 2000, it denied a petition for rulemaking from Sinclair Broadcast Group requesting the change. Because of continued adoption of the 8VSB based ATSC standard in the U.S., a switch to COFDM is now unlikely.

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. Template:Ref

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 often pulls ahead of COFDM because of this. In urban areas, however, COFDM still offers better reception than 8VSB.

ATSC 8VSB receivers (not including the display) remain very expensive as of 2005. While some can be obtained at the bargain basement price of US$100, most are priced somewhere between $350 and $750.

While ATSC 8VSB consumer receivers for use with HDTV Television sets are available, test and measuring instruments for ATSC 8VSB receivers remain elusive and are extremely expensive. The lowest priced is full featured Equipment from Modulation Sciences, which is about 20% of the cost of Spectrum analyzers sold today with 8VSB capability. Template:Ref

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 due to the fact that 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.

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