8VSB

In the ATSC digital television standard, used primarily in North America, 8VSB is the modulation method used for broadcast; that is, it specifies how the radio signal fluctuates to convey information. By comparison, QAM is the modulation method used for cable. The specifications for a cable-ready television, then, might state that it supports 8VSB (for broadcast TV) and QAM (for cable TV).

8VSB is an 8-level vestigial sideband modulation. 8VSB is capable of transmitting three bits (2³=8) per symbol by amplitude modulating a sinusoidal carrier to an intermediate frequency. The resulting signal is then band-pass filtered with a Nyquist filter to remove redundancies in the side lobes, and then again modulated to the broadcast frequency.

Vestigial sideband modulation (VSB) is a modulation method which attempts to eliminate the spectral redundancy of pulse amplitude modulated (PAM) signals. It is well known that modulating a real data sequence by a cosine carrier results in a symmetric double-sided passband spectrum. The symmetry implies that one of the sidebands is redundant, and thus removing one sideband with an ideal brickwall filter should preserve the ability for perfect demodulation. As brickwall filters with zero transition bands cannot be physically realized, the filtering actually implemented in attempting such a scheme leaves a vestige of the redundant sideband, hence the name “VSB”.

In the 6 MHz (megahertz) channel used for broadcast ATSC, 8VSB carries a symbol rate of 10.76 Mbaud, a gross bit rate of 32 Mbit/s, and a net bit rate of 19.39 Mbit/s of usable data. The net bit rate is lower 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 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 de facto industry standard instead.

^{[dubious – discuss]}

A significant advantage of 8VSB for broadcasters is that it requires much less power 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. Part of the advantage is the lower peak to average power ratio needed compared to COFDM. An 8VSB transmitter needs to have a peak power capability of 6 db (four times) its average power. 8VSB is also more resistant to impulse noise. Some stations can cover the same area while transmitting at an effective radiated power of approximately 25% 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. Reed–Solomon error correction is the primary system used to retain data integrity.

In summer of 2005, the ATSC published standards for Enhanced VSB, or E-VSB [1]. Using forward error correction, the E-VSB standard will allow DTV reception on low power handheld receivers with smaller antennas in much the same way DVB-H does in Europe, but still using 8VSB transmission.

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.^[1] As a result, it denied in 2000 a petition for rulemaking from Sinclair Broadcast Group requesting that broadcasters be allowed to choose between 8VSB or COFDM as is most appropriate for their area of coverage.^[2] The FCC report also acknowledged that COFDM would "generally be expected to perform better in situations where there is dynamic multipath," such as mobile operation or in the presence of trees that are moving in high winds. Since the original FCC report, further improvements to VSB reception technologies as well as the introduction of E-VSB option to ATSC have reduced this challenge somewhat.

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. Most analog terrestrial transmissions in the US were turned off in June 2009, and 8VSB tuners are common to all new TVs, further complicating a future transition to COFDM.

Yet, a Centris study released in February 2008 revealed "serious 'gaps' in digital TV signal coverage across the country "when taking into account 'outdoor receiving antenna sensitivity and multipath interference.'" As a result, the Centris study states, "certain households - for example: those that are not elevated; are surrounded by trees; or have set-top antennas instead of roof-top antennas; among other factors - are at higher risk of having limited or no signal coverage. Centris surveys reveal that 75% or more of over-the-air households have only set-top antennas."^[3]

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. ^[4]

However, there were questions whether the COFDM receiver selected for these tests − a transmitter monitor[2] 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.[3]

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. Newer 8VSB receivers, however, are 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.

• ATSC tuner

• What exactly is 8-VSB anyway?