8VSB
Encyclopedia
8VSB is the modulation
method used for broadcast in the ATSC digital television standard. ATSC and 8VSB modulation is used primarily in North America
; in contrast, the DVB-T standard uses COFDM.
A modulation method specifies how the radio signal fluctuates to convey information. ATSC and DVB-T specify the modulation used for over-the-air digital television; 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. In essence, it converts a binary
stream into an octal
representation by amplitude modulating
a sinusoidal
carrier to one of eight levels. 8VSB is capable of transmitting three bits (23=8) per symbol
; in ATSC, each symbol includes two bits from the MPEG transport stream which are trellis modulated
to produce a three-bit figure. The resulting signal is then band-pass filtered with a Nyquist filter
to remove redundancies in the side lobes, and then shifted up to the broadcast frequency.
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.
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
http://www.tvtechnology.com/features/atsc/F_Whitaker.shtml. 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 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 that broadcasters be allowed to choose between 8VSB or COFDM as is most appropriate for their area of coverage.
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.
However, there were questions whether the COFDM receiver selected for these tests − a transmitter monitorhttp://www.computermodules.com/pdf/dtvm2000t.pdf 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.http://www.sbgi.net/dtv/newer/Receiver%20Anamoly%20Retest.pdf
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. Several "enhanced" VSB systems were in development, most notably E-VSB
, A-VSB
, and MPH
. The deficiencies in 8VSB in regards to multipath reception can be dealt with by using additional forward error-correcting codes, such as that used by ATSC-M/H
for Mobile/Handheld reception.
It should also be noted that the vast majority of USA TV stations use COFDM for their studio to transmitter links and news gathering operations.
It should also be noted that these are point-to-point communication links and not broadcast transmissions.
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
method used for broadcast in the ATSC digital television standard. ATSC and 8VSB modulation is used primarily in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
; in contrast, the DVB-T standard uses COFDM.
A modulation method specifies how the radio signal fluctuates to convey information. ATSC and DVB-T specify the modulation used for over-the-air digital television; by comparison, QAM
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
is the modulation method used for cable
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
. 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. In essence, it converts a binary
Binary
- Mathematics :* Binary numeral system, a representation for numbers using only two digits * Binary function, a function in mathematics that takes two arguments- Computing :* Binary file, composed of something other than human-readable text...
stream into an octal
Octal
The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Numerals can be made from binary numerals by grouping consecutive binary digits into groups of three...
representation by amplitude modulating
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
a sinusoidal
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
carrier to one of eight levels. 8VSB is capable of transmitting three bits (23=8) per symbol
Symbol rate
In digital communications, symbol rate is the number of symbol changes made to the transmission medium per second using a digitally modulated signal or a line code. The Symbol rate is measured in baud or symbols/second. In the case of a line code, the symbol rate is the pulse rate in pulses/second...
; in ATSC, each symbol includes two bits from the MPEG transport stream which are trellis modulated
Trellis modulation
In telecommunication, trellis modulation is a modulation scheme which allows highly efficient transmission of information over band-limited channels such as telephone lines...
to produce a three-bit figure. The resulting signal is then band-pass filtered with a Nyquist filter
Nyquist filter
A Nyquist filter is an electronic filter used in TV receivers to equalize the video characteristics. The filter is named after the Swedish–US engineer Harry Nyquist .- VSB :...
to remove redundancies in the side lobes, and then shifted up to the broadcast frequency.
Modulation Technique
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”.Throughput
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 correctionForward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
codes. The eight signal levels are selected with the use of a trellis encoder
Trellis modulation
In telecommunication, trellis modulation is a modulation scheme which allows highly efficient transmission of information over band-limited channels such as telephone lines...
. There are also similar modulations 2VSB
2VSB
In telecommunications, 2VSB is an abbreviation for 2-level vestigial sideband modulation, a transmission method capable of transmitting one bit at a time.Other faster but less rugged forms include 4VSB, 8VSB, and 16VSB....
, 4VSB
4VSB
4VSB is an abbreviation for 4-level vestigial sideband modulation, capable of transmitting two bits at a time. Other faster but less rugged forms include 8VSB and 16VSB. While 2VSB is more rugged, it is also slower....
, and 16VSB
16VSB
16VSB is an abbreviation for 16-level vestigial sideband modulation, capable of transmitting four bits at a time.-How it works:Other slower but more rugged forms of VSB include 2VSB, 4VSB, and 8VSB...
. 16VSB was notably intended to be used for ATSC digital cable
Digital cable
Digital cable is a generic term for any type of cable television distribution using digital video compression or distribution. The technology was originally developed by Motorola.-Background:...
, but quadrature amplitude modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
(QAM) has become the de facto industry standard instead.
Power saving advantages
A significant advantage of 8VSB for broadcasters is that it requires much less power to cover an area comparable to that of the earlier NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
system, and it is reportedly better at this than the most common alternative system, COFDM
Orthogonal frequency-division multiplexing
Orthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio...
. 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
Effective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...
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
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
to achieve this. Reed–Solomon error correction
Reed–Solomon error correction
In coding theory, Reed–Solomon codes are non-binary cyclic error-correcting codes invented by Irving S. Reed and Gustave Solomon. They described a systematic way of building codes that could detect and correct multiple random symbol errors...
is the primary system used to retain data integrity.
In summer of 2005, the ATSC published standards for Enhanced VSB, or E-VSB
E-VSB
E-VSB or Enhanced VSB is an optional enhancement to the original ATSC Standards that use the 8VSB modulation system used for transmission of digital television. It is intended for improving reception where signals are weaker, including fringe reception areas, and on portable devices such as...
http://www.tvtechnology.com/features/atsc/F_Whitaker.shtml. Using forward error correction
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
, the E-VSB standard will allow DTV reception on low power handheld receivers with smaller antennas in much the same way DVB-H
DVB-H
DVB-H is one of three prevalent mobile TV formats. It is a technical specification for bringing broadcast services to mobile handsets. DVB-H was formally adopted as ETSI standard EN 302 304 in November 2004. The DVB-H specification can be downloaded from the official DVB-H website...
does in Europe, but still using 8VSB transmission.
Disputes over ATSC's use
For some period of time, there had been a continuing lobby for changing the modulationModulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
for ATSC to COFDM, the way DVB-T
DVB-T
DVB-T is an abbreviation for Digital Video Broadcasting — Terrestrial; it is the DVB European-based consortium standard for the broadcast transmission of digital terrestrial television that was first published in 1997 and first broadcast in the UK in 1998...
is transmitted in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, and ISDB-T in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. However, the FCC
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
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
Sinclair Broadcast Group
The Sinclair Broadcast Group is an American telecommunications company that operates the largest number of local television stations in the United States. Headquartered in Hunt Valley, Maryland, it owns a total of 57 stations across the country in 35 primarily small and medium markets, many of...
requesting that broadcasters be allowed to choose between 8VSB or COFDM as is most appropriate for their area of coverage.
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
E-VSB
E-VSB or Enhanced VSB is an optional enhancement to the original ATSC Standards that use the 8VSB modulation system used for transmission of digital television. It is intended for improving reception where signals are weaker, including fringe reception areas, and on portable devices such as...
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.
8VSB vs COFDM
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 monitorhttp://www.computermodules.com/pdf/dtvm2000t.pdf 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.http://www.sbgi.net/dtv/newer/Receiver%20Anamoly%20Retest.pdf
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
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
(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. Several "enhanced" VSB systems were in development, most notably E-VSB
E-VSB
E-VSB or Enhanced VSB is an optional enhancement to the original ATSC Standards that use the 8VSB modulation system used for transmission of digital television. It is intended for improving reception where signals are weaker, including fringe reception areas, and on portable devices such as...
, A-VSB
A-VSB
A-VSB or Advanced VSB is a modification of the 8VSB modulation system used for transmission of digital television using the ATSC system. One of the constraints of conventional ATSC transmission is that reliable reception is difficult or impossible when the receiver is moving at speeds associated...
, and MPH
MPH (ATSC)
MPH inband mobile digital television is a technology jointly developed by Harris Corporation, LG Electronics, Inc. and its U.S. research subsidiary, Zenith Electronics...
. The deficiencies in 8VSB in regards to multipath reception can be dealt with by using additional forward error-correcting codes, such as that used by ATSC-M/H
ATSC-M/H
ATSC-M/H is a standard in the USA for mobile digital TV, that allows TV broadcasts to be received by mobile devices.....
for Mobile/Handheld reception.
It should also be noted that the vast majority of USA TV stations use COFDM for their studio to transmitter links and news gathering operations.
It should also be noted that these are point-to-point communication links and not broadcast transmissions.