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. 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.
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