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102 Everything You Ever Wanted to Know About DVD<br />
A DVD-Video disc can have up to eight audio tracks (streams) associated<br />
with each video track (or each video angle). Each audio track can be in one<br />
of three formats:<br />
• Dolby Digital (AC-3) 1 to 5.1 channels<br />
• MPEG-2 audio 1 to 5.1 or 7.1 channels<br />
• PCM 1 to 8 channels.<br />
Two additional optional formats are provided: DTS and Sony Dynamic<br />
Digital Sound (SDDS). Both require the appropriate decoders and are not<br />
supported by all players.<br />
The “.1” refers to a low-frequency effects (LFE) channel that connects to<br />
a subwoofer. This channel carries an emphasized bass audio signal.<br />
Linear PCM is uncompressed (lossless) digital audio, the same format<br />
used on CDs and most studio masters. It can be sampled at 48 or 96 kHz<br />
with 16, 20, or 24 bits per sample, with 1 to 8 channels used. (Audio CDs are<br />
limited to 44.1 kHz at 16 bits.) The maximum bit rate is 6.144 Mbps, which<br />
limits sample rates and bit sizes when five or more channels are used.<br />
It’s generally felt that the 120 dB dynamic range of 20 bits, combined<br />
with a frequency response of around 22,000 Hz from 48 kHz sampling, is<br />
adequate for high-fidelity sound reproduction. However, additional bits and<br />
higher sampling rates are useful in audiophile applications, studio work,<br />
noise shaping, advanced digital processing, and three-dimensional sound<br />
field reproduction. DVD players are required to support all the variations of<br />
LPCM, but many subsample 96 kHz down to 48 kHz, and some may not<br />
use all 20 or 24 bits. The signal provided on the digital output for external<br />
digital-to-analog converters may be limited to less than 96 kHz and less<br />
than 24 bits.<br />
Dolby Digital is multichannel digital audio, using lossy AC-3 coding technology<br />
from original PCM source with a sample rate of 48 kHz at up to 24<br />
bits. The bitrate is 64 kbps to 448 kbps, with 384 or 448 being the normal<br />
rate for 5.1 channels, and 192 being the typical rate for stereo (with or without<br />
surround encoding). (Most Dolby Digital decoders support up to 640<br />
Kbps, so nonstandard discs with 640 kbps tracks play on many players.)<br />
The channel combinations are (front/surround) 1/0, 1�1/0 (dual mono), 2/0,<br />
3/0, 2/1, 3/1, 2/2, and 3/2. The LFE channel is optional with all eight combinations.<br />
For details, see the Advanced Television Systems Committee<br />
(ATSC) document A/52 at www.atsc.org/document.html. Dolby Digital is the<br />
format used for audio tracks on almost all DVDs.<br />
MPEG audio is multichannel digital audio, using lossy compression from<br />
original PCM format with a sample rate of 48 kHz at 16 or 20 bits. Both
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