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Dolby Digital (AC-3)
Dolby Digital is a
5.1-channel surround sound
format, and is the standard for DVD-Video. Dolby Digital features up to five discrete (independent) channels (front center, front left, front right, surround left, surround right; giving it the "5" designation) of full
frequency sound
(with respect to the range of human
hearing, which ideally ranges from 20 Hz to 20,000 Hz), plus a sixth channel for low frequency effects
(LFE). The LFE signal is usually reserved for the subwoofer speaker(s), or those speakers capable of reproducing
low frequency ranges. The low frequency effects channel gives Dolby Digital the ".1" designation. The ".1" signifies that the sixth channel is not full frequency, as it contains only deep bass frequencies (3 Hz to 120 Hz).
Dolby Digital is a lossy audio
encoding scheme that supports up to 5.1 channels of discrete
audio. In actuality, a Dolby Digital soundtrack may be composed of
less than 5.1 channels. For example, Dolby Digital 1.0 is mono
audio (single track of audio information), while Dolby Digital 2.0 is
stereo (in which Dolby Surround
or Dolby Surround Pro-Logic
can be encoded). Dolby Digital 4.0 have discrete audio signals for
the center, left, and right channels, plus a mono audio signal that is
common for the surround channels. Dolby Digital 5.0 is like Dolby
Digital 5.1, but lacks a discrete audio signal for the low frequency
effects channel.
For DVD-Video, Dolby Digital encodes these multi-channel
soundtracks at either the 448 kbit/sec or 384 kbit/sec data rates.
For Blu-ray Disc, Dolby Digital encodes these
multi-channel soundtracks at 640 kbit/sec.
Keep in mind that "Dolby Digital" does not necessarily
equate to "Dolby Digital 5.1". "Dolby
Digital" generically refers to the encoding scheme as discussed in
the paragraph above. Only when it says "Dolby Digital
5.1" explicitly can you be sure that the soundtrack consists of 5.1
discrete channels.
To take advantage of Dolby Digital surround sound, you must have either:
(1) Dolby Digital decoder in the DVD player and a "Dolby Digital ready"
AV receiver
or pre-amplifier, or
(2) Dolby Digital AV receiver or pre-amplifier and use the DVD player's digital audio output.
(This option is generally better since it
allows the DVD player to pass the raw digital audio signal to the
receiver or
pre-amplifier, which theoretically
is less subject to interference and degradation.)
DTS Digital Surround is a competing
format that also supports up to "5.1" discrete
channels.
Also see: Surround Sound
Formats page for a full explanation,
Dolby Digital Plus,
Dolby Pro Logic IIz,
THX
Surround EX, DTS Digital Surround
Additional Information and References: Dolby's
Dolby Digital
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