Dolby Digital (AC-3) - TimeForDVD.com

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. Dolby Digital encodes these multi-channel soundtracks at either the 448 kbit/sec or 384 kbit/sec data rates.

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" receiver or pre-amplifier, or
(2) Dolby Digital 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.

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