Dolby Digital

Dolby Digital is the trademark for Dolby Laboratories' AC-3 lossy audio compression (or data reduction) system. It is a system for coding and decoding (codec) digital audio sound so that it occupies less space on the recording medium. It is termed lossy because a perceptual coding scheme is utilized which attempts to remove information that is inaudible: for example, when a louder sound masks a quieter sound. (e.g., it is not possible to hear the noise on a poor quality tape recording of heavy metal.) In this sense it is an example of lossy data compression as these frequencies are not restored on playback.

Its main application is for multi-channel audio, but it supports anywhere from 1.0 channels (mono) to 5.1 channels (full surround) and also dual channel (1+1).

"5.1" surround sound consists of:

• The 5 full-range (10 Hz-22 kHz) channels:
  • 3 front channels (left, center and right)
  • And 2 surround side channels (left, right).
• Plus the "0.1" limited range (10 Hz-120 Hz) Low Frequency Effect channel LFE, that carries deep bass sound effects to the subwoofer.

Dolby Digital 5.1 EX (often, incorrectly, referred to as 6.1) provides a rear centre channel, using a matrix decoder, for the rear surround speakers in a cinema; in a similar fashion to Dolby Pro Logic's extraction of a front centre channel from a 2 channel source. In a home theater application, the additional center rear information is reproduced by two rear surround speakers. It should be noted that Dolby Digital EXis a second stage in the decoding chain: it is a "bolt on" to the core 5.1 audio compression standard, in order to provide a backward-compatible sixth channel that could not otherwise be accommodated.

*Dolby, Dolby Digital, and ProLogic are registered trademarks of Dolby Laboratories