A Dolby Digital® decoder is a device which decodes signals sent in the most popular format for digital surround sound. The most common version of this format involves five individual surround channels plus a separate channel for low frequency sounds carried by a subwoofer. However, there are also several other versions of Dolby Digital®.
The most common use of a Dolby Digital® decoder is as part of a home entertainment system. In most cases, the decoder will form part of a device known as a receiver which connects to one or more audio sources and then distributes the sound to the various speakers. Most receivers will also have a radio built in, while some can also handle video signals. An alternative to a receiver is an all-in-one unit in which the decoder and audio inputs are built into the subwoofer, which then connects to the speakers. A Dolby Digital® decoder could also be built into a computer sound card.
The standard form of Dolby Digital involves carrying separate signals for left, center, right, left surround and right surround channels, plus a sub-woofer. The surround channels are often referred to as rear channels, though many audio experts suggest the relevant speakers should be placed to the side of the listener rather than behind them. Dolby Digital® is sometimes known as AC-3 or, when used in television broadcasts, ATSC A/52.
Variations on Dolby Digital® include Dolby Digital EX® and Dolby Digital Surround EX®. Both of these produce a signal for an extra speaker, which sits behind the listener. The difference is that the former artificially creates the sound for the channel by "deducing" it from the existing signal, while the latter involves the extra sound channel being specifically built into the signal in the original recording.
There are several other formats which may be covered by a Dolby Digital® decoder. Dolby Digital Live® creates surround sound "on the fly" from a video game. Dolby Digital Plus®, which requires dedicated equipment to play back, stores more information to make a better quality sound.
Most Dolby Digital® decoders will also be able to handle Dolby Pro Logic®, an older format which carries surround sound in a standard stereo signal, but only has a single surround or rear channel. A later variation on this is Dolby Pro Logic II®. This simulates full Dolby Digital® from any stereo source rather than using information which has been specifically encoded to provide a surround source. Unlike some similar simulation techniques, Dolby Pro Logic II® simply splits the original sound between the various speakers rather than artificially creating sound.