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As you may already know, audio information on DVD is stored digitally. Sound is an inherently analogue medium, so must be converted into computer data (long strings of 1s and 0s) before this can take place. Much as the message "PEACE ON EARTH" can be translated into dozens of spoken languages (English, French, Korean, etc.) this sound can be translated into data using many different languages or 'formats'.
Computer users might be familiar with formats such as MP3 or WAV -- these are simply different ways of expressing audio in the form of computer data. In the field of digital audio there are numerous ways audio can be turned into data. Each format has its own advantages and disadvantages, but the goal is always the same: to reproduce the original signal as accurately as possible within the limitations and constraints imposed on each format.
In the world of DVD, there are two primary 'multichannel' (that is, more than the conventional one to two audio channel) formats. One is called DTS (Digital Theater Systems) and the other is called Dolby Digital. DTS has been discussed above, so I will focus on the most common format, Dolby Digital.
What is Dolby Digital?
Dolby Digital is an audio compression system introduced to theatres back in 1992 for the film Batman Returns. In late 1995 the system made its appearance on LaserDisc, DVD's immediate predecessor, with the release of Clear and Present Danger. It first appeared on DVD in late 1996 (in Japan), and early the following year in the US.
Dolby Digital is a very flexible system, allowing anywhere from 1 to 5 full-range audio channels with an optional 'LFE' (Low Frequency Effects, or bass) channel for each variant. Dolby Digital also consumes very little 'space' (data) on a DVD, and a typical Dolby Digital 5.1 soundtrack requires less than a third of the space required by a CD's two channels. This small size leaves lots of space for multiple soundtracks, extras, or better video.
Dolby Digital soundtracks can be so small because they are 'compressed', much like a computer .zip file (but in this case 'AC-3'); although unlike a computer's compression system some audio information is 'thrown away' to save additional space. This may sound bad but this information shouldn't be audible so makes little to no difference to the system's overall sound quality.
Dolby Digital 2.0 is the most common use of the format, the '2' indicating the number of full range channels and the '0' indicating the lack of an LFE channel.
Dolby Digital 2.0 is usually used to present stereo soundtracks on DVD, which can additionally be recorded in Dolby Surround (a technique that squeezes four audio channels into two, and is intended to be processed by a Pro Logic or Pro Logic 2 decoder).
Dolby Digital 5.1 is the second most common use of Dolby Digital on DVD, presenting five full-range audio channels and an LFE channel. Three of these channel are arranged across the front soundstage (left, centre and right) and two across the rear soundstage (left surround and right surround; Dolby Digital Surround EX adds an additional back-surround channel blended into the two surround channels). The LFE channel is directed to a subwoofer, or if one is unavailable to a system's main speakers.
Other variants such as Dolby Digital 4.1 are much less common, appearing on a few DVDs such as Big Trouble in Little China. Dolby Digital 4.1 usually indicates three front soundstage channels and a single surround channel that is fed to both surround speakers.
Along with PCM audio (the same kind of audio used by the venerable CD) Dolby Digital is one of DVD's audio 'standards', which means every DVD must contain either a Dolby Digital or PCM soundtrack. Any type of Dolby Digital will do, even a mono Dolby Digital 1.0 soundtrack.
Because Dolby Digital soundtracks are so much smaller than PCM soundtracks, most DVDs include a Dolby Digital soundtrack rather than PCM.
Adam
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