DOLBY DIGITAL (AC-3)

Cinema sound entered the digital age with the development of Dolby Digital by Dolby Laboratories in the late 1980's. The first movie release in the theaters to feature the format was Batman Returns in 1992. Dolby Digital carries six channels of 'discrete', or separate, digital audio information - one for each of the five main speaker arrangements and a sixth low frequency bass channel to feed an active sub-woofer. This has given rise to the term '5.1 Channel' surround sound, where the '.1' represents the sub channel. Dolby Digital became the most common and most advanced cinema sound format quite quickly, a position that the format still holds today. Many refer to the format as AC-3, the name of the third generation Audio Coding hardware used to encode and decode the sound channels. Soundtracks featuring Dolby Digital for home cinema use began with the adoption of the format as the sound system for digital television in various countries around the world starting in 1993, followed by the release of True Lies and Clear And Present Danger on Dolby Digital encoded Laserdisc in 1995. Also in 1995, the format was also selected to be the surround sound format for the new DVD Video discs.

The advances over the previous industry standard were numerous. Firstly, a digital track takes up less space when recorded on to any given format, making it possible to encode the soundtrack using relatively little space. As a result, it was possible to include discrete channels for each of the rear surround speakers, enabling them to act in full range stereo rather than limited range mono. The Low Frequency Effects (LFE) channel provided information for a separate active sub-woofer in order to enhance and punctuate atmospheric effects such as thunderstorms, explosions or the low rumble of a passing spacecraft.

From an early stage, a method was needed to enable the Dolby Digital soundtrack to be included on the filmstrip in addition the the normal four channel Dolby Stereo track so that movies could be presented in cinemas that had not yet been fitted with the required Dolby Digital sound system. Traditional soundtracks were recorded on the filmstrip in the gap between the picture area and the film sprockets. The digital channels were encoded between the sprockets themselves.