Introduction| Mono| Dolby Stereo| Dolby Surround|
Dolby Pro-Logic| Dolby Pro-Logic II| Dolby Digital|
Dolby Digital Surround-EX| DTS| DTS-ES| THX| Sony SDDS|

Dolby Digital (AC-3)

Dolby Laboratories developed the Dolby Digital sound format in the late 1980's. Dolby Stereo had evolved into Dolby Spectral Recording (SR) and it was this format that was first developed into a digital form, called Dolby SR Digital. Dolby Digital was eventually released in 1992 with the theatrical presentation of Batman Returns and has since become the most widely used cinema sound format. Many people refer to Dolby Digital as 'AC-3', as this is the short name for the third generation Audio Coding technology used to encode and decode the digital data that makes up the soundtrack. The Dolby Digital system carries 6 separate, or 'discrete' audio channels that is known as Dolby Digital 5.1. The channels are used to drive speakers placed around the listening room - front-left, front-right, front-centre, surround-left and surround-right. The sixth channel, the '0.1' of the 5.1 title, is a limited bandwidth Low Frequency Effects (LFE) channel used to send deep bass information to an active sub-woofer. Although Dolby Digital is used primarily to describe this 5.1 channel format, the Dolby Digital is used to encode and decode a number of sound configurations, such as two-channel stereo, four-channel Dolby surround and even one-channel mono tracks. The main difference with 5.1 when compared to analogue surround formats is with the rear channels. Dolby Digital has two descrete rear channels that operate in stereo, with each channel being able to handle full-frequency signals at volumes as loud as the front speakers. The use of Dolby Digital for home cinema began in 1993 when the format was first used as the sound system for several digital television services around the world, and later in 1995 with the release of True Lies and Clear And Present Danger on NTSC laserdisc. In 1995 the format was also selected to be the primary sound format for use on DVD-video, which at the time was in the late stages of pre-production Digital data in any form can be compressed into a size that is a fraction of the original, and this fact is one of the main reasons why Dolby Digital has been so successful. A full six-channel soundrack could be encoded onto a film strip of a digital disc media without being restricted by the same capacity limitations found on analogue media. Laserdisc is a large digital disc and could store Dolby Digital due to the available storage capacity. Similarly, DVD-video has a vast storage capacity and can carry multiple 5.1 soundtracks if required. In the movie theatres, the digital data can be encoded on the filmstrip on the 'spare' area between the side sprockets. By doing this, a movie cound be shipped to theatres with the existing Dolby Stereo track in addition to the new six-channel digital format.