The Evolution of Dolby Digital Surround Sound.
By Roger Ward

Dolby For The Cinema

Dolby's entry into film sound really is a story that began in the early 50's when the first successful multi-channel sound formats took off. The first few sound formats were a matrixed four channel stereo format, unlike the 2-channel sound format of the home. By the late 50's there were 2 such film formats, 4-track Cinemascope sound (35mm) and 6-track Todd-AO sound. The sound itself was produced onto the magnetic strip that a reel of film has and is then played back sort of like a home cassette player. With the decline in cinema in the 60's & 70's these formats were dumped because they were seen as too expensive for a time when the movie business was in a huge slump.

In the mid 70's Dolby Labs introduced a new sound standard upon the movie going world, Dolby Stereo. Instead of using the Metal strip in the film, the new process used the optical soundtrack technology on (35mm) film that had been used to put mono sound on movies for years. Originally the process tried to put the 4-channels or Left, Center, Right and Surround channel onto the optical print, but it produced too much noise and didn't work very well and a way needed to be found to put the four channels of information onto just 2 sound channels. Dolby Labs came up with matrixing techniques that encoded the 4-channels into the 2-channel system that would highly compatible with the optical track on the film print. When a movie was played back on a compatible Dolby processor the 2 channels would be converted in a 4-channel sound mix Left, Center, Right & Surround. The new sound technology turned out to be so successful and more cost effective that it was established as the dominant sound format for movies. Over the years the format has been refined.

In 1986 Dolby Lab's introduced Dolby Stereo-SR (Spectral Recording), Dolby SR was basically the same as Dolby Stereo, but produced better sound fidelity and noise reduction. In the late 80's Dolby Labs undertook the process of putting digital sound onto theatrical prints and Dolby Digital was born. The process involves placing the digital optical track between the sprockets of the film to provide 5.1-channels of discrete digital audio, Left, Center, Right, Left Surround, Right Surround & Subwoofer channels, but the process is also backwardly compatible with Dolby Stereo and the soundtrack can down mix to the standard 4-channel format. Dolby Digital debuted in 1992 with the theatrical release of Batman Returns and has since became the de-facto sound format in the movies, just like Dolby Stereo did over a decade before.

Dolby Sound For The Home

Dolby's sound format for the home has basically evolved from it's larger cinema counterpart. Stereo reproduction in movies at home really began when the first 2-channel stereo videocassettes came out in the early 80's, by this time stereophonic sound was the norm and people were expecting more than just basic stereo sound and Dolby Labs saw this need. In 1982 they released Dolby Surround a home version of Dolby Stereo which contained the same 4-channel matrixed sound format that Dolby Stereo uses in the theatre. By the late 80's the need was seen to advance the home cinema sound and thus Dolby Surround Pro-Logic was born. Where Dolby Stereo for the home created a phantom center speaker from the 2 front speakers and would only sound convincing to people sitting in front of the TV, Pro-Logic included a center speaker and well as the Right, Left & Surround speakers to lock the center channel in position and it would sound as if the dialog would be coming from the center, even if you were sitting on an angle from the TV. The process also allowed you to place the front speakers further apart than before to create a larger sound field.

By the mid 90's the home version of Dolby Digital was born, the system uses the Left, Center, Right, Left Rear & Right Rear as well as the .1 subwoofer channels. Originally Dolby Digital (AC-3) was only available on American NTSC Laserdiscs that were encoded in DD5.1. Now DVD's and soon Digital TV broadcasts are all encoded in Dolby Digital and soon a new sound format is on the way from Dolby Labs, Dolby Digital EX.