Speakers' Corner: Surround sound explained

Sound in the cinema
Audio recording and reproduction technology has largely been driven by the film industry, which is responsible for today's surround-sound systems.

Back in 1939, Disney's pioneering film animation Fantasia featured the soundtrack as an animated 'character' and made the first public use of multi-track recording and multi-channel surround-sound playback, which Disney called Fantasound. Unfortunately, the Second World War halted further development for many years.

The earliest sound systems for film used a synchronised phonograph disc for the sound, but this soon gave way to an optical stripe running along the edge of the film and this system, using multiple stripes and extended by digital mosaic patterns to support multiple audio formats, is still in use.

When combined with the picture on the same reel of film, this was originally a mono system. This was expanded to two tracks for stereo and then to multi-track through simultaneous synchronised playback of multiple reels of film.

Prints are also made with magnetic stripes on the film, which produces good results but is more expensive and less convenient than optical striping.

In 1975, Dolby laboratories introduced Dolby Stereo for film soundtracks. Dolby Stereo is an analogue system that matrix-encodes four audio tracks onto two optical tracks on the film. It's rather more than stereo because the four tracks are used to feed left, centre and right loudspeakers behind the screen.

Ambient 'surround' sound is fed from the fourth channel to a number of loudspeakers positioned around the sides and back of the theatre. This system was the forerunner of the current 5.1 format.