5.1 ("five point one") is the common name for six channel surround sound multichannel audio systems. 5.1 is now the most commonly used layout in both commercial cinemas and home theaters. It uses five full bandwidth channels and one low frequency enhancement channel (the "point one").[1] Dolby Digital, Dolby Pro Logic II, DTS, and SDDS are all commonly 5.1 systems. 5.1 is also the standard surround sound audio component of digital broadcast and music.[2]
All 5.1 systems use the same speaker channels and configuration, having a front left and right, a center channel, and two surround channels.
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5.1 dates back to the 1970s (1976) when Dolby Labs modified the track usage of the six analogue magnetic soundtracks on Todd-AO 70mm film prints. Instead of the 5 screen channels and one surround channel of the Todd-AO format, Dolby Stereo 70 mm Six Track provided three screen channels, two surround channels and a low-frequency enhancement channel.
When digital sound was applied to 35mm release prints, starting with Batman Returns in 1992, the same 5.1 layout was adopted. The ability to provide 5.1 sound had been one of the key reasons for using 70mm for prestige screenings. The provision of 5.1 digital sound on 35mm significantly reduced the use of the very expensive 70mm format.
Digital 5.1 sound was later used on Laserdisc and DVD starting with the Laserdisc version of Clear and Present Danger in 1995.[3] Blu-ray and Digital cinema both have eight-channel capability which can be used to provide either 5.1 or 7.1 surround sound. 7.1 is an extension of 5.1 that uses four surround zones: two at the sides and two at the back.
A system of digital 5.1 surround sound had also been used in 1987 at the famous Parisian Cabaret The Moulin Rouge, created by French engineer Dominique Bertrand. To achieve such a system, in 1985 a dedicated mixing console had to be designed in cooperation with Solid State Logic, based on their 5000 series, and dedicated speakers in cooperation with APG.[4] The console included ABCDEF channels. Respectively: A left, B right, C centre, D left rear, E right rear, F bass. The same engineer had already developed a similar 3.1 system in 1973, for use at the official International Summit of Francophone States, in Dakar, Senegal.
Regarding music the main goal of 5.1 surround sound is a proper localization and equability of all acoustic sources for a centered positioned audience. Therefore ideally five speakers of same dimension should be used.
For play-back of 5.1 music recommendations of the International Telecommunication Union (ITU) have been released and propose following configuration (ITU-R BS 775):[5]
Placing speakers to meet these requirements, in order to provide optimum sound quality, is often at odds with the space constraints of the average living room.