5.1 surround sound
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 effects channel (the "point one"). 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.
5.1 dates back to 1976 when Dolby Labs modified the track usage of the six analogue magnetic soundtracks on Todd-AO 70 mm film prints. Instead of the five 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 35 mm 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 70 mm for prestige screenings. The provision of 5.1 digital sound on 35 mm significantly reduced the use of the very expensive 70 mm format.
Digital 5.1 sound was later used on Laserdisc and DVD starting with the Laserdisc version of Clear and Present Danger in 1995. 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 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. 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.
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 matched speakers should be used.
- five speakers of the same size for front, center and surround
- identical distance from the listeners for all five speakers
- angle adjustment regarding viewing direction of audience: center 0°, front ±22.5° for movies ±30° for music, surround ±110°
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.
See also 
- "Understanding Surround Sound Formats". Crutchfield.com. 2009-09-23. Retrieved 2011-03-07.
- What is 5.1?. 5dot1.com. Retrieved on 27 January 2013.
- "AC3 audio format". Retrieved 2011-03-07.
- "Recherche & Dévelopement". Retrieved 2011-03-07.
- Thomas Lund (2000), Enhanced Localization in 5.1 Production, Audio Engineering Society, retrieved 2011-03-07