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Surround sound is the concept of expanding the spatial imaging of audio playback from 1 dimension (mono/Left-Right) to 2D or 3D.
This is often performed for a more realistic audio environment, actively implemented in cinema sound systems, technical theatre, home entertainment, video arcades, computer gaming, and a growing number of other applications.
Many popular surround sound formats have evolved over the years. They include ambisonics, quadraphonic, Dolby 5.1 Surround sound, and DTS.
Surround sound can be created using several methods. The simplest to understand uses several speakers around the listener to play audio coming from different directions. Another approach involves processing the audio using psychoacoustic sound localization methods to simulate a 3D sound field using headphones. The third approach, wave field synthesis, uses a very large number of speakers to generate the "audio hologram" of the original audio in the whole room (as of 2004 the only commercial implementation of this is Iosono).
Surround sound is not limited to placement of speakers along a flat (2-dimensional) plane. Vertically-located audio sources can be considered.
Specifications 5.1 uses: - Five full-range audio channels:
- Three for speakers at the front (stereo left and right, plus centre mono)
- Two for surround speakers at the side or rear (stereo left and right)
- A sixth, low-frequency effects (LFE) channel carries supporting deep bass sound effects, ranging from 10 Hz to 120 Hz, which can for example be used by a subwoofer.
5.1 EX includes a third surround channel that can be decoded at the listener's option for playback over additional surround speakers placed behind the viewers (rear speakers). The additional center rear information is split between the left and right back surround channels.
7.1 would use two additional speakers, although no consumer home cinema applications currently exist for it. Some computers are capable of outputting a discrete 7.1 signal.
Note: A distinction is made between the number of discrete channels encoded in the original signal, and the number of channels that are reproduced for playback; these can be added using matrix decoding. A distinction is also made between the number of channels reproduced for playback, and the number of speakers over which these channels are played.
A 5.1 EX signal, for example, contains 5.1 discrete channels of information. Parts of the signal are then passed through a matrix decoder to create a 6.1 channel playback signal. The 6.1 channels are then properly played over 7.1 channels (i.e. 7 speakers and a subwoofer).
Notation This notation, e.g. '5.1', reflects the number of full range, discrete channels; including a ".1" to reflect the limited range of the LFE channel.
e.g. 5 full-range channels + 1 LFE channel = 5.1
It can also be expressed as the number of full-range channels in front of the listener, separated by a slash from the number of full-range channels beside or behind the listener, separated by a decimal point from the number of limited-range LFE channels.
e.g. 3 front channels + 2 side channels + an LFE channel = 3/2.1
This notation can then be expanded to include the notation of Matrix Decoders. Dolby Digital EX, for example, has a sixth full-range channel incorporated into the two rear channels with a matrix. This would be expressed:
3 front channels + 2 rear channels + 3 channels reproduced in the rear in total + 1 LFE channel = 3/2:3.1
Note: The term stereo, although popularised in reference to two channel audio, can also be properly used to refer to surround sound.
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