Surround sound

Surround sound is the concept of expanding the spatial imaging of audio playback from one dimension (mono/Left-Right) to two or three dimensions.

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 discrete 5.1 Surround sound on DVD-Audio (DVD-A) or SACD (Super Audio CD), ambisonics, quadraphonic, Dolby 5.1 Surround sound, DTS, DVD-Video (DVD-V), and MP3 Surround.

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, which is based on Huygens' principle, attempts to reconstruct the recorded soundfield wavefronts within the listening space and so might be regarded as a form of "audio hologram". There are two related forms of this approach,the first of which, Ambisonics, provides an exact reconstruction at a central point and a less and less accurate reconstruction as you move away from this point. The second form, wave field synthesis or WFS, produces a soundfield which, whilst not absolutely accurate anywhere, has an even error field over the whole area. WFS (of which two commercial systems are available, one from the Swiss company sonic emotion and one from Iosono) requires a large number of loudspeakers and a considerable amount of computing power to produce its results whereas Ambisonics, for which there is a significant amount of both free and commercial software available (as well as some hardware from, for instance, Meridean) requires far fewer resources, at least in its simplest form (this is no longer so true for more recent developments such as Near Field Compensated Higher Order Ambisonics^* but for the present Ambisonics has a far greater market penetration in the domestic arena and especially amongst musicians involved in electronic and computer music. Some consumer electronic devices (AV receivers, stereos, and computer soundcards) have Digital signal processors or digital audio processors features built into them to simulate surrround sound from stereo sources.

Though generally the province of big-budget movie productions and sophisticated video games, some consumer camcorders (particularly DVD-R based models from Sony) have surround sound capability either built-in or available as an add-on. Though considered by camcorder reviewers to be of dubious utility, it is nevertheless one of the few ways that someone not using professional equipment can create surround sound. (The MiniDV spec does allow up to four channels of sound, making it theoretically possible for such camcorders, but it is seldom implemented that way.)

Specifications

4.0 Surround (matrixed Quadraphonic)

Extracts 4 audio channels from a stereo source:

Two for speakers at the front - left (L) and right (R).
Two for surround speakers at the rear - surround left (L_S) and surround right (R_S).
Describes the early matrixed Surround Quadraphonic, Quadraphonic Stereo and other systems.
Surround Sound speaker placement: Four identical speakers placed equidistant around a central listening position.

4.1 Surround (matrixed Prologic)

Extracts 4 audio channels from a stereo source::

Three for speakers at the front - left (L), center (C) and right (R).
One for surround speakers at the rear - limited frequency mono surround channel (S).
One optional low-frequency channel using a sub-woofer.
Describes the Dolby Prologic matrixed Surround systems, with mono rear surround channels.
Surround Sound speaker placement: The front speakers should be placed at the edges of the screen, toed in to face the central listening location, and the tweeters should be ear hight. The center speaker should be placed behind the screen (when using projection) or over or under a tv, and as close to ear high as possible. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and can be of smaller size, due to the frequency limitations of the surround channel.

5.1 Surround (matrixed Prologic II)

Extracts 5 audio channels from a stereo source:

Three for speakers at the front - left (L) and right (R) at 22-30°, and center (C).
Two for surround speakers at 90-110° to the side or rear - stereo surround left (L_S) and surround right (R_S).
One low-frequency channel using a sub-woofer.
Describes the Dolby Prologic II matrixed Surround systems, with stereo rear surround channels.
Surround Sound speaker placement: The front speakers should be placed at the edges of the screen, toed in to face the central listening location, and the tweeters should be ear hight. The center speaker should be placed behind the screen (when using projection) or over or under a tv, and as close to ear high as possible. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a di-pole construction.

5.1 Surround (discrete Dolby Digital, DTS)

Delivers six audio channels:

Three for speakers at the front - left (L) and right (R) at 22-30°, and center (C).
Two for surround speakers at 90-110° to the side or rear - surround left (L_S) and surround right (R_S).
A Low-Frequency Effects channel carries supporting deep bass sound effects, ranging from 10 Hz to 80 Hz, which can for example be used by a subwoofer.
Surround Sound speaker placement: Surround Sound speaker placement is different for both music and movie content. For music speakers are placed in a circle around the listener. The center channel has 0º offset, left and right are offset ± 30º, and the left/right surrounds are offset by ±110º. Also all speakers should be, monopole, equidistant to the listener, and all delay (ms) calculations on the surround decoder should be turned off (0ms).

For movie surround, the front speakers should be placed at the edges of the screen, toed in to face the central listening location, and the tweeters should be ear hight. The center speaker should be placed behind the screen (when using projection) or over or under a tv, and as close to ear high as possible. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a di-pole construction.

6.1 Surround (discrete Dolby Digital EX, DTS-ES)

6.1, which was developed by Gary Rydstrom and Anthony Grimani and is typically delivered as Dolby Digital EX or DTS-ES, adds a Surround back channel to the 5.1 setup. The additional surround back information is stored in the Dolby Digital EX format as a matrixed signal, meaning that audio that is in phase and equally in the Sl and Sr channels will be played in the Surround Back channel. DTS-ES allows for a discrete Surround back signal, but can also be matrixed. Some 6.1 playback systems do use dual-mono Sb channels.

7.1 Surround (discrete SDDS system (Sony Dynamic Digital System))

7.1 would use two additional speakers, although no consumer home cinema applications currently exist for it. Some computers and video game systems are capable of outputting a discrete 7.1 signal, and a number of mid-range and high-end receivers support it if it is available.

7.1 systems often refer to playing a 6.1 signal over a 7.1 surround setup. This is usually accomplished by duplicating the surround channel (6th channel) to the additional 7th channel. In practice, this improves envelopment of sound where a big space exists between the rear surround speakers.

The 7.1 system is also known as the SDDS system (Sony Dynamic Digital System), developed by Sony for large cinema halls. These incorporate Left Center (LC) and Right Center (RC) channels between the Left and Center channels and the Right and Center channels respectively.

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.

Additionally, 5.1, 6.1, and 7.1 formats make use of bass management, which allows sound that is below the abilities of main channels (5.0 channels) to be redirected to the subwoofer, which is designed to handle that frequency range. There are notation differences between the pre-bass-managed signal and once it has passed through bass manager. For example, in 5.1, the channels are referred to as L, R, C, LFE, Sl, and Sr. However, once passing through the bass manager, they are referred to as L, R, C, Sub, Sl, and Sr.

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.

Music Artists

Jean Michel Jarre, AERO album
Diatonis, Highway 1 album
Andreas Paolo Perger - 5.1 surround guitar as artistical challenge, link to the personal homepage of the artist

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