SURROUND SOUND

'Multichannel audio' is the name for a variety of techniques for expanding and enriching the sound of audio playback by recording additional sound channels that can be reproduced on additional speakers. 'Surround sound' refers to the application of multichannel audio to channels "surrounding" the audience (generally some combination of left surround, right surround, and back surround) as opposed to "screen channels" (center, [front] left, and [front] right).
Surround sound systems are used in cinema sound systems, home entertainment systems such as "home theater" systems, video arcade games, computer games, technical theatre, and a growing number of other applications.
Consumer surround sound formats include sound on Video DVDs and HDTV broadcasts
encoded as Dolby Digital or DTS;
the competing DVD-Audio (DVD-A) and Super Audio CD (SACD) formats;
and MP3 Surround.
Theatrical film 5.1 surround formats include Dolby Digital, DTS, and
Sony Dynamic Digital Sound (SDDS).
Surround sound hardware is mostly used by big-budget movie productions and sophisticated video games. However, some consumer camcorders (particularly DVD-R based models from Sony) have surround sound capability either built-in or available as an add-on. Some consumer electronic devices (AV receivers, stereos, and computer soundcards) have digital signal processors or digital audio processors built into them to simulate surround sound from stereo sources.
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Creating surround sound

Surround sound can be created using several methods. The simplest method uses several speakers placed around the listener to play audio coming from different directions. A second approach is to process the audio using psychoacoustic sound localization methods to simulate a 3-D sound field using headphones.
A third approach, 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". One form, wave field synthesis(WFS) produces a soundfield which has an even error field over the whole area. WFS commercial systems made by the Swiss company ''sonic emotion'' and by ''Iosono'', require a large number of loudspeakers and a considerable amount of computing power to produce its results.
Ambisonics is another form based on Huygens' principle. Ambisonics provides an exact reconstruction at a central point, and a less accurate reconstruction as you move away from this point. There is a significant amount of both free and commercial software available for Ambisonics, and Ambisonics has significant market penetration in the consumer market, especially with musicians who use electronic and computer music. In addition, Ambisonics is standard in hardware surround products offered by Meridian Audio, Ltd. In its simplest form, Ambisonics consumes few resources . However, this is no longer true for more recent developments such as Near Field Compensated Higher Order Ambisonics.^[1] Some years ago it was shown that, in the limit, WFS and Ambisonics converge.^[2]
Surround sound can also be synthesized from stereo sources, see for example Penteo.

Mapping channels to speakers

In most cases, surround sound systems have relied on the mapping of each source channel to its own loudspeaker. Matrix systems recover the number and content of the source channels and apply them to their respective loudspeakers. With discrete surround, the transmission medium allows for (at least) the same number of channels that the source and destination utilize.
However, one-to-one channel to speaker mapping is not the only way of transmitting surround information. The transmitted signal may encode the information defining the original soundfield to a greater or lesser extent; this is ''rendered'' at the replay end by a decoder which generates the number of loudspeaker feeds required to suit the number of speakers available for replay and their configuration. This "replay device independent" encoding is analogous to the process of encoding and decoding an Adobe PostScript file, where the file describes the page and is rendered according to the resolution of the output device. Audio rendering is used in Ambisonics and WFS systems. Meridian Lossless Packing contains elements of this capability.

Bass management

Surround replay systems may make use of ''bass management'', the fundamental principle of which is that bass content in the incoming signal, irrespective of channel, should be directed only to loudspeakers capable of handling it, whether the latter are the main system loudspeakers or one or more special low-frequency speakers called subwoofers.
There is a notation difference before and after the bass management system. Before the bass management system there is a Low Frequency Effects (LFE) channel. After the bass management system there is a subwoofer signal. A common misunderstanding is the belief that the LFE channel is the "subwoofer channel". The bass management system may direct bass to one or more subwoofers (if present) from ''any'' channel, not just from the LFE channel. Also, if there is no subwoofer speaker present then the bass management system can direct the LFE channel to one or more of the main speakers.

Low Frequency Effects (LFE) channel

The ''Low Frequency Effects channel'', or ''LFE'', is a source of some confusion in surround sound. The LFE channel was originally developed to carry extremely low "sub-bass" cinematic sound effects (e.g., the loud rumble of thunder or explosions) on their own channel. When loud sub-bass effects are on a different channel, this allows theaters to control the volume of the sub-bass effects, so that it suits the size of their sound reproduction system and the acoustic environment of their cinema. As well, independent control of the sub-bass effects reduced the problem of intermodulation distortion in analog movie sound reproduction.
In the original movie theater implementation, the LFE was a separate channel fed to one or more subwoofers. However, home replay systems may not have a separate bass speaker (subwoofer) that is able to handle the sub-bass effects. As a result, modern home surround decoders and systems often include a bass management system that allows bass on any channel (main or LFE) to be fed only to the loudspeakers that can handle low-frequency signals. ''The salient point here is that the LFE channel is not the "subwoofer channel"''; there may not even be a subwoofer, and if there is it may be handling a good deal more than effects.^[3]
Some record labels such as Telarc and Chesky have argued that LFE channels are not needed in a modern digital multichannel entertainment system. They argue that all available channels have a full frequency range and, as such, there is no need for an LFE in surround music production, because all the frequencies are available in all the main channels. These labels sometimes use the LFE channel to carry a height channel, underlining its redundancy for its original purpose.

Surround sound specifications

The descriptions of surround sound specifications below distinguish between the number of discrete channels encoded in the original signal and the number of channels reproduced for playback. The number of channels reproduced for playback can be changed by using matrix decoding. A distinction is also made between the number of channels reproduced for playback and the number of speakers used to reproduce (each channel may refer to a group of speakers). The graphics to the right of each specification description represent the number of channels, not the number of speakers.

3.0 Channel Surround (analog matrixed: Dolby Surround)

Extracts 3 audio channels from a specially encoded two-channel source:

★ Two channels for speakers at the front - left (L) and right (R).

★ One channel for surround speaker or speakers at the rear - surround (S).

★ Describes the numerous matrixed (pre- Pro Logic) surround processors.

★ Speaker placement: (3 speakers in total) Three identical speakers placed equidistant around a central listening position. If two rear speakers are used they should be placed high on side walls, slightly behind the listening position, and should be of bi-pole construction.

4.0 Channel Surround (analog matrixed/discrete: Quadraphonic)

Extracts 4 audio channels from either a specially encoded two-channel source or a four-channel source:

★ Two channels for speakers at the front - left (L) and right (R).

★ Two channels for surround speakers at the rear - surround left (LS) and surround right (RS).

★ Describes the early matrixed systems and discrete Quadraphonic surround systems. Source media, usually records or tape, is often branded 4 channel stereo.

★ Speaker placement: (4 speakers in total) 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 height. Rear channel speaker should be placed behind the listening position to form a square with the front speakers, and should have a normal high-quality monopolar construction.

4.0 Channel Surround (analog matrixed: Dolby Pro Logic)

Extracts 4 audio channels from a specially encoded two-channel source:

★ Two channels for speakers at the front - left (L) and right (R).

★ One channel for speaker at the center - center (C).

★ One channel for both surround speakers at the rear - mono surround channel (S).

★ Describes the Dolby Pro Logic matrixed surround system. Source media, usually VHS, Laser Disc or television broadcast, CableTV/Satellite is often branded with "Dolby Surround" logo. This is the encoding used on the analog optical track for theatrical motion picture films.

★ Speaker placement: (4 channels in total) 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 height. 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. Surround channel speakers should be placed high on side walls, slightly behind the listening position, and should be of bi-pole construction.

5.1 Channel Surround (analog matrixed: Dolby Pro Logic II)

Extracts 5 audio channels from either a specially encoded two-channel or a stereo source:

★ Two channels for speakers at the front - left (L) and right (R).

★ One channel for speaker at the center - center (C).

★ Two channels for surround speakers at the rear - surround left (LS) and surround right (RS).

★ One low-frequency effects channel (LFE).

★ Describes the Dolby Pro Logic II matrixed surround system. Source media is often branded with "Pro Logic II" logo. Most modern theatrical motion picture films include a digital soundtrack in this format.

★ Surround sound speaker placement: (6 speaker channels in total) surround sound speaker placement is different for both music and movie content.
For music, speakers should be placed closest to the left and right of the listening position, close to that of the 6.1 surround speakers.
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 height. The center speaker should be placed behind the screen (when using projection) or over or under a TV, and as close to ear height as possible. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a normal high-quality monopolar construction.

5.1 Channel Surround (digital discrete: Dolby Digital, DTS, SDDS)

Delivers 5 discrete audio channels and 1 LFE channel from a 6 channel source:

★ Two channels for speakers at the front - left (L) and right (R).

★ One channel for speaker at the center - center (C).

★ Two channels for surround speakers at the rear - surround left (LS) and surround right (RS).

★ One low-frequency effects channel (LFE).

★ Describes the Dolby Digital, Digital Theater System (DTS), and Sony Dynamic Digital Sound (SDDS) systems. Source media, usually DVD and sometimes Laser Disc or satellite/digital cable is often branded with "Dolby Digital" and/or DTS logos.

★ DTS uses a higher sampling rate than Dolby Digital, thus DTS uses less compression and achieves higher fidelity than Dolby Digital.

★ Surround Sound speaker placement: (6 speaker channels in total) 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 (0 ms).
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 height. The center speaker should be placed behind the screen (when using projection) or over or under a TV, and as close to ear height as possible. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a normal high-quality monopolar construction.

6.1 Channel Surround (analog matrixed: Dolby Pro Logic IIx)

Extracts 6 audio channels and 1 low-frequency channel from either a specially encoded two-channel or a stereo source:

★ Two channels for speakers at the front - left (L) and right (R).

★ One channel for speaker at the center - center (C).

★ Two channels for surround speakers at the sides - side left (LS) and side right (RS).

★ One channel for surround speakers at the rear - back surround channel (BS).

★ One low-frequency channel to drive a sub-woofer.

★ Describes the Dolby Pro Logic IIx matrixed surround system. Source media is the same as both Dolby Pro Logic and Dolby Pro Logic II.

★ Surround Sound speaker placement: (8 speakers in total) surround sound speaker placement is different for both music and movie content.
For music, speakers placement is best as shown in the diagram, tweeters should be pointed closest to the listening position.
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 height. The center speaker should be placed behind the screen (when using projection) or over or under a TV, and as close to ear height as possible. Side channel speakers should be placed high on side walls, to the left and right of the listening position, equidistant from the front speakers and the rear speakers. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a normal high-quality monopolar construction.

6.1 Channel Surround (digital partially discrete: Dolby Digital EX)

Delivers 5 audio channels, 1 extracted audio channel and 1 LFE channel from a 6 channel source:

★ Two discrete channels for speakers at the front - left (L) and right (R).

★ One discrete channel for speaker at the center - center (C).

★ Two channels for surround speakers at the sides - left surround (LS) and right surround (RS). The discrete LS and RS channels are dematrixed into LS, RS, and back surround (BS).

★ One channel for surround speakers at the rear - back surround channel (BS).

★ One low-frequency effects channel (LFE).

★ Describes the Dolby Digital EX discrete/matrixed hybrid Surround system. Source media, usually DVD is often branded with "Dolby Digital EX" logo. This format is used in some theatrical motion picture films.

★ Surround Sound speaker placement: (8 speaker channels in total) Surround Sound speaker placement is different for both music and movie content.
For music, speaker placement is unknown
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 height. The center speaker should be placed behind the screen (when using projection) or over or under a TV, and as close to ear height as possible. Side channel speakers should be placed high on side walls, to the left and right of the listening position, equidistant from the front speakers and the rear speakers. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a normal high-quality monopolar construction.

6.1 Channel Surround (digital discrete: DTS-ES)

Delivers 6 discrete audio channels and 1 LFE channel from a 7 channel source:

★ Two channels for speakers at the front - left (L) and right (R).

★ One channel for speaker at the center - center (C).

★ Two channels for surround speakers at the sides - side left (LS) and side right (RS).

★ One channel for surround speakers at the rear - back surround channel (BS).

★ One low-frequency effects channel (LFE).

★ Describes the DTS ES discrete Surround system. Source media, usually DVD is often branded with "DTS ES" logo. In theatrical motion picture film, this format does not exist, and the name "DTS-ES" refers to the above hybrid format used for Dolby Digital EX.

★ Surround Sound speaker placement: (8 speaker channels in total) Surround Sound speaker placement is different for both music and movie content.
For music, speakers placement is unknown
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 height. The center speaker should be placed behind the screen (when using projection) or over or under a TV, and as close to ear height as possible. Side channel speakers should be placed high on side walls, to the left and right of the listening position, equidistant from the front speakers and the rear speakers. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a normal high-quality monopolar construction.

7.1 Channel Surround (digital discrete: Dolby Digital Plus, DTS-HD)

Delivers 7 audio channels and 1 LFE channel from an 8 channel source:

★ Two channels for speakers at the front - left (L_F) and right (R_F).

★ One channel for speaker at the center - center (C).

★ Two channels for surround speakers at the sides - side left (L) and side right (R).

★ Two channels for surround speakers at the rear - surround left (LS) and surround right (RS).

★ One low-frequency effects channel (LFE).

★ Describes the Dolby Digital Plus discrete Surround system. Source media, usually HD-DVD and sometimes Blu-Ray is often branded with "Dolby Digital Plus" and/or DTS-HD logos.

★ Surround Sound speaker placement: (8 speaker channels in total) Surround Sound speaker placement is different for both music and movie content.
For music, speaker placement is unknown.
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 height. The center speaker should be placed behind the screen (when using projection) or over or under a TV, and as close to ear height as possible. Side channel speakers should be placed high on side walls, to the left and right of the listening position, equidistant from the front speakers and the rear speakers. Rear channel speakers should be placed high on side walls, slightly behind the listening position, and should have a normal high-quality monopolar construction.

10.2 Channel Surround

Main articles: 10.2

'10.2' is the surround sound format developed by THX creator Tomlinson Holman of TMH Labs and University of Southern California (schools of Cinema/Television and Engineering). Developed along with Chris Kyriakakis of the USC Viterbi School of Engineering, ''10.2'' refers to the format's promotional slogan: "Twice as good as 5.1". Advocates of 10.2 argue that it is the audio equivalent of IMAX.
10.2 augments the LS (left surround) and RS (right surround) channels by two point surround channels that can more finely manipulate sound - allowing the mixer to shift sounds in a distinct 360° circle around the movie watcher.
The 14 discrete channels are:

★ Five front speakers: Left Wide, Left, Center, Right and Right Wide

★ Five surround channels: Left Surround Diffuse, Left Surround Direct, Back Surround, Right Surround Diffuse and Right Surround Direct

★ Two LFE channels: LFE Left, LFE Right

★ Two Height channels: Left Height, Right Height
The ''.2'' of the 10.2 refers to the addition of a second subwoofer. The system is bass managed such that all the speakers on the left side use the left sub and all the speakers on the right use the right sub. The Center and Back Surround speaker are split among the two subs. The two subs also serve as two discrete LFE (Low Frequency Effects) channels. Although low frequencies are not localizable, it was found that splitting the bass on either side of the audience increases the sense of envelopment.

22.2 Channel Surround

Main articles: 22.2

'22.2' is the surround sound component of Ultra High Definition Video
(Super Hi-vision TV with 4320 scanning lines), and has been developed by NHK
Science & Technical Research Laboratories. As its name suggests, it uses 24
speakers. These are arranged in three layers: A middle layer of ten speakers,
an upper layer of nine speakers, and a lower layer of three speakers and two
sub-woofers. The system was demonstrated at Expo 2005, Aichi,
Japan, the NAB 2006 conference,
Las Vegas, and at
IBC 2006, Amsterdam,
Netherlands.

Notation

This notation, e.g. '5.1', reflects the number of full range 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, as it strictly means "solid" sound. However this is no longer a common usage and "stereo sound" is almost exclusively used to describe 2 channel left and right sound.

References

1. Spatial Sound Encoding Including Near Field Effect: Introducing Distance Coding Filters and a Viable, New Ambisonic Format
2. Further Investigations of High Order Ambisonics and Wavefield Synthesis for Holophonic Sound Imaging
3. Multichannel Music Mixing by Dolby Laboratories, Inc.

External links

★ Speaker Setup

★ Ambisonia.com (Repository for surround sounds)

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