* Significantly changed from the previous version
** New in this version
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http://www.omg.unb.ca/~mleese/By anonymous FTP from pacific.cs.unb.ca (131.202.35.4) in the directory /pub/ambisonic. The file is called faq_v2_2.html or something similar, and is suitable for viewing with Mosaic and other WWW browsers. PostScript and text files derived from this HTML source file can be found in the same directory.
http://www.york.ac.uk/inst/mustech/3d_audio/ambison.htm
http://jrusby.uoregon.edu/mustech.html
Also, if sufficiently popular, it will be posted periodically to the Usenet newsgroups rec.audio.tech, rec.audio.opinion, rec.audio.misc and rec.audio.pro.
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This Frequently Asked Questions draws heavily from the Gerzon 1977b and Fellgett 1981 references. Any opinions it contains, however, are my own.
The FAQ still contains a few `?'s. If you can complete any of them, please let me know. If you have more information, please e-mail, post or fax it to me.
Many thanks,
Martin Leese
Department of Geodesy and Geomatics Engineering
University of New Brunswick
Post Office Box 4400
Fredericton, N.B.
Canada E3B 5A3
E-mail: MLeese@atlantic.cs.unb.ca
WWW:
http://www.omg.unb.ca/~mleese/
Phone: (506) 453-4969
Fax: (506) 453-4943
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Then there was stereo with directional information spread along a line in front of the listener.
In real life, of course, sound reaches our ears from all directions. Surround sound attempts to improve the realism of the perceived sound by providing information from all directions, not just from in front of the listener.
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Ambisonic technology is based on a meta-theory (a theory of theories) of sound localisation developed by Michael A Gerzon when he was with the Mathematical Institute, University of Oxford. (See the Gerzon 1992a reference for 63 pages of intractable mathematics.) Ambisonic decoder design aims to satisfy simultaneously and consistently as many as possible of the mechanisms of localisation used by the ear/brain. The Gerzon theory takes account of non-central as well as central listening positions.
In a practical decoder the spherical harmonic direction signals, W, X, Y and Z, are passed through a set of variable gain shelf filters designed to match the different requirements of the ear/brain at low and high frequencies (below and above about 700 Hz). The speaker feeds are then derived by passing the outputs from the shelf filters through an amplitude matrix. An important aspect of Ambisonic decoder technology is that it is only at this final stage of processing that the number and layout of speakers is considered.
The listening area for Ambisonic Surround Sound is comparable with that for conventional stereo, but larger.
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The fact that the Z component can be recorded creates the possibility of periphonic reproduction although, in practice, this would require speakers to be nailed to the ceiling and floor.
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In contrast, Ambisonics attempts to recreate for the listener the complete sound field of the original performance. A particular number of speakers is not assumed and the technology can use various numbers and speaker layouts. The Ambisonic Surround Sound system strives for a fusion of the channels/speakers so that they all conspire to locate sounds in their correct positions.
A problem with the quadraphonic systems was that they did not work, and could never have been made to work because they were based on false premisses. (This is explained in detail in the Sommerwerk 1984 reference, part I.) In contrast, Ambisonics does work.
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I have, however, studied the excellent article Introduction to [Dolby] surround sound, written by Bob Niland (rjn@csn.net) and available by anonymous FTP from ftp.csn.org in the file /Laserdisc/ld03. This suggests that:
Dolby Motion Picture matrix encoding (Dolby MP) is an encoding system, derived from the SQ quadraphonic system, designed for motion picture sound-tracks. Four channels are encoded; left, centre, right and a mono surround channel. Dolby Stereo is the result of this encoding. Dolby Surround is a decoding process designed to decode Dolby Stereo in the living room. Dolby Pro-Logic is an active decoding process, also designed for the living room. LucasFilm THX is an enhancement of Dolby Pro-Logic.I have also studied technical publications on Dolby Surround AC-3 which are available from Eileen Tuuri of Dolby Laboratories Inc (ert@dolby.com). These suggest that:
Dolby AC-3 is a digital encoding technique that exploits "audio masking" to achieve high bit-rate reductions. Dolby AC-3 can be used to encode between 1 and 5.1 audio channels. Dolby Stereo Digital film sound format uses Dolby AC-3 to encode 5.1 audio channels onto filmstock. The 5 channels, left, centre, right, right surround and left surround, are all full bandwidth. The .1 channel is a band limited (20 Hz to 120 Hz) bass effects channel. Dolby Surround Digital, also called Dolby Surround AC-3, is the consumer equivalent of Dolby Stereo Digital film sound and is also based on Dolby AC-3 coding of 5.1 channels.The difference between Ambisonic Surround Sound and the Dolby surround systems is that the former is designed for the realistic reproduction of music while the latter, to enhance the enjoyment of motion pictures.
The following description of Dolby MP is due to Rustan Finndin:
Dolby MP is the leading encoding standard in the film industry for storing multi-channel surround sound on two sound-channels. Three front channels - right, centre and left are used for positioning the dialog and other sounds coming from the screen, and a fourth (and sometimes a fifth) surround channel with speakers beside and behind the audience are used for background music and sound effects.Section headings are shown in the Contents list.Most movies are recorded using this standard and thus video tapes and laserdiscs often hold the information.
A home amplifier with a Dolby Surround or Dolby Pro-Logic decoder extracts the different channels of sound from the Stereo signal.
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A key part of Ambisonic decoding is the use of shelf filters. These give different gains at low and high frequencies to satisfy different location mechanisms used by the ear/brain. (The change occurs at around 700 Hz.) Dolby MP is a derivative of the SQ quadraphonic system and, unfortunately, SQ decoding requires that no shelf filters be used. As a consequence, SQ (and therefore Dolby MP) decoders cannot be designed to give full Ambisonic results; there is even a mathematical theorem to this effect.
Integrex Limited manufactured an Ambisonic decoder that could decode almost every quadraphonic system, including SQ. The design of this decoder has been published (see the Gerzon 1977b reference), and the SQ mode used Ambisonic technology only to give more freedom over the number and layout of speakers.
Dolby MP will always suffer from poor sound images for sounds in inter-speaker directions, and non-existent sound imaging to the rear and side of the listener. While not a serious impediment to the enjoyment of motion picture video, these limitations do make Dolby MP encoding unsuitable for music. I am old enough to remember the SQ quadraphonic system; it sucked.
It is also strange that the Dolby MP crowd seem determined to repeat the mistakes made by the quadraphonic crowd, namely a desire for channel separation (which merely calls attention to individual speakers as sources of sound) and a penchant for active decoders (which cause fatigue in listeners).
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Because Dolby AC-3 does not use matrix mixing the five audio channels have complete separation. Unfortunately, this is not sufficient for realistic surround sound reproduction. In conventional stereo, the ability to perceive sounds from inter-speaker directions is poor when the speakers are to the rear of the listener and non-existent when they are to one side. (See the Gerzon 1985 or Fellgett 1981 reference. Better still, try it yourself!) This means that any surround sound system which relies on stereo imaging between adjacent speakers must fail. This is as true for Dolby Surround AC-3 as it was true for the quadraphonic systems.
The solution is to convert the 5.1 channels into W, X and Y signals, by projecting each of the five stereo pairs onto 72 degrees of the horizontal sound stage, and to then use Ambisonic technology to reproduce the soundfield. I am no longer sure whether this is technically feasible.
As with Dolby MP, this is not a serious impediment to the enjoyment of motion picture video. It does, however, make Dolby Surround AC-3 unsuitable for music.
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UHJ is a set of engineering specifications which gives consumer formats, C-Formats, for encoding the W, X, Y and Z direction signals into different numbers of channels. The first task performed by an Ambisonic decoder is to decode the UHJ signals back into W, X, Y and Z. It should be stressed that Ambisonics consists of much more than the UHJ encoding specifications.
The reason the UHJ specifications are needed is because it is impossible to obtain reasonable surround sound from using only two of the four B-Format signals. This is possible with UHJ, and UHJ also gives excellent stereo when listened to without an Ambisonic decoder. The disadvantage of UHJ is that both encoding and decoding require the use of 90 degree phase shifters.
The UHJ set consists of BHJ, SHJ, THJ and PHJ, having 2, 2.5, 3 and 4 channels respectively. The PHJ encoded channels are called Left, Right, T and Q. To produce SHJ or THJ the Q channel is omitted. For BHJ, both T and Q are left out.
In practice, only two-channel UHJ (BHJ) encoded material has ever been released. For this reason UHJ has become a synonym for BHJ, and UHJ is the symbol you will see on LPs and CDs.
Readers interested in seeing the set of encoding/decoding equations should consult the appendices of the Gerzon 1985 reference.
UHJ is more symbolism than acronym. The `U' stands for Universal, and is taken from the UMX quadraphonic system which pioneered the technique of using supplementary channels to enhance directional resolution. The `H' represents the BBC's Matrix H and their work on mono and stereo compatible matrices. The `J' is taken from System 45J, the name of the original two-channel Ambisonic system. (The `J' symbolised the 90 degree phase shifters used in System 45J encoders and decoders.)
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SHJ specifies how W, X and Y can be encoded into 2.5 channels, where the .5 channel is of reduced bandwidth (5 kHz). The original intention was to provide the .5 channel in broadcasting by additional modulation of the 38 kHz sub-carrier. Presumably RDS, Minicall, etc, kills this possibility.
THJ specifies how W, X and Y can be encoded into three channels. This is the "no-compromise" horizontal C-Format.
PHJ specifies how W, X, Y and Z can be encoded into four channels and is the "no-compromise" periphonic C-Format. Periphonic reproduction, however, would require speakers nailed to the ceiling and floor.
BHJ, SHJ, THJ and PHJ are all inter-compatible. That is to say, to go from one member of the set to the next you add or delete additional signals without changing those that remain. A beauty of this is that each member of the UHJ set is mono and stereo compatible. In addition, a BHJ decoder, for example, can decode SHJ, THJ and PHJ material simply by ignoring the extra channel(s).
Version 1.0 of this FAQ suggested encoding a third channel into the `subcodes' of a CD to give THJ or SHJ. It transpires that the bandwidth available in the subcodes is less that 2 kHz using 16 bits, so this ain't gonna fly.
Jeffrey Silberman has pointed out that it is possible to use "buried data" to encode a third channel of reduced bandwidth onto a CD such that an existing CD player is unaware the channel exists. This would allow SHJ encoded CDs to be produced that are completely compatible with conventional stereo CD players. That is to say, a stereo system would produce stereo, a BHJ decoder would produce surround sound, and an SHJ decoder (fed from a CD player with special digital electronics) would produce even better surround sound. All this from the same CD! The technique is too complicated to describe here, and interested readers should consult the Gerzon and Craven 1995 reference.
Peter Knight has pointed out that the CD format specification includes a four channel quad format which would be suitable for PHJ encoded material. The problem, of course, is that existing CD players are not quad CD "aware" and would produce a mishmash if asked to play a quad CD. He also has pointed out that quad CDs have to be spun twice as fast as stereo CDs and have only half the playtime.
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By Unicorn-Kanchana. Telephone anyone? Do they still exist?
Unicorn-KanchanaBy Music from York. Telephone anyone? Do they still exist?
12 Hillgate Place
London W8 7SJ
United Kingdom
Music from YorkBy Libra Realsound. They may only have released UHJ encoded cassettes. Address/telephone anyone?
PO Box 66
Lancaster LA2 6HS
United Kingdom
By IMF Electronics (now TDL Electronics Limited).
By Hyperion Records Limited? I am not sure whether they released records or just sold them. Telephone anyone? Do they still exist?
Hyperion Records LimitedSection headings are shown in the Contents list.
PO Box 25
Eltham
London SE9 1AX
United Kingdom
Nimbus RecordsUHJ encoded CDs have also been released by the following labels. Can anybody provide current addresses?
Wyastone Leys
Monmouth
Gwent NP5 3SR
United Kingdom
Tel: +44-1600-890682
Fax: +44-1600-890779Nimbus Records
PO Box 7746
Charlottesville
VA 22905 7746
USA
Tel: +1-804-985-8555 (also 1-800-944-1341)
Fax: +1-804-985-3953
E-mail: NimbusCD@aol.com
York AmbisonicsThese record labels are also supposed to have released UHJ encoded CDs. Can anyone expand the acronyms or supply addresses?
Unicorn-Kanchana
Collins Classics
EMI
Ondine
Pony Canyon
Finlandia
DKPSection headings are shown in the Contents list.
SNH
AMB
Minim Electronics Limited marketed a range of three decoders (the AD 7, AD 8 and AD 10) and also a printed circuit board module for enthusiasts to incorporate into their own stuff.
IMF Electronics assisted in the development of Ambisonics and manufactured a decoder, the D20B. The company still exists under the name TDL Electronics Limited, but I do not know whether they have any current interest in Ambisonics.
TDL Electronics LimitedUp to 1981, Integrex Limited offered an Ambisonic decoder that could decode SQ, QS and UMX as well as UHJ. Integrex were still listed in the UK Electronic Yellow Pages in December 1994, but the telephone number is not recognised. Does anybody know if they still exist?
Unit 2, Pilot Trading Estate
High Wycombe
Buckinghamshire
United Kingdom
Tel: +44-1494-441191
Fax: +44-1494-461803
Integrex LimitedAbacoid Limited produced a Professional Ambisonic Decoder, the PAD 9211. Does anyone know if they still exist?
Portwood Industrial Estate
Church Gresley
Derbyshire DE11 9PT
United Kingdom
Tel: +44-1284-551551
Abacoid LimitedTroy manufactured an Ambisonic Processor [Decoder], the TA 110P, for use in the car. Does anyone know if they still exist?
110A St Margarets Road
Hanwell
London W7 2HF
United Kingdom
Tel: +44-181-840-2076
TroyNAD developed inexpensive Ambisonic decoders. Does anyone know if they were ever offered for sale?
PO Box 246
London SW18 5JS
United Kingdom
Tel: +44-181-871-4211
NADAnd one for our antipodean readers, Audiotech Industries offered an Ambisonic decoder. Details anyone?
683 High Street
North Finchley
London N12
United Kingdom
Tel: +44-181-446-3199
Audiotech IndustriesSection headings are shown in the Contents list.
PO Box 8756
Auckland
New Zealand
Tel: +64-9-587-386
By Minim Electronics Limited. When I last spoke to them in January 1995, they were about to revamp their offerings.
Minim Electronics LimitedAudio Design manufacture a Reference Ambisonic Decoder. This is intended for professional use, and is priced accordingly. The decoder is distributed by Cepiar Limited.
Lent Rise Road
Burnham
Slough SL1 7NY
United Kingdom
Tel: +44-1628-663724
Cepiar LimitedIn March 1993, Sound & Vision reviewed the Onkyo TX-SV909PRO A/V Receiver which incorporates an Ambisonic decoder (see References).
Debden House
11 St Ethelwold's Close
Ely
Cambridgeshire CB6 3AX
United Kingdom
Tel: +44-1353-666818
Fax: +44-1353-666819
Meridian's 565 Digital Surround Processor incorporates an Ambisonic decoder.
Meridian Audio LimitedMaplin Electronics plc offer an Ambisonic decoder kit. This was apparently described in a 1994 issue of Electronics - The Maplin Magazine. Can somebody track this down please?
14 Clifton Road
Huntingdon
Cambridgeshire PE18 7EJ
United Kingdom
Tel: +44-1480-434334
Fax: +44-1480-459934
E-mail: jrs@meridian.win-uk.netMeridian America Inc
3800 Camp Creek Pkwy.
Bldg. 2400, Suite 112
Atlanta, GA 30331
USA
Tel: +1-404-344-7111
Fax: +1-404-346-7111
Maplin Electronics plcIf anyone knows of other commercial Ambisonic decoders, will they please let me know. I am particularly interested to hear about stand alone Ambisonic decoders that are not bundled together with home theatre decoders.
PO Box 3
Rayleigh
Essex SS6 8LR
United Kingdom
Tel: +44-1702-552911/554155/554161
Fax: +44-1702-553935
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Ambisonics has suffered from some or all of the following:
Ambisonic technology attempts to record and reproduce the complete sound field experienced during the original performance. It can reproduce this sound field using various numbers of speakers and speaker layouts. These include 4 speakers driven by 3 power-amps, 4 speakers driven by 4 power-amps and 6 speakers driven by 4 power-amps. For reproduction using only 4 speakers, a layout control compensates for different aspect ratios. Commercial Ambisonic decoders in addition provide distance compensation for when the inter-speaker distances are small.
The speaker feeds are each a simple weighted sum of the W, X, Y and Z signals after they have passed through the variable gain shelf filters. Readers interested in seeing the equation should consult the appendices of the Gerzon 1985 reference.
Ambisonic technology does place restrictions on the choice and placement of speakers. Specifically:
Auditorium decoders which can drive between 8 and 128 speakers are available (from Cepiar Limited). For domestic use the limiting factors are the cost of the necessary speakers and power-amps, and the practical problem of squeezing them into your living room.
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If you want to know what Ambisonic Surround Sound sounds like, but cannot arrange to hear it for yourself, the results of an extensive domestic listening test are described in the Attewell 1982 reference.
Further information on Ambisonics is available from the Ambisonic and `B'-format User Group (ABUG) run by Donald G Maclennan. ABUG is an informal group of about 180 devotees. Donald keeps a database of members' interests and he is happy to put people whose interests overlap in touch with each other. The membership includes people from the pro sound industry, journalism, academia and individuals like me who are just interested.
Donald G MaclennanThis FAQ is aimed at the domestic listener. Two areas of Ambisonic technology which have been omitted deliberately are recording and studio processing. These topics deserve an FAQ of their own, but somebody else will have to create it; I have done enough. Readers interested in these topics might like to join a mailserv list devoted to surround sound recording practice and research. To join, send a request to Jimmy T Rusby (jrusby@darkwing.uoregon.edu). The list is called 3daudio, but the title is a misnomer as the list is intended to support all types of surround sound recording.
51 Hillside Avenue
Plymouth
Devon PL4 6PS
United Kingdom
Tel: +44-1752-663229
Fax: by appointment
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Prof P B Fellgett, 1981, "Ambisonics", in New Electronics, May issue, pages 38, 44, 46 and 48.
An extensive reference list on Ambisonics, compiled by Michael A Gerzon and others, is available on the WWW (see Section 1). I am also happy to e-mail copies to people.
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Many thanks to my beta testers for their helpful comments:
Peter Knight (P.J.Knight@bradford.ac.uk)Many thanks to the following people for responding to previous versions of this FAQ:
Jun Guo (jguo@u.washington.edu)
john 015 (CC015012@brownvm.brown.edu)
Jeffrey Silberman (ambisonx@well.sf.ca.us)
Jimmy T Rusby (jrusby@darkwing.uoregon.edu)
Dave Malham (dgm2@unix.york.ac.uk)
Brian Lenharth (brianl@lsid.hp.com)
David S Monett of Monett Media Inc
Peter G Craven (peter@occl-ox.demon.co.uk)
Jeffrey Silberman maintains one of the very few
B-Format studios in the USA, Aural Adventures®. He would love to hear
about any others.
[ Please note that Jeffrey's studio has been renamed. "Aural
Adventures" is the registered trademark of Bill Barnes and is unconnected
with Jeffrey Silberman. ]
Aural Adventures®David S Monett runs Monett Media Ambisonics. This is a wanttodo entity that will record anything worth recording (in their opinion) any place for free, or near free, provided the recordings are never subject to commercial release or broadcast use.
21 Millay Place
Mill Valley
CA 94941
USA
Tel: +1-415-380-8273
Fax: +1-415-380-9414
E-mail: ambisonx@well.sf.ca.us
Monett Media AmbisonicsSection headings are shown in the Contents list.
400 Colony Square
Suite 200
Atlanta
Georgia 30361
USA
Tel: +1-404-870-9009 (ask for Ambisonics)
Fax: +1-404-870-9005
As it [UHJ encoding] is the first system design based on a complete mathematical analysis of both system theory and human phychoacoustics it is possible to say with some confidence that no system appreciably different from it can exceed its performance in optimal surround-sound decoding, so that no further system change is ever likely to be needed for horizontal encoding.Nothing has happened in the last 18 years to suggest this statement is wrong.
[From the Gerzon 1977a reference, pages 38 and 39]
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