→UHJ format: Fixed broken link
'''Ambisonics''' is a surround sound system first developed in the 1970s.
Its main difference from other surround techniques is that it separates
== Ambisonics and 5.1 ==
Ambisonics and conventional 5.1 surround sound are very different. 5.1 is a
set speaker feeds, the signal only being fully defined for sounds coming
rear of the listener and not-at-all when they are to one side. You can
demonstrate this for yourself by performing
exper.html a very simple experiment].
Pair-wise mixing did not work in the quadraphonic era and it will not work
now. Such an absolute statement can be made because the way that humans
Ambisonics, the positions of the speakers are unknown
''and are of no interest''. Further, when Ambisonics is decoded to speaker
, all of the speakers cooperate to localise a sound in its correct position. The speakers all contribute to the creation of a single coherent soundfield.
=== Ambisonics to 5.1 ===
== B-Format ==
B-Format is a single coherent soundfield composed of a set of related
channels. The number of channels used depends on whether the soundfiled
=== Channel correlation ===
The correlation between B-Format channels depends on the content.
(which [[OggPCM]] supports) consists of an
omni-directional component, called W, and three figure-of-eight
components pointing forward, left and up, called X, Y, Z.
One problem with B-Format is that it is big on phase. The phase relationships between the different B-Format channels are important if the resulting soundfield is to correctly "gel". This may be a problem when B-Format channels are compressed using lossy compression.
==== Coupling recommendations ==== <small>Shamefully mangled by [[User:Gmaxwell|Gmaxwell]] from [http://lists.xiph.org/pipermail/vorbis-dev/2007-February/018655.html Richard Lee's Feb 4th vorbis-dev post]</small> <blockquote>The Ambisonic B-format WXYZ channels should be should be lossless coupled together over most of the frequency range. This will give the most gain as W "usually" has everything that XYZ has. A perfect B-format signal has scaled versions of XYZ in W.<br/> We allow "4 phase coupling" at higher frequencies. A perfect B-format set of WXYZ should only have real relation to each other. <br/> This falls down at LF and close sources and sometimes when you are recording organ pedal notes. But lossless coupling at LF is cheap. This also falls down for diffuse field. It is possible that the pseudo random differences in response of a real Soundfield in different directions above 10khz may preserve enough of the "random" nature of reverb to be acceptable without needing full random phase.<br/> '''The deviations from perfection can take 2 forms''': #Minimum Phase deviations of XYZ from W. These start happening above about 6kHz with present Soundfields. #Random Phase deviations of XYZ from W. These occur at High Frequencies when a real Soundfield Mike no longer has flat frequency response in all directions. Above 10kHz with present Soundfields. B-format from Ambisonically panned material (as opposed to natural Soundfield recordings) will be perfectly in phase so this recommendation will not result in loss. This needs investigating. Presently no Ambisonic effects units use "phase" at High Frequencies but this should not be ruled out. *Allowing "4 phase" above 18kHz, like default Dolby Digital at 448kb/s, is likely to be undetectable on present Soundfield recordings. *Allowing "4 phase" down to 10kHz might force the random phase deviations of a real Soundfield mike to be real and MAY result in better results. This needs investigating. *"Point phase" coupling should be avoided </blockquote>
There is a file specification in use for downloadable B-Format files
www. ambisonia.com/ Members/ mleese/ file- format-for-b-format/ ".amb" specification].
=== Limitations of the ".amb" specification ===
www. ambisonia.com/ Members/ mleese/ file- format-for-b-format/ ".amb" specification]
for downloadable B-Format files is based on the WAVE-EX format. There are
85 pieces available in this format
[http://www.ambisonia.com for free download]. Most of these are
first-order full-sphere soundfields. (The same website also has details of
Members/ etienne/ambisonic-software/ ad hoc software decoders].)
Some of the limitations of the specification are:
| 1|| 1||full-sphere||'''f'''|| 4||WXYZ
| 2|| 0||horizontal||'''hh'''|| 5||
| 2|| 1||mixed-order||'''fh'''|| 6||
| 2|| 2||full-sphere||'''ff'''|| 9||WXYZRSTUV
| 3|| 0||horizontal||'''hhh'''|| 7||
| 3|| 1||mixed-order||'''fhh'''|| 8||
| 3|| 2||mixed-order||'''ffh'''|| 11||WXYZRSTUVPQ
response pointing in ''any'' direction. Hence, it is possible to synthesize
''all'' coincident stereo mic techniques. Two popular stereo techniques are
''Blumlein Mid-Side'' and ''
Blumlien Crossed Pairs''.
==== Blumlein Mid-Side ====
==== Blumlein Crossed
Left = (X + Y)/sqrt(2) /* (Left, Right) are just the (Y, X) */
recorded. A good suggestion is to not specify a ''particular'' default
channel conversion; instead, simply specify that there must be one. If one
has to be specified then Blumlein Crossed
Pairs is the simpler.
== UHJ format ==
B-Format is the main format for Ambisonic files. However, B-Format is
not mono- or stereo-compatible. This is why the UHJ hierarchical system
(Left, Right), but not without loss. It is also important for the Ambisonic
decoder to be aware that the B-Format channels were recovered from
cox. net/ surround/ uhjdisc/ ambindex. htm two-channel UHJ LPs and CDs]
have been released. Three- and four-channel UHJ recordings have never been
where j is a +90 degree phase shift
For three- and four-channel UHJ:
There is a file specification for downloadable two-channel UHJ files
www. ambisonicbootlegs. net/ Members/ mleese/file-format-for-uhj/ ".uhj" specification], but it is not currently in use.
=== Limitations of the ".uhj" specification ===
www. ambisonicbootlegs. net/ Members/ mleese/file-format-for-uhj/ ".uhj" specification]
for downloadable two-channel UHJ files is based on the WAVE or WAVE-EX
format. A UHJ chunk is added to the file to indicate it is UHJ. As
=== Default channel conversions from UHJ ===
Converting a UHJ file to a mono file is straightforward. Use Mono =
(Left + Right)/2.
Converting a UHJ file to a stereo file is even easier. Use Left = Left, Right = Right, and discard T and Q if present.
== G-Format ==
problem with playing G-Format recordings (and Ambisonics in general) is that the speakers used need to be phase matched. This is because, in Ambisonics, all speakers cooperate to localise sounds in any particular direction; there are no "surround speakers" as such. The easiest way to accomplish this is to use identical speakers. Unfortunately, many home theatre systems include a centre-front speaker which is different from the other speakers. It is noticeable that the G-Format recordings commercially released on DVD-A by [http://www.wyastone.co.uk/nrl/dvd.html Nimbus Records] are for four speakers in a square, the centre-front speaker being unused.
files can also contain conversion coefficients to recover the original B-Format channels. The recovered B-Format channels can then be
fed to a decoder in the listener's living room, and so accommodate a
speaker arrangement different from the one used when the G-Format file
was produced. Each B-Format channel is recovered using a weighted
combination of the speaker feeds in the G-Format file.
=== Default channel conversions from G-Format ===
== Resources on Ambisonics ==
*There is a set of [http://en.wikipedia.org/wiki/Ambisonics Wikipedia articles on Ambisonics].
*Of particular relevance is the [http://
www. ambisonicbootlegs. net/ Members/ mleese/ file- format-for-b-format/ ".amb" specification] in use for downloadable B-Format files. However the ".amb" spec has some limitations which it would be useful to overcome.*There is also the [http:// www. ambisonicbootlegs. net/ Members/ mleese/file-format-for-uhj/ ".uhj" specification] for downloadable two-channel UHJ files, but it is not currently in use. The ".uhj" spec also has some limitations which it would be useful to overcome.
*[http://members.tripod.com/martin_leese/Ambisonic/ This website] has many pages on Ambisonics (including at the bottom links to other Ambisonic websites).
*[http://www.ambisonic.net/ Ambisonic.Net website] includes a detailed series of descriptive and practical articles on current and past Ambisonic techniques with links to tools, other sites and additional material.
www. ambisonicbootlegs.net/ Members/ricardo Richard Lee's page on Ambisonics] , a few articles and samples.