→Limitations of the ".amb" specification: Clarify
'''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.
if the resulting soundfield is to correctly "gel". This may be a problem
when B-Format channels are compressed using lossy compression.
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:
#It is limited to 4 GByte files (2 GBytes if somebody screwed up).
#It is limited to third-order soundfields and below. While third-order looks like a lot (16 channels), there already
exists a prototype mic that can record up to fourth-order (25 channels).
#No compression (particularly lossless).
| 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 ==
In Ambisonics, all speakers cooperate to localise sounds in any particular direction; there are no "surround speakers" as such. Because of this, best results when playing G-Format recordings (and Ambisonics in general) are obtained when the speakers are matched. 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.
An easy way to cope with this is adopted on G-Format recordings commercially
released on DVD-A by [http://www.wyastone.co.uk/nrl/dvd.html Nimbus Records].
They use 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. Obviously if a B-Format version of the file exists then it can be fed to the decoder directly without the need for G-Format.
File formats for G-Format include all multi-channel formats that contain
speaker feeds. However, these will not contain information to allow the
B-Format channels to be recovered. A ".amg" file format (based on WAVE-EX) for downloadable G-Format files, which will allow the B-Format channels to be recovered, is
=== 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. ambisonia.com/ 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. ambisonia.com/ 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. ambisonia.com/ Members/ricardo Richard Lee's page on Ambisonics] , a few articles and samples.