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1,027 bytes removed, 18:13, 18 June 2013
Limitations of the ".amb" specification: Clarify
This page is part of the XiphWiki, and is aimed at people developing file
formats and associated software for Ambisonics. For an general introduction
to Ambisonics, please go to the
[[Wikipedia:Ambisonics|Wikipedia page on Ambisonics]].
'''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
[ 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
feeds, all of the speakers cooperate to localise a sound in its correct positionso, for example, when the speakers on the left push those on the right pull. 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 ===
Compression techniques typically make use of channel correlation to
remove redundancy from the audio data, and so improve the compression
The correlation between B-Format channels depends on the content.
Four-channel B-Format (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.
==== Coupling recommendations ====
<small>Shamefully mangled by [[User:Gmaxwell|Gmaxwell]] from [ 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
There is a file specification in use for downloadable B-Format files
called the
[ Format_file_format.html ".amb" specification].
=== Limitations of the ".amb" specification ===
The [ Format_file_format.html ".amb" specification]
for downloadable B-Format files is based on the WAVE-EX format. There are
currently over 85 200 pieces available in this format
[ for free download]. Most of these are
first-order full-sphere soundfields. (The same website also has details of
[ Playback_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 are already exists a prototype mic research rigs that can record up to reproduce fourth-order (25 channels).
#No compression (particularly lossless).
| 1|| 1||full-sphere||'''f'''|| 4||WXYZ
| 2|| 0||horizontal||'''hh'''|| 5||WXYRSWXYUV
| 2|| 1||mixed-order||'''fh'''|| 6||WXYZRSWXYZUV
| 2|| 2||full-sphere||'''ff'''|| 9||WXYZRSTUV
| 3|| 0||horizontal||'''hhh'''|| 7||WXYRSPQWXYUVPQ
| 3|| 1||mixed-order||'''fhh'''|| 8||WXYZRSPQWXYZUVPQ
| 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 Blumlein Crossed PairsPair''.
==== Blumlein Mid-Side ====
==== Blumlein Crossed Pairs Pair ====
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 Pair 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
two-channel UHJ(because of the need to apply different shelf filters).
Several hundred
[http://members.coxtripod.netcom/surroundmartin_leese/uhjdiscAmbisonic/ambindexuhjhtm.htm txt two-channel UHJ LPs and CDs]
have been released. Three- and four-channel UHJ recordings have never been
commercially released.
where j is a +90 degree phase shift
Note that two-channel UHJ requires the player to use different shelf filters than for B-Format (or for three- and four-channel UHJ).
For three- and four-channel UHJ:
There is a file specification for downloadable two-channel UHJ files
called the
[http://wwwmembers.ambisonicbootlegstripod.netcom/Membersmartin_leese/mleese/file-format-for-uhjAmbisonic/ UHJ_file_format.html ".uhj" specification], but it is not currently in use.
=== Limitations of the ".uhj" specification ===
The [http://wwwmembers.ambisonicbootlegstripod.netcom/Membersmartin_leese/mleese/file-format-for-uhjAmbisonic/ UHJ_file_format.html ".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)/sqrt(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 ==
A G-Format file contains an Ambisonic soundfield pre-decoded to a set of
speaker feeds. This allows listeners who do not own an Ambisonic decoder
to enjoy Ambisonics. When producing the G-Format file, the sound engineer
creates a set of speaker feeds for a particular number and arrangement of
speakers. This is typically four speakers arranged in a square, although
a centre-front speaker fed with X at a low level is often useful. Other
speaker arrangements are also possible,
A G-Format file is any common multi-channel surround file containing an Ambisonic soundfield pre-decoded to its speaker feeds. This allows listeners who do not own an Ambisonic decoder to enjoy Ambisonics. The sound engineer creates a set of speaker feeds for a particular number and arrangement of speakers. This is typically four speakers arranged in a square. Other speaker arrangements are also possible 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 [ Nimbus Records].
They use four speakers in a square, the centre-front speaker being unused.
If a mismatched centre-front speaker is used, it should be fed at a very
low level. Centre-front = 0.1*X has been used successfully.
=== Recovering B-Format from G-Format files can also contain conversion coefficients ===It is sometimes possible to recover the original B-Format channelsfrom the G-Format speaker feeds. 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. The conversion coefficients required for the B-Format recovery depend on the particular speaker arrangment chosen by the sound engineer. (Obviously , if a B-Format version of the file also 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 automatically recovered. A [ ".amg" file format ] (based on WAVE-EX) for downloadable G-Format files, which will allow the B-Format channels to be automatically recovered, has been proposed. Such file formats have the advantage of storing the conversion coefficients at the time the G-Format file is created. This is the only time the required information is under developmentreadily available.
=== Default channel conversions from G-Format ===
== Resources on Ambisonics ==
*There is a set of [ Wikipedia articles on Ambisonics].
*Of particular relevance is the [ Format_file_format.html ".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 [ UHJ_file_format.html ".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.
*[ This website] has many pages on Ambisonics (including at the bottom links to other Ambisonic websites).
*[ 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.
*[http://wwwambisonic.ambisonia.cominfo/Membersinfo/ricardo .html Richard Lee's page on Ambisonics], a few contains articles on shelf filters and samplesthe design of Ambisonic decoders[[Category:Developers stuff]]

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