Ambisonic File Formats
From Ambisonia
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Ambisonic files can come in a number of different formats.
The main one is called B-Format, the other
formats being derived from this. UHJ format is mono- and
stereo-compatible. G-Format is a set of speaker feeds,
so can be enjoyed in surround sound without the need for a
decoder in the living room.
Contents |
[edit] 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 is horizontal-only or full-sphere, and on the order. These B-Format channels are transmission channels, not speaker feeds. Listening to B-Format requires a decoder in your living room.
[edit] File formats for B-Format
The following file formats support B-Format:
- ".amb" (based on WAVE-EX)
- OggPCM
- Apple Core Audio Format (CAF)
[edit] B-Format releases
There are currently over 100 pieces available in the ".amb" format for free download. Most of these are first-order full-sphere soundfields.
[edit] 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 was developed. Depending on the number of channels available, the UHJ system can carry more or less information, but at all times it is fully mono- and stereo-compatible. Up to four channels (Left, Right, T, Q) may be used. (The T-channel can also be band-limited but, as this "2½-channel UHJ" was only ever used for FM radio transmission, it will not be discussed further.)
To listen to UHJ files in surround requires a decoder in your living room. Also, UHJ is restricted to first-order soundfields, either horizontal (two- and three-channel UHJ) or full-sphere (four-channel UHJ).
[edit] File formats for UHJ format
The following file formats support UHJ format:
[edit] UHJ releases
Several hundred two-channel UHJ LPs and CDs have been released. Three- and four-channel UHJ recordings have never been released commercially.
[edit] 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 or five speakers arranged in a regular pentagon. Other speaker arrangements are also possible.
G-Format 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.
[edit] File formats 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. The ".amg" file format (based on WAVE-EX) for downloadable G-Format files, which will allow the B-Format channels to be recovered, has been proposed. OggPCM also supports preferential unmatrixing that can be used to both recover B-Format from G-Format, and to allow G-Format to be played back in legacy systems if Ambisonics capability is not present.
[edit] G-Format releases
A few G-Format recordings on DVD-A for four speakers in a square have been released commercially by Nimbus Records.
[edit] Other file formats
Other file formats which could be used for Ambisonics are discussed in the article Other File Formats.
