Ambisonics

Ambisonics is a surround sound system first developed in the 1970s. Its main difference from other surround techniques is that it separates transmission channels from speaker feeds, the speaker feeds being derived using a decoder situated in the living room. Decoders can be implemented in either hardware or software. Typically more speakers are used than transmission channels, and the more speakers used then the more stable the resulting soundfield. Speakers can be arranged in a number of configurations, regular polygons being the most popular.

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.

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. Some numbers of channels are tabulated below.

Channel correlation

The correlation between B-Format channels depends on the content. Four-channel B-Format (which Ogg supports) consists of an omni-directional component, called W, and three figure-of-eight components pointing forward, left and up, called X, Y, Z. (Three-channel, horizontal-only B-Format simply omits the Z channel.) This means that anything in X also appears in W. Same for Y and Z. Also, if content comes from Front-Left then it appears equally in X and Y. Same for content from Front-Right, Back-Left, Back-Right; only the relative polarity changes. So there can be a lot of correlation between B-Format channels, but it is content dependent.

There is a file specification in use for downloadable B-Format files called the ".amb" specification.

Limitations of the ".amb" specification

The ".amb" specification for downloadable B-Format files is based on the WAVE-EX format. There are currently over 75 pieces available in this format for free download. Most of these are first-order full-sphere soundfields. Some of the limitations of the specification are:

1. It is limited to 4 GByte files (2 GBytes if somebody screwed up).
2. 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.
3. No compression (particularly lossless).

The reason that the ".amb" file specification is limited to third-order and below is because it uses the number of channels to uniquely define the soundfield order. Unfortunately this simple and elegant scheme does not work above third-order as ambiguities creep in. (One ambiguity is illustrated in the table below.)

A more general file format will have to use something else, such as Malham notation, or storing both the horizontal-order and height-order. There is a one-to-one correspondence between Malham notation and the pair of orders, and either can generate the number of channels.

Malham notation

Malham notation specifies the order of a B-Format soundfield using a string of characters, each character being either f (for full-sphere) or h (for horizontal). The first character in the string specifies the type of the first-order components, the second character the type of the second-order components, etc.

Horizontal order	Height order	Soundfield_type	Malham notation	Number of_channels	Channels
1	0	horizontal	h	3	WXY
1	1	full-sphere	f	4	WXYZ
2	0	horizontal	hh	5	WXYRS
2	1	mixed-order	fh	6	WXYZRS
2	2	full-sphere	ff	9	WXYZRSTUV
3	0	horizontal	hhh	7	WXYRSPQ
3	1	mixed-order	fhh	8	WXYZRSPQ
3	2	mixed-order	ffh	11	WXYZRSTUVPQ
3	3	full-sphere	fff	16	WXYZRSTUVKLMNOPQ
4	0	horizontal	hhhh	9	extra channels unlabled

Default channel conversions from B-Format

Converting a B-Format file to a mono file is straightforward. Use Mono = W*sqrt(2).

Converting a B-Format file to a stereo file is more difficult. The "proper" way to do this is to convert the W,X,Y channels to two-channel UHJ. Unfortunately this requires the use of wide-band 90-degree phase shifters. In the digital domain these are usually implemented as convolution filters.

Assuming 90-degree phase shifters are unavailable then the problem is one of choice. Starting from B-Format, it is possible to synthesize any mic 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

Mid = (W*sqrt(2)) + X  /*This is a cardioid response pointing forward*/
Left = Mid + Y
Right = Mid - Y

Blumlein Crossed Pairs

Left = (X + Y)/sqrt(2)
Right = (X - Y)/sqrt(2)

Which conversion to stereo is better depends on the material and how it was 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 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).

Converting B-Format channels to UHJ channels, and vice versa, requires the use of wide-band 90-degree phase shifters. In the digital domain these are usually implemented as convolution filters. Conversion between four-channel B-Format (W, X, Y, Z) and four-channel UHJ (Left, Right, T, Q) can be accomplished without loss of information. The same with three-channel to three-channel (W, X, Y) <=> (Left, Right, T). It is possible to recover three-channel B-Format (W, X, Y) from two-channel UHJ (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.

Several hundred two-channel UHJ LPs and CDs have been released. Three- and four-channel UHJ recordings have never been commercially released.

There is a file specification for downloadable two-channel UHJ files called the ".uhj" specification, but it is not currently in use.

Limitations of the ".uhj" specification

The ".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 unrecognized chunks are always skipped, use of this chunk maintains stereo compatibility. Some of the limitations of the specification are:

1. It is limited to 4 GByte files (2 GBytes if somebody screwed up).
2. It is limited to two-channel UHJ files. Three- and four-channel UHJ are not accommodated.
3. No compression.

The ".uhj" spcecification is only defined for two-channel UHJ to maintain stereo compatibility. While it would be possible to add the UHJ chunk to three- and four-channel WAVE or WAVE-EX files, the recommendations from Microsoft for playing such files is that the audio device should render the extra channels to output ports not in use. This can happen even when the extra channels are masked off. Because of this, three- and four-channel WAVE or WAVE-EX files can not be made stereo compatible.

In the Ogg world, it should be possible to use default channel conversions to ensure that three- and four-channel UHJ files remain stereo compatible.

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

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. However, other speaker arrangements are possible.

In addition, the G-Format file contains conversion coefficients which can be used 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.

Details will follow later. A file format for downloadable G-Format files is being developed. As soon as it has been critiqued by the Ambisonic community, it will be described here (assuming it survives).

Resources on Ambisonics

• There is a set of Wikipedia articles on Ambisonics.
• Of particular relevance is the ".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" 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).