Last-modified: 21 January 1998
Copyright: © 1994, 1995, 1996, 1998 by Martin J Leese
Distribution is unlimited
A major advantage of Ambisonic Surround Sound is that recording and studio processing are disengaged from reproduction. The former produce and operate on the W, X, Y and Z channels, but these can be reproduced through any number of speakers. The more speakers which are used the better, as this gives a larger listening area and a more stable sound localisation. Using more speakers also improves the illusion that the speakers have vanished; that is to say, the listeners hear a single seamless sound field. For horizontal surround sound a minimum of four speakers is required.
Ambisonic technology places restrictions on the choice and placement of speakers. Specifically:
Some diagrams would be useful here.
The disadvantage of using only four speakers is that sounds with spiky waveforms (audience applause, harpsichords, oboes) tend to be drawn away from their correct location and towards a speaker. Using five or six speakers gives considerably more robust side and rear imaging.
For periphonic (full-sphere) surround sound a minimum of six or eight speakers is required, driven from at least four or five power-amps, respectively. Readers interested in seeing the possible speaker layouts should consult the Gerzon 1980 reference.
Auditorium decoders that can drive between 8 and 128 speakers are available (from Cepiar Limited). For domestic use the limiting factors are the cost of the necessary speakers and power-amps, and the practical problem of squeezing them into your living room. (The use of Ambisonics in auditoria is described in the Malham 1992 reference.)
The speaker feeds are each a simple weighted sum of the W, X, Y and Z signals after they have passed through the shelf filters. Readers interested in seeing the equation should consult the appendices of the Gerzon 1985 reference.
With the "old-type" Ambisonic decoders described above, the apparent position of the centre-front sound image varies as the listener moves from one side of the living room to the other. This is not a problem when reproducing music, but when used with TV the on-screen sounds can become misaligned with the on-screen pictures. "New-type" Ambisonic decoders, described in the Gerzon and Barton 1992 reference, produce a stable centre-front sound image that solves this problem. These decoders can be used with irregular speaker layouts, either five speakers with one at the centre-front or six speakers with two at the centre-front. Alternatively, it is possible to use an additional channel to stabilise the centre-front sound image, as described in the Gerzon 1992b reference. These newer types of Ambisonic decoders are not yet commercially available.