Seminar 6th March 2012 4 p.m. Room 13/3019
Ambisonics Old and New
Dr Peter Craven
Freelance
- Web page
- http://www.southampton.ac.uk/engineering/research/centres/isvr.page
- Categories
- Acoustics
- Submitter
- Marcus Erridge
Ambisonics is the surround-sound system that was developed at the University of Reading in the 1970s as a response to various ill-conceived ‘Quadraphonic sound systems' that were being promoted by large companies. The theory was developed by Michael Gerzon, who applied the mathematics of axiomatic quantum theory to audio for the first time, to general astonishment.
Ambisonics differs from currently popular surround-sound systems such as ‘5.1’ in that it does not specify a specific loudspeaker layout or even how many speakers will be used. Instead, the recording contains information about the directions of original sounds; questions such as how many speakers being a completely separate playback decision and possibly different for each listener.
Ambisonic encoding was based on spherical harmonics, typically giving ‘rotational invariance’ (equal directional resolution all round the listener). This was revolutionary compared to ‘quadraphonic’ matrix systems that assumed a forward-facing listener. However, the media of the time imposed severe limitations on the number of channels so only first-order spherical harmonics could be conveyed, and Ambisonics thus gained a reputation in some quarters for poor image stability and definition.
Today the focus of academic interest is HOA or ‘High Order Ambisonics’ and the earlier systems have been dubbed POA: ‘plain old Ambisonics’ or even ‘poor old Ambisonics’. HOA generally uses spherical harmonics of third order or higher, with the aim of reconstructing an original soundfield over a finite listening area. Given practical limitations on the number of channels, some compromise is necessary, and while POA would have compromised using psychoacoustic criteria, HOA usually emphasises only the physical accuracy of the reconstruction. Were Gerzon still alive, he may well have argued that ‘High Order Ambisonics’ is not really Ambisonics at all.