Ambisonics

Ambisonics (also Ambisonic) is a process for recording and reproducing a sound field. This audio technology was developed in Great Britain in the 1960s and 1970s and was largely pioneered by Michael A. Gerzon and Peter Fellgett . In contrast to the channel-oriented transmission method, there is no fixed number of loudspeakers for playback. The respective signals are calculated according to mathematical specifications from the transmitted values for sound pressure and sound velocity for each individual loudspeaker position.

Physical principle

In the basic version, known as First Order Ambisonics B format , the information is transmitted in the W, X, Y and Z channels. W contains the pure sound pressure component, recorded with an omnidirectional (omnidirectional) microphone. The signals X, Y and Z are the pressure gradient components in the associated spatial axes.

${\ displaystyle W = 1}$

${\ displaystyle X = {\ sqrt {2}} \ cos \ phi}$

${\ displaystyle Y = {\ sqrt {2}} \ sin \ phi}$

They are recorded with microphones whose figure-of-eight characteristic is aligned in the corresponding axis. The aim of the method is to reconstruct the recorded sound pressure with the associated vector of the sound velocity from these signals at the listening position. For a rectangular loudspeaker arrangement in the horizontal plane, the individual loudspeaker signals are derived from the angles to the central axis:

{\ displaystyle P_ {n} = W + X / ({\ sqrt {2}} \ cos \ theta _ {n}) + Y / ({\ sqrt {2}} \ sin \ theta _ {n}) \ ,}

The virtual positions of the sound source can not only be recorded with the soundfield microphone, an ambisonics coder can display the mono signal at freely selectable positions in the three-dimensional sound field.

Advantages of the procedure

One advantage of the method is that the elevation plane can also be decoded according to mathematical relationships for any loudspeaker position. As a result, a three-dimensional sound field is generated with just four transmission channels. In this sound field, no spatial axis is preferred; all loudspeakers contribute their share. Conventional surround sound methods are still two-dimensional even with six transmission channels.

As the number of decoded loudspeaker channels increases, the sound field becomes more stable. It can then even be perceived by listeners outside the loudspeaker arrangement. The loudspeakers do not have to be positioned at fixed positions in a regular rectangle. This allows better adaptation to the practical reproduction conditions.

The stability of the spatial image can be further increased with additional transmission channels. The process always remains downward compatible. There is simply no need to decode individual channels. The UHJ hierarchy was developed to ensure compatibility for playback on mono and stereo systems. This includes a format with four channels, which maps the entire B format, an exclusively horizontal 3-channel format, a 2½-channel format in which the third channel only covers half the bandwidth and a 2-channel format. Even with two transmission channels it is possible to establish a horizontal surround sound reproduction, whereby the reproduction remains stereo-compatible without a decoder. Today, UHJ is usually equated with the 2-channel format (actually BHJ).

Recordings in Ambisonics-G format can be played on conventional surround sound systems. The decoder is switched to the transmission path on the recording side. A decoding is decoded to the standard positions of conventional surround sound speakers in living rooms. The advantages of the process are only used to a limited extent. Increasingly, this method is also to be used in the recording area in conventional productions for improved spatial imaging.

Remaining problems

The breakthrough of the process has not yet succeeded; Ambisonics is largely unknown today.

In larger playback rooms, four loudspeakers in the azimuth plane are not sufficient for stable playback. The listener must have a relatively high proportion of direct sound so that the advantages of the method can be recognized.

Research and market maturity

Although most of the patents on the process have now expired, the further development of the approach is still the subject of research.
Above all, the High Order Ambisonics (HOA) processes are further developed to greater perfection. These decoders can now be implemented on a software basis. As with the related method of wave field synthesis, WFS, Ambisonics may have prospects of replacing the conventional channel-based procedures. To date, however, the system has not been commercially successful.

Individual evidence

↑ Richard Elen: Ambisonics: The Surround Alternative (PDF; 653 kB)

Web links

Commons : Ambisonics - collection of images, videos and audio files

Ambisonics IEM Graz (PDF file; 169 kB)
Ambisonics website

[1] Richard Elen: Ambisonics: The Surround Alternative (PDF; 653 kB)