ambisonics

Ambisonics is a technique for encoding sound information on a number of channels to reproduce a "3 dimensional" sound field. In the basic version, sound information is encoded into 4 channels: W, X, Y and Z. This is called the Ambisonics B-format. The W channel is the general loudness of the signal. The X, Y and Z channels are the directional components in three dimensions. These directional components are based on a spherical harmonic decomposition of the soundfield. The loudspeaker signals are derived by using a linear combination of these four channels and is dependent on the actual position of the speaker in relation to the center of an imaginary sphere going through all available speakers. For the simplest (two dimensional) case (no height information) and placing the loudspeakers on a circle, we can derive the loudspeaker signals from the using W, X and Y channels:

P_n = \frac{1}{N} (\sqrt2 W + k X \cos\theta_n + k Y \sin\theta_n)

where

N

is the number of speakers,

n

is the speaker under consideration,

k

is a constant which can be between 1 and 2 and

\theta_n

is the angle at which the speaker is located on the circle. The larger values of

k

make for better performance at the centre of the array, at the expense of the size of the sweet spot. More generally, the full three dimensional case is given by:

P_n = \frac{1}{N} (\sqrt2 W + k X \cos\theta_n \cos\phi_n + k Y \sin\theta_n \cos\phi_n + k Z \sin\phi_n)

where

\phi_n

is the angle above or below the horizontal plane. Readers should note that the coordinate system used in Ambisonics follows the right hand rule convention with positive X pointing forwards, positive Y pointing to the left and positive Z pointing upwards. Horizontal angles run anticlockwise from due front and vertical angles are positive above the horizontal, negative below. Proponents of this technique assert that is a superior form of surround sound for a number of reasons:

it allows better localization of sound outside of the sweet spot
it only requires 4 speakers to generate a horizontal sound field, whereas typical home cinema surround systems use 5 main speakers and 1 special speaker, the subwoofer which reproduces the LFE (Low Frequency Effects) channel.
it only requires 6 speakers to generate a full 3D sound field.
only 4 channels of information are required for transmission and storage, though replay can be by more than 4 loudspeakers as it is possible to derive a signal for each speaker position using appropriate circuitry or software.
the loudspeakers do not have to be positioned in a rigid setting.

Many Ambisonic recordings have been made using a special microphone - the soundfield microphone. This microphone has also become popular with recording engineers, since it can be reconfigured electronically or via software to provide different 3 D polar responses either during or after recording. The Ogg Vorbis project has shown interest in implementing ambisonics as a means for including surround sound in their project. Research is ongoing, particularly into "higher-order" systems that provide better localisation over larger areas.

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