First-order Ambisonics and B-Format
In the basic version, known as first-order Ambisonics, sound information is encoded into four channels: W, X, Y and Z. This is called Ambisonic B-format. The W channel is the non-directional mono component of the signal, corresponding to the output of an omnidirectional microphone. The X, Y and Z channels are the directional components in three dimensions. They correspond to the outputs of three figure-of-eight microphones, facing forward, to the left, and upward respectively. (Note that the fact that B-format channels are analogous to microphone configurations does not mean that Ambisonic recordings can only be made with coincident microphone arrays.)
The B-format signals are based on a spherical harmonic decomposition of the soundfield and correspond to the sound pressure (W), and the three components of the pressure gradient (X, Y, and Z) (not to be confused with the related particle velocity) at a point in space. Together, these approximate the sound field on a sphere around the microphone; formally the first-order truncation of the multipole expansion. This is called "first-order" because W (the mono signal) is the zero-order information, corresponding to a sphere (constant function on the sphere), while X, Y, and Z are the first-order terms (the dipoles), corresponding to the response of figure-of-eight microphones – as functions, to particular functions that are positive on half the sphere, and negative of the other half. This first-order truncation is only an approximation of the overall sound field (but see Higher-order Ambisonics).
The loudspeaker signals are derived by using a linear combination of these four channels, where each signal is dependent on the actual position of the speaker in relation to the center of an imaginary sphere the surface of which passes through all available speakers. In more advanced decoding schemes, spatial equalization is applied to the signals to account for the differences in the high- and low-frequency sound localization mechanisms in human hearing. A further refinement accounts for the distance of the listener from the loudspeakers.
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