Ambisonics Decoding

MAT 240D

Direct Decoding (projection)

Project the spherical harmonics directly

unto the loudspeakers:

For first order without height:

where n is the loudspeaker index.

Velocity and Energy localization vectors

• Described in Gerzon's article: Gerzon, M. A. (1992). General Metatheory of Auditory Localisation. In 92nd AES Convention.
• The direction of each indicates the direction of the expected localization perception, while the magnitude indicates the quality of the localization. In "natural" hearing from a single source, the magnitude of each vector should be exactly 1, and the direction of the vectors is the direction to the source.
  

Energy, Pressure and vectors

Decoding Goals

• Constant amplitude gain for all source directions
• Constant energy gain for all source directions
• At low frequencies, correct reproduced wavefront direction and velocity
• At high frequencies, maximum concentration of energy in the source direction
• Matching high- and low-frequency perceived directions

Velocity and Energy vectors

• In first-order Ambisonics the zeroth-order component represents the sound pressure, and the three first-order components represent the acoustic particle velocity.

• If these components are reproduced exactly, then the sound will be correct at the center.

• However, it is not possible to get the first-order components correct except at a single point and not practical to get them correct at higher frequencies, where the wavelengths become smaller than the size of the human head.

The task of the decoder is to create the best perceptual impression that the soundfield is being reproduced accurately given the loudspeaker array being used.

Classic decoders

Split the signal in two bands

(typically with phase aligned shelf filters)

Treat each band separately:

The lower band maximized r.sub.v

The upper band maximized r.sub.E

"Vienna" decoders

US Patent 5757927

Decoder arranged to satisfy where the r.sub.v varies with azimuth, and, preferably, the directional gain pattern of the pressure signal P varies with frequency. Typically, for decoders having better front-stage than back-stage image stability, the back-sound gain divided by front-sound gain for the pressure signal will have a smaller value at low frequencies (for which r.sub.v typically equals 1) than at higher frequencies (for which typically r.sub.E is maximised with a greater value for front- stage sounds than for back-stage sounds).

Vienna decoder

Other optimizations

Add a gain offset to avoid out of phase speakers

Compensate distance differences with delays