This thesis deals with the acoustic field representation for spatial reproduction over loudspeakers or headphones, applied to the multimedia domain, including new applications for browsing in virtual 3D-scenes on Internet. This domain combines the reproduction of pre-existent complex sound fields (e.g. in the multi-channel format) and constructive spatialisation tasks (3D pan-pot and room effects). The question of the representation should give an answer to the variability of a number of parameters: transmission bit-rate, user resources, listening conditions, sound or audio-visual materials handled, interactive rendering.
We study thoroughly the ambisonic approach, based on the spherical harmonic decomposition of the acoustic field. Its well known first order restricted form processes a minimal, directional sound field encoding (WXYZ components of the B-format). A decoder optimised for a centred or an extended listening area, leads to a "variable geometry" rendering (for 2D or 3D loudspeaker rigs, or headphones via binaural techniques). The consideration of higher order components introduces the concept of "variable resolution" representation.
After giving a thorough account of acoustic and psychoacoustic bases, spatialisation strategies (stereo, surround, binaural, etc.) and the localisation theories implied in ambisonic decoding (Gerzon), we generalise to all higher order all Ambisonic aspects, from the encoding to the decoding. The original decoding forms (Gerzon, Malham) are generalised into three solution families. Objective evaluations supported by informal listening experiments confirm the contribution of the higher orders and the optimised solutions, in terms of sound image precision and robustness. Implemented among other spatialisation techniques (including artificial reverberation) within a real-time interface on a PC, Ambisonic has been successfully applied to source manipulation and mixing (with mono, multi-channel and B-format as input sources).
Ambisonic extension to higher orders concerns many application fields
and should develop in the near future thanks to the current studies and
Spatialisation - 3D-Sound - Surround - Multimedia - 3D-browsing
- Ambisonic(s) - B-format - Localisation theory - Velocity
vector - Energy vector - Psychoacoustic decoding - Sound field representation
- Spherical harmonic decomposition - Directionnal sampling - Scalability
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