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Jérôme Daniel, France Telecom R&D

Workshop « From 5.1 to Sound Field Synthesis..."  AES 120th Convention, Paris 2006 Higher Order Ambisonics: promises and reality. Jérôme Daniel, France Telecom R&D. Front (X). Left (Y). Right. Back. Traditional 1st order Ambisonics: B-Format encoding. Panoramic sound recording

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Jérôme Daniel, France Telecom R&D

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  1. Workshop « From 5.1 to Sound Field Synthesis..." AES 120th Convention, Paris 2006Higher Order Ambisonics: promises and reality Jérôme Daniel, France Telecom R&D

  2. Front (X) Left (Y) Right Back Traditional 1st order Ambisonics: B-Format encoding • Panoramic sound recording • Coincident omni (W) and bidirectional (X,Y) microphones • Front-back, Left-Right separation • Directional information = amplitude relationships • Description of wave propagation direction & speed  localization • Independent of any loudspeaker layout

  3. Front (X) + = = + + + - B-Format Left (Y) Right - Back Reproduction over loudspeakers : spatial decoding • Simulate any coincident mic setup • Recombine B-Format directivity patterns • Decoding operation: matrix signals W,X,Y • One virtual microphone per loudspeaker • ... as many as wanted, but... • … sound image blur remains the same

  4. Front (X) Left (Y) Right Back Reproduction over loudspeakers : spatial decoding • Simulate any coincident mic setup • Recombine B-Format directivity patterns • Decoding operation: matrix signals W,X,Y • One virtual microphone per loudspeaker • ... as many as wanted, but... • … sound image blur remains the same • Optimized decoding for localization • HF (>600- 700 Hz) • Concentrate energy contributions in the expected direction ( less altered ILD, ITD) • (LF < 600-700z) • Reproduce true wave propagation • at the listener scale ( good ITD) • minimise opposite contributions Compromise for large area [Malham] Optimize localization at the sweet spot [Gerzon]

  5. "Traditional" 1st order Ambisonics: pros & cons • Pros • Compact multichannel format (no redundancy) • Spatial homogeneity • Acoustic fidelity (regarding propagation properties) • Easily extended to 3D (additional Z) • Flexibility: sound field transformation; reproduction setups • Commercialized B-Format microphones (eg SoundField™) • Cons • Blurred / unstable sound images ("tiny" sweet spot) • Not well adapted to irregular/unbalanced loudspeaker arrangements (esp. ITU setup) • Limitations due to low directivity of usual mikes, esp. at LF • ... that’s why non-coincident microphone approaches might be preferred

  6. Front (X) Left (Y) Right Back Introducing Higher Order Ambisonics (HOA) • Increase angular discrimination in spatial encoding • add directivities with "faster" angular variation 2nd order 3rd order 4th order 1st order

  7. Front (X) Left (Y) Right Back Introducing Higher Order Ambisonics (HOA) • Increase angular discrimination in spatial encoding • add directivities with "faster" angular variation • Increase angular selectivity of loudspeakers’ contributions • selective virtual microphone directivities • better use of narrowed loudspeakers + + + + = = = =

  8. Front (X) Left (Y) Right Back Introducing Higher Order Ambisonics (HOA) • Increase angular discrimination in spatial encoding • add directivities with "faster" angular variation • Increase angular selectivity of loudspeakers’ contributions • selective virtual microphone directivities • better use of narrowed loudspeakers 3rd order 4th order 1st order 2nd order

  9. Rendering properties of higher spatial resolution • Acoustic reconstruction • Enlarged sweet area  "Holophony" [Nicol, Daniel] • Enhanced distance encoding  control of the wave curvature spherical wave (R=1m) (gaussian pulse) monochromatic plane wave (f=600Hz) 1st order 5th order 10th order 2nd order • Quality of sound images: localization clues for a centred listener good reconstruction (good ITD) up to flim blur angle due to HF clues alteration (ILD&ITD) above flim

  10. [Craven, 2003] [Laborie et al] Compatibility with irregular/unbalanced arrangements • Synthesize directivities adapted to ITU inter-loudspeaker angles • From 4th order ambisonics [Craven, 2003] • Using 5th order resolution [Laborie et al]: better front channels separation • Possible decoding criterion (among others): imitate pair-wise pan-pot

  11. Compatibility with irregular/unbalanced arrangements • Synthesize directivities adapted to ITU inter-loudspeaker angles • From 4th order ambisonics [Craven, 2003] • Using 5th order resolution [Laborie et al]: better front channels separation • Possible decoding criterion (among others): imitate pair-wise pan-pot • 4th order decoding over enriched ITU setup (5+2+1) • C (0°), L&R(+-30°), SL&SR(+-120°) … + L&R(+-70°) … + B (180°) • Demonstration on a 8-loudspeaker setup (kindly provided by Cabasse) ◊="energy vector" (* = target, ie ideal sound image)

  12. K HOA signals K N LdSpk signals “Virtualization”: HRTF filtering Head-tracker Reproduction over headphones Extension to 3D encoding and reproduction Reproduction over a 3D rig Encoding into 3D HOA Format • 3D encoding and decoding • Dynamic binaural reproduction • Virtual loudspeakers  doesn’t sound so good • Enhanced method: better efficiency (CPU) & rendering • Sound field rotation driven by head-tracker • Demo : Poster session P31, Tuesday, 14:00 - 15:30 Sound Field Rotation Spatial decoding (similar to 2D)

  13. First conclusion on Higher Order Ambisonics • Pros • Scalablemultichannel format • Spatial homogeneity • Acoustic fidelity + "high spatial definition"Wave field reconstruction • Easily extended to 3D – Efficient binaural spatialisation • Even more flexibility: sound field transformation; reproduction setups, including irregular arrangements like ITU • Cons • nothing?  • What do we need in practice? • HOA (or « high spatial resolution ») microphone systems • Spatial processing tools

  14. Higher Order Ambisonics Microphone Systems • Synthesis of Spherical Harmonics • Extension of differential microphones: Pressure gradient and higher order derivatives using non-coincident acoustic sensors! • Non concentric sensor distribution (Trinnov) • Distribution over a rigid sphere (FT) • [Meyer, Elko, Kubli] [Rafaely] [Ward, Abhayapala]… • Trade-off on the size of the array • bigger is better to have spatial resolution at LF • smaller is better to reduce spatial aliasing (at HF) • A few words on FT prototype • Designed for "proof of concept" (homogeneous 3D) • 32 sensors  4th order 3D (and even 5th order 2D) • Objective measurements & validation [Moreau et al]  Poster session P31, Tuesday, 14:00 - 15:30

  15. Tools and applications • Recording and mixing tools • Prototypes of HOA mic (FT, Trinnov) • Suite of VST plug-ins  demo • Use in common audio edition tools, or interactive audio progr. • Applications • Music, documentary, fictions • Sharing of events/ambiances (eg familial use), teleconferences • Interactive audio and multi-media: • A flexible multi-channel 3D audio format • Games, Virtual/Mixt Reality • News nodes for virtual scene description in MPEG4 (AudioBIFSV3) • label a multi-channel stream as a HOA content (AudioChannelConfig) • a new kind of sound object that describes a Surrounding Sound Field (SurroundingSound)

  16. Demonstrations • Loudspeaker reproduction • Reproduction of 4th order 3D recordings over enriched ITU setup (5 to 8 ldspk) • Acknowledment: Many thanks to Cabasse and R&D manager Yvon Kernéis • Head-tracked binaural reproduction • [Moreau et al] Poster session P31, Tuesday, 14:00 - 15:30 • Could also be shown after this workshop

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