D1 - 21/04/23France TélécomRecherche & Développement
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
#2
Traditional 1st order Ambisonics: B-Format encoding
s Panoramic sound recording
QCoincident omni (W) and
bidirectional (X,Y) microphones
QFront-back, Left-Right separation
QDirectional information
= amplitude relationships
QDescription of wave propagation
direction & speed
localization
QIndependent of any loudspeaker
layout
Front (X)
Back
Left (Y)
Right
#3
Reproduction over loudspeakers : spatial decoding
s Simulate any coincident mic setup
QRecombine B-Format directivity patterns
QDecoding operation: matrix signals W,X,Y
QOne virtual microphone per loudspeaker
Q... as many as wanted, but...
Q… sound image blur remains the same
+
-= + = +-
+
B-Format
Front (X)
Back
Left (Y)
Right
#4
Reproduction over loudspeakers : spatial decoding
s Simulate any coincident mic setup
QRecombine B-Format directivity patterns
QDecoding operation: matrix signals
W,X,Y
QOne virtual microphone per loudspeaker
Q... as many as wanted, but...
Q… sound image blur remains the same
s Optimized decoding for localization
(LF < 600-700z)Reproduce true wave propagation at the listener scale ( good ITD)
HF (>600- 700 Hz)Concentrate energy contributions in the expected direction ( less altered ILD, ITD)
Front (X)
Back
Left (Y)
Right
minimise opposite contributions
Compromise for large area [Malham]Optimize localization at the sweet spot [Gerzon]
#5
"Traditional" 1st order Ambisonics: pros & cons
s ProsQCompact multichannel format (no redundancy)
QSpatial homogeneity
QAcoustic fidelity (regarding propagation properties)
QEasily extended to 3D (additional Z)
QFlexibility: sound field transformation; reproduction setups
QCommercialized B-Format microphones (eg SoundField™)
s ConsQBlurred / unstable sound images ("tiny" sweet spot)
QNot well adapted to irregular/unbalanced loudspeaker arrangements (esp. ITU setup)
QLimitations due to low directivity of usual mikes, esp. at LF
Q... that’s why non-coincident microphone approaches might be preferred
#6
Introducing Higher Order Ambisonics (HOA)
s Increase angular discrimination in
spatial encoding
Qadd directivities with "faster"
angular variation
Front (X)
Back
Left (Y)
Right
1st order 2nd order 3rd order 4th order
#7
Introducing Higher Order Ambisonics (HOA)
s Increase angular discrimination in
spatial encoding
Qadd directivities with "faster"
angular variation
s Increase angular selectivity of
loudspeakers’ contributions
Qselective virtual microphone
directivities
Qbetter use of narrowed
loudspeakers
Front (X)
Back
Left (Y)
Right
+ + + +
= = = =
#8
Introducing Higher Order Ambisonics (HOA)
s Increase angular discrimination in
spatial encoding
Qadd directivities with "faster"
angular variation
s Increase angular selectivity of
loudspeakers’ contributions
Qselective virtual microphone
directivities
Qbetter use of narrowed
loudspeakers
Front (X)
Back
Left (Y)
Right
1st order 2nd order 3rd order 4th order
#9
Rendering properties of higher spatial resolution
s Acoustic reconstruction
QEnlarged sweet area "Holophony" [Nicol, Daniel]
QEnhanced distance encoding control of the wave curvature
monochromatic plane wave (f=600Hz)
1st order 2nd order 5th order 10th order
s Quality of sound images: localization clues for a centred listener
spherical wave (R=1m)(gaussian pulse)
Order M 1 2 3 4
flim 700 Hz 1300 Hz 1900 Hz 2500 Hz
E 45° 30° 22.5° 18°
good reconstruction (good ITD) up to flim
blur angle due to HF clues alteration (ILD&ITD)above flim
#10
Compatibility with irregular/unbalanced arrangementss Synthesize directivities adapted to ITU inter-loudspeaker angles
QFrom 4th order ambisonics [Craven, 2003]
QUsing 5th order resolution [Laborie et al]: better front channels
separation
QPossible decoding criterion (among others): imitate pair-wise pan-pot
[Craven, 2003]
[Laborie et al]
#11
Compatibility with irregular/unbalanced arrangementss Synthesize directivities adapted to ITU inter-loudspeaker angles
QFrom 4th order ambisonics [Craven, 2003]
QUsing 5th order resolution [Laborie et al]: better front channels
separation
QPossible decoding criterion (among others): imitate pair-wise pan-pot
s 4th order decoding over enriched ITU setup (5+2+1)QC (0°), L&R(+-30°), SL&SR(+-120°) … + L&R(+-70°) … + B (180°)
QDemonstration on a 8-loudspeaker setup (kindly provided by Cabasse)
◊ = "energy vector" (* = target, ie ideal sound image)
#12
Extension to 3D encoding and reproduction
s 3D encoding and decoding
s Dynamic binaural reproduction
QVirtual loudspeakers doesn’t sound so good
QEnhanced method: better efficiency (CPU) & rendering
QSound field rotation driven by head-tracker
QDemo : Poster session P31, Tuesday, 14:00 - 15:30
Encoding into 3D HOA Format Reproduction over a 3D rig
Reproduction over headphones
Spatial decoding(similar to 2D)
Head-tracker
“Virtualization”:HRTF filtering
K N LdS
pk signals
K
HO
A
signals
SoundField
Rotation
#13
First conclusion on Higher Order Ambisonics
s ProsQScalable multichannel format
QSpatial homogeneity
QAcoustic fidelity + "high spatial definition"Wave field reconstruction
QEasily extended to 3D – Efficient binaural spatialisation
QEven more flexibility: sound field transformation;
reproduction setups, including irregular arrangements like ITU
s ConsQnothing?
s What do we need in practice?QHOA (or « high spatial resolution ») microphone systems
QSpatial processing tools
#14
Higher Order Ambisonics Microphone Systems s Synthesis of Spherical Harmonics
QExtension of differential microphones:
Pressure gradient and higher order derivatives
using non-coincident acoustic sensors!
QNon concentric sensor distribution (Trinnov)
QDistribution over a rigid sphere (FT)
Q[Meyer, Elko, Kubli] [Rafaely] [Ward, Abhayapala]…
QTrade-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)
s A few words on FT prototype
QDesigned for "proof of concept" (homogeneous 3D)
Q32 sensors 4th order 3D (and even 5th order 2D)
QObjective measurements & validation [Moreau et al]
Poster session P31, Tuesday, 14:00 -
15:30
#15
Tools and applications
s Recording and mixing toolsQPrototypes of HOA mic (FT, Trinnov)
QSuite of VST plug-ins demo
QUse in common audio edition tools, or interactive audio progr.
s ApplicationsQMusic, documentary, fictions
QSharing of events/ambiances (eg familial use), teleconferences
QInteractive 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
s Loudspeaker reproductionQReproduction of 4th order 3D recordings over enriched ITU setup (5 to 8 ldspk)
QAcknowledment:
Many thanks to Cabasse
and R&D manager Yvon Kernéis
s Head-tracked binaural reproduction Q[Moreau et al] Poster session P31, Tuesday, 14:00 - 15:30
QCould also be shown after this workshop