Transcript HOA Workshop "5.1 and beyond" AES 120

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
France Télécom
Recherche & Développement
D1 - 29/10/2015
Traditional 1st order Ambisonics: B-Format
encoding Front (X)
s
Panoramic sound recording
QCoincident omni (W) and bidirectional
(X,Y) microphones
Left
(Y)
Right
QFront-back, Left-Right separation
QDirectional information
= amplitude relationships
QDescription of wave propagation
direction & speed  localization
QIndependent of any loudspeaker layout
Back
#2
Reproduction over loudspeakers : spatial
decoding Front (X)
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...
Right Q… sound image blur remains the same
Left
(Y)
Back
+
=
+ - =
+
-
+
B-Format
#3
Reproduction over loudspeakers : spatial
decoding Front (X)
Left
(Y)
s
Simulate any coincident mic setup
s
Optimized decoding for localization
Right
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
Back
(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)
Optimize localization at the sweet spot [Gerzon]
minimise opposite contributions
Compromise for large area [Malham]
#4
"Traditional" 1st order Ambisonics: pros & cons
s
Pros
s
Cons
QCompact 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™)
QBlurred / 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
#5
Introducing Higher Order Ambisonics (HOA)
Front (X)
s
Increase angular discrimination in
spatial encoding
Qadd directivities with "faster" angular
Left
(Y)
Right
variation
Back
1st order
2nd order
3rd order
4th order
#6
Introducing Higher Order Ambisonics (HOA)
Front (X)
s
Increase angular discrimination in
spatial encoding
Qadd directivities with "faster" angular
Left
(Y)
variation
Right
s
Increase angular selectivity of
loudspeakers’ contributions
Qselective virtual microphone directivities
Qbetter use of narrowed loudspeakers
Back
+
+
=
+
=
+
=
=
#7
Introducing Higher Order Ambisonics (HOA)
Front (X)
s
Increase angular discrimination in
spatial encoding
Qadd directivities with "faster" angular
Left
(Y)
variation
Right
s
Increase angular selectivity of
loudspeakers’ contributions
Qselective virtual microphone directivities
Qbetter use of narrowed loudspeakers
Back
1st order
2nd order
3rd order
4th order
#8
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
5th order
2nd order
s
10th order
spherical wave (R=1m)
(gaussian pulse)
Quality of sound images: localization clues for a centred listener
Order M
1
2
3
4
flim
700 Hz
1300 Hz
1900 Hz
2500 Hz
aE
45°
30°
22.5°
18°
good reconstruction (good ITD)
up to flim
blur angle due to
HF clues alteration (ILD&ITD)
above flim
#9
Compatibility with irregular/unbalanced arrangements
s 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
[Laborie et al]
[Craven, 2003]
#10
Compatibility with irregular/unbalanced arrangements
s 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)
#11
Extension to 3D encoding and reproduction
Reproduction over a 3D rig
Encoding into 3D HOA Format
Spatial
decoding
(similar
to 2D)
N LdSpk
signals
K
K HOA
signals
Sound
Field
Rotation
“Virtualization”:
HRTF filtering
s
s
3D encoding and decoding
Head-tracker
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
Reproduction over
headphones
#12
First conclusion on Higher Order Ambisonics
s
Pros
QScalable 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
Cons
s
What do we need in practice?
Qnothing? 
QHOA (or « high spatial resolution ») microphone systems
QSpatial processing tools
#13
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
#14
Tools and applications
s
Recording and mixing tools
s
Applications
QPrototypes of HOA mic (FT, Trinnov)
QSuite of VST plug-ins  demo
QUse in common audio edition tools, or interactive audio progr.
QMusic, 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)
#15
Demonstrations
s
Loudspeaker reproduction
QReproduction 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
#16