Microphone Array Design and Beamforming · Microphone Array Design and Beamforming Heinrich Löllmann Multimedia Communications and Signal Processing [email protected] with

Microphone Array Design and Beamforming

Heinrich Löllmann

Multimedia Communications and Signal Processing

[email protected]

with contributions from Vladi Tourbabin and Hendrik Barfuss

EUSIPCO Tutorial on Embodied Audition for Robots

August 31, 2015

Overview

Introduction

Microphone Array Design

• measure for array performance

• array design for Spherical Harmonics (SHs)

• new robot head design

Beamforming

• robust least-squares beamformer design

• HRTF-based design for robots

• evaluation for Automatic Speech Recognition (ASR)

Conclusions

H. Löllmann: Microphone Array Design and Beamforming

2

Introduction Microphone Array Design Beamforming Conclusions

Introduction

Beamforming is used by robots to enhance the performance of

the automatic speech recognition (ASR)

Numerous publications about beamformers design and related

approaches, e.g., [Brandstein & Ward, 2001, Van Trees 2002,

Benesty et al, 2008]

• fixed beamformer designs

• adaptive beamforming

• blind source separation (BSS)

...

Which issues are special for a beamformer design in the

context of robot audition?


3


Design Aspects for Robot Audition

Construction of the robot (head)

• How many microphones are needed/feasible?

• What are the ‘optimal’ microphone positions?

Beamforming with head microphones

• influence of the head (no free-field)

• presence of ego-noise ( tutorial talk on signal enhancement)

• influence of robot movements, especially head rotations

Localization and tracking of the desired speaker

tutorial talks by C. Evers and R. Horaud


4


Optimal Microphone Positions

Many robot systems use 2 microphones to mimic the human

auditory system [Argentieri et al., 2013]

Can we do better than with only two ears?

How can we determine the optimal microphone placement?

Measure for array performance in dependence of sensor positions

presented in [Tourbabin & Rafaely, 2014]


5


Model for generalized head-related transfer function (HRTF)

Generalized HRTF Matrix


6

complex amplitude of far-field source

complex pressure amplitudes transfer function between source j and sensor l



Representation for D sources, M sensors, K frequency points

Compact matrix formulation


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Singular Value Decomposition (SVD) of matrix with generalized

head related transfer functions (GHRTFs)

Observation

• information to construct from mainly contained by the most dominant

eigenvectors

• high effective rank of GHRTF matrix indicates a good sensor placing


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The effective rank of the GHRTF matrix is given by

Optimal microphone positions obtained by

Solution can be found by genetic algorithm optimization

Optimal Sensor Placement


9


Optimal Sensor Placement

Simulation example

• effective rank of GHRTF matrix for single positions on head surface

Relation between effective rank and beamformer robustness as

well as DOA estimation accuracy derived in [Tourbabin & Rafaely,

2014]


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Array Design for Spherical Harmonics (SH)

A nearly ball-shaped robot head with many microphones

motivates array designs in the Spherical Harmonic (SH) domain

An approach related to the previous method developed for this

case

• optimal position founds by minimizing the aliasing level for different

positions

• outline of this concept provided by the appendix, but a detailed

treatment of SH exceeds this tutorial


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New Head Array Design for Nao Robot


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Possible regions for microphone

placement (green)

• determined by mechanical

constraints of the manufacturer

Positions considered for the

optimization

• 327 positions on a simulated head




13

Simulation results (BG University) Layout for new Nao head (Aldebaran)

• slight deviations from optimal position due

to mechanical constraints



First prototype head


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Beamformer Design for Robot Audition

Review: Filter-and-Sum Beamformer

Beampattern


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Robust Least-Squares Frequency-Invariant (RLSFI) Design

[Mabande et al, 2009]

Robust Beamformer Design


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Design Example

RLSFI beamformer design for different WNG thresholds

• beampattern for free-field response

• trade-off between spatial selectivity and robustness (WNG)


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Design Example

RLSFI beamformer design for

• beampattern for HRTF-based response

• reduced spatial selectivity and distortions in look direction

HRTFs should be considered in the design!


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HRTF-based RLSFI Design [Barfuss et al, 2015]

HRTF-Based Beamformer Design


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HRTF-based RLSFI design for different WNG thresholds

• distortionless response in look direction

• similar spatial selectivity as for free-field design

Design Example


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HRTF-based design free-field design


Experimental Evaluation

Setup

• ASR Pocket Sphinx trained on clean speech of GRID corpus

• test corpus contained 200 utterances

• signal quality evaluated by frequency weighted segmental SNR (fwSegSNR)

[Hu & Loizou, 2008]

• interference at 45o and desired speaker between 0o and 180o

• room impulse responses measured for a T60 of 190ms


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Experimental Evaluation

Results

• scenario with speaker

at 30o and 60o most

challenging

• better performance for

HRTF-based design in

almost all cases


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Conclusions

Beamformer design for robot audition requires tailored

algorithms and designs

Optimal microphone positions

• can be determined by the effective rank of GHRTF matrix

• can be found for a SH design by minimizing the aliasing level

Beamformer design based on common free-field assumption

leads to inferior results

• knowledge about the HRTF of the robot should be incorporated in the

design


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Array Design for Spherical Harmonics (SHs)

How to find the best microphone positions for SHs

► Concept [Tourbabin et al., 2015]


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Appendix

Array Design for Spherical Harmonics

► Example for matrix

• aliasing level for sensor positions given by ratio of highest

off-line element and corresponding diagonal element

► Optimal microphone positions obtained by


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Appendix

Maximal possible WNG

• f or free-field (delay-and-sum beamformer)

• lower maximal WNG for HRTF-based design

White Noise Gain for HRTF-based Design


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Appendix

References

[Argentieri et al., 2013] S. Argentieri, A. Portello, M. Bernard, P. Danés, and B. Gas, “Binaural

Systems in Robotics,” in The Technology of Binaural Listening, J. Blauert, Ed., Modern Acoustics and

Signal Processing, pp.225–253, Springer

[Brandstein & Ward, 2001]: M. Brandstein and D. Ward (Eds.): Microphone Arrays, Springer, 2001

[Van Trees 2002]: H. L. Van Tress: Optimum Array Processing (Detection, Estimation and Modulation

Theory, Part IV), Wiley Intersience

[Benesty et al., 2008] J. Benesty, J. Chen, and Y. Huang: Microphone Array Signal Processing,

Springer

[Tourbabin & Rafaely, 2014] V. Tourbabin and B. Rafaely: “Theoretical Framework for the Optimization

of Microphone Array Configuration for Humanoid Robot Audition”, IEEE Trans. on Audio, Speech, and

Language Processing, vol. 22, no.12

[Mabande et al., 2009] E. Mabande, A. Schad, and W. Kellermann: “Design of Robust Superdirective

Beamformers as a Convex Optimization Problem”, IEEE Intl. Conf. on Acoustics, Speech, and Signal

Processing (ICASSP), Taipei, Taiwan

[Barfuss et al., 2015] H. Barfuss, C. Huemmer, G. Lamani, A. Schwarz, and W. Kellermann: “HRTF-

Based Robust Least-Squares Frequency-Invariant Beamforming”, Workshop on Applications of Signal

Processing to Audio and Acoustics (WASPAA), New Paltz, NY, USA

[Hu & Loizou, 2008] Y. Hu and P.C. Loizou: “Evaluation of Objective Quality Measures for Speech

Enhancement”, IEEE Trans. on Audio, Speech, and Language Processing, vol.16, no.1, pp.229-238


27

Appendix

Acknowledgment


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The research leading to these results has received funding from the

European Union's Seventh Framework Programme (FP7/2007-2013)

under grant agreement no. 609465.

Microphone Array Design and Beamforming · Microphone Array Design and Beamforming Heinrich Löllmann Multimedia Communications and Signal Processing [email protected] with

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