An Integrated Approach for Noise Reduction and Dynamic Range Compression in Hearing Aids

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An Integrated Approach for Noise Reduction and

Dynamic Range Compression in Hearing Aids

Kim Ngo1 _{Simon Doclo}1,2 _{Ann Spriet}1,3

Marc Moonen1 _{Jan Wouters}3 _{Søren Holdt Jensen}4 1

2_{NXP Semiconductors, Corporate I&T-Research, Belgium} 3_{Dept. of Neurosciences, KU Leuven, Belgium}

4

Dept. of Electronic Systems, AAU, Denmark

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Outline

• Introduction - hearing aids, hearing impairment

• Noise Reduction (NR)

• Dynamic Range Compression (DRC)

• Problem statement - Why integration is needed ?

• Integration of NR and DRC

• Results

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Introduction

Sensorineural hearing loss

• Threshold of hearing raised as a result of the hearing loss

• Threshold of loudness discomfort remains the same

• Reduced dynamic range (threshold to discomfort)

• Reduced frequency selectivity

• Difficulty hearing conversations and understanding speech

Background Noise

• Understanding speech in noise is a major problem

• multiple speakers, fans, traffic etc.

• Reduces the intelligibility of speech

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Introduction

Background Noise

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Introduction

Background Noise

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Introduction

Background Noise

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Introduction

Background Noise

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Noise Reduction

Increase Signal-to-noise ratio and improve speech intelligibility !

Single-channel Noise Reduction • Limited benefit with single-channel

• Exploits only spectral and temporal differences

Multichannel Noise Reduction

• Exploits spatial diversity of speech and noise

• GSC, Beamformer, MWF etc. Noise Desired signal + ... ... z[k] x1[k] x2[k] xM[k] wM[k] w2[k] w1[k] z[k] = M X l=1 wTl [k]xl[k] =wT[k]x[k]

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Noise Reduction

• GSC, Beamformer, MWF etc. Noise Desired signal + ... ... z[k] x1[k] x2[k] xM[k] wM[k] w2[k] w1[k] z[k] = M X l=1 wTl [k]xl[k] =wT[k]x[k]

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Noise Reduction

• GSC, Beamformer, MWF etc. Noise Desired signal + ... ... z[k] x1[k] x2[k] xM[k] wM[k] w2[k] w1[k] z[k] = M X l=1 wT l [k]xl[k] =wT[k]x[k]

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Dynamic Range Compression

Map the wide dynamic range of speech into the reduced dynamic range of a hearing aid user !

• Automatically adjust the gain based on the intensity level

• High intensity attenuated - low intensity amplified

• Weak sounds audible - loud sounds not uncomfortably loud

Power (dB) Gain Model

CR/CT Release time Attack time Input spectrum Output Spectrum Gain adjustment 0 20 40 60 80 100 120 20 40 60 80 100 120 output SPL (dB) input SPL (dB) CR CT

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Dynamic Range Compression

Map the wide dynamic range of speech into the reduced dynamic range of a hearing aid user ! • Automatically adjust the gain based on the intensity level

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Dynamic Range Compression

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Dynamic Range Compression

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Problem statement

Motivation

• The general problems of NR and DRC are not new

• These areas are usually developed and evaluated independently

• Integrating these blocks has not achieved much attention

• Analyse any undesired effects in the integration process

• Evaluation of an integrated scheme is also unknown

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Problem statement

Motivation

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Problem statement

Motivation

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Problem statement

Motivation

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Problem statement

Motivation

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Problem statement

Motivation

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Problem statement

Existing mehtod

• Hearing aids typically use a serial concatenation

• Negatively affects the NR stage

• Residual noise after NR is amplified by the DRC

• Counteract and limit functionality of other algorithms

NR NR DRC Noisy speech Noisy speech Noisy speech DRC DRC NR / Integrated Enhanced speech Enhanced speech Enhanced speech

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Integration of NR and DRC

Dual-Dynamic Range Compression

• Using switchable compression characteristic

• Ps

DRC,dB- speech DRC (speech dominant case)

• P_DRC,dBn - noise DRC (noise dominant case)

Pdual,dB = (1 − β) · PDRC,dBn + β · P

s DRC,dB

βreflects the amount of speech/noise contribution in each segment

0 20 40 60 80 100 120 0 20 40 60 80 100 120 Dual−DRC1 output SPL (dB) input SPL (dB) PDRC,dB s P DRC,dB n β 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Dual−DRC2 output SPL (dB) input SPL (dB) P DRC,dB s P DRC,dB n β

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Integration of NR and DRC

• Ps

s DRC,dB

0 20 40 60 80 100 120 0 20 40 60 80 100 120 Dual−DRC1 output SPL (dB) input SPL (dB) PDRC,dB s P DRC,dB n β 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Dual−DRC2 output SPL (dB) input SPL (dB) P DRC,dB s P DRC,dB n β

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Integration of NR and DRC

• Ps

s DRC,dB

0 20 40 60 80 100 120 0 20 40 60 80 100 120 Dual−DRC1 output SPL (dB) PDRC,dB s P DRC,dB n β 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Dual−DRC2 output SPL (dB) input SPL (dB) P DRC,dB s P DRC,dB n β

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Integration of NR and DRC

• Ps

s DRC,dB

40 60 80 100 120 Dual−DRC1 output SPL (dB) PDRC,dB s β 40 60 80 100 120 Dual−DRC2 output SPL (dB) P DRC,dB s β

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Integration of NR and DRC

Dual-Dynamic Range Compression • Speech and noise detection

• Input/output of NR • Additional information to DRC NR DRC Dual−DRC αN R(ω, k) = Ps out,N R(ω, k) + Pout,N Rn (ω, k) Ps in,N R(ω, k) + P n in,N R(ω, k)

• Threshold function(speech/noise contribution)

β = 8 < : β = 1 if αN R≥ αmax β = 0 if αN R≤ αmin β = αNR−αmin αmax−αmin otherwise ₀ 1 1αNR β Pn DRC,dB Pdual,dB αmin Ps DRC,dB αmax

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Integration of NR and DRC

• Input/output of NR • Additional information to DRC NR DRC Dual−DRC αN R(ω, k) = Ps out,N R(ω, k) + Pout,N Rn (ω, k) Ps in,N R(ω, k) + P n in,N R(ω, k)

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Integration of NR and DRC

• Input/output of NR • Additional information to DRC NR DRC Dual−DRC αN R(ω, k) = Ps out,N R(ω, k) + Pout,N Rn (ω, k) Ps in,N R(ω, k) + Pin,N Rn (ω, k)

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Integration of NR and DRC

• Input/output of NR • Additional information to DRC NR DRC Dual−DRC αN R(ω, k) = Ps out,N R(ω, k) + Pout,N Rn (ω, k) Ps in,N R(ω, k) + Pin,N Rn (ω, k)

8 _{β = 1} _{if α} N R≥ αmax 1 β P Ps DRC,dB

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Results

Experimental Set-up

• Multi-microphone NR used is the well known GSC

• 2-microphone behind-the-ear hearing aid

• speech and the noise sources are located at 0◦_{and 120}◦ • Multi-talker babble noise at 0dB input SNR

• Input level 65dB at the hearing aid microphones

• fast-acting DRC with a gain of 30dB

Objective Quality Measures

• Intelligibility-weighted signal-to-noise ratio

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Results

Experimental Set-up

• Multi-microphone NR used is the well known GSC

• 2-microphone behind-the-ear hearing aid

• speech and the noise sources are located at 0◦_{and 120}◦ • Multi-talker babble noise at 0dB input SNR

• Input level 65dB at the hearing aid microphones

• fast-acting DRC with a gain of 30dB

Objective Quality Measures

• Intelligibility-weighted signal-to-noise ratio

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Results

Experiment 1 (Effect of CR on ∆SNR)

• ∆SNR decrease as CR increases

• Dual-DRC1 improve at low CR

• Dual-DRC2 improve at high CR

• Difference is the impact of β 0₁ ₂ ₃ ₄ ₅ 5 10 15 20 Compression Ratio (CR) ∆ SNR (dB) Serial concatenation Dual−DRC1 Dual−DRC2 Experiment 2 (Effect of CR on SD) • SD increase as CR increases

• SD for dual-DRC2 can be controlled

• SD decreases for dual-DRC1

• Low impact of β at high CR 4₁ ₂ ₃ ₄ ₅ 6 8 10 12 14 Compression Ratio (CR) SD (dB) Serial concatenation Dual−DRC1 Dual−DRC2

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Results

Experiment 1 (Effect of CR on ∆SNR) • ∆SNR decrease as CR increases

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Results

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Results

• SD decreases for dual-DRC1 8

10 12 14

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Conclusion

• A serial concatenation of NR and DRC has undesired effects

• Counteract NR performance (degradation of SNR improvement)

• Introducing an integrated approach for NR and DRC

• Dual-DRC concept using switchable compression characteristic

• Speech/noise detection using the input/output power ratio of NR

• Dual-DRC resulted in an SNR improvement compared to a serial concatenation of NR and DRC

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Conclusion

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Conclusion

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Conclusion

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