SIGNAL PROCESSING IN HEARING AIDS October 2003. Signal Processing in Hearing Aids “An Australian...

SIGNAL PROCESSING IN HEARING AIDS

October 2003

Signal Processing in Hearing Aids

“An Australian research and technology company that

develops new solutions for the expanding digital

hearing aid and communications industry.”


Bionic Ear Institute

A Venture Capital Backed Spin-Off from CRC HEAR


Electric hearing with

Cochlear implant

Acoustic hearing with

Hearing aid

A new amplification scheme so that cochlear implants and hearing aids can be used more effectively together


The research goals were:

To make soft sounds more audible.

To ensure that loud sounds were comfortable.

To improve speech intelligibility in noise.

To maintain high sound quality.


Cochlear Implant Hearing Aid

Research

1997-2002

Mipsy (desk-top) Mipsy (desk-top)

Audallion (wearable)

DSP hearing aids

ProductsSPrint™

Cochlear LtdCommercial hearing aids

Digital Signal Processors for ADROTM


Investment: ($5 million in 2 years)Main shareholders: Rothschilds Biosciences, Nanyang Ventures


Audiologists

Dynamic Team

Research reputation

Excellent research partners

Sound processing expertise

Scientists

Business Team Engineers


Advances in Hearing Aid Technology


ADRO™ ADRO is hearing aid software based on a revolutionary technology that significantly enhances hearing aid performance for hearing impaired people.

Fitting software for ADROADROFit

ADRO plus:

Feedback suppression

Directional microphone

Noise reduction

Products and services

Premium hearing aid package

Customisation Products customised to OEM’s needs


Strategic Alliance

Established with the only independent manufacture of low power

digital signal processing chips to hearing aid and communications

industry


95% seek better hearing of speech against background noise

88% seek better sound quality

83% would like to hear more soft sounds

ADRO achieves the improvements wearers really want

Recently collected market surveys of hearing aid users show:

: from Marke Trak VI - Kochkin, 2002)


ADRO is an alternative approach that automatically adjusts the sounds to suit all types of hearing loss


A real life situation


ADRO improves intelligibility in any background noise

Multi-band compression hearing aid

Same hearing aid using ADROTM


0

10

20

30

40

50

60

70

80

90

100

MIL

D1

MOD1S

MOD2S

MOD3S

MOD4

MOD5

MOD6S

MOD7

MOD8S

MOD9

MOD10

MOD11

MOD12

S

MOD13

MOD14

SEV1

SEV3

SEV4

SEV5

Subject

Sc

ore

(%

)

ADROComp

ADRO Significantly Improved Performance forSentences in Babble


4.5% ** 11.9%*** 14.2%***

In the same clinical trial, ADRO improved performance for sentences in quiet

** p<0.01 *** p<0.001

0

20

40

60

80

100

High Mid Low

Relative Speech Level

Se

nte

nc

e S

co

re (

%)

ADRO Compression


ADRO improves intelligibility at a distance.

Multi-band compressionhearing aid

Hearing aid using ADROTM

TECHNICAL ASPECTS


Introduction

What are the effects of a hearing loss?

DSP in hearing aids

– Dspfactory’s Toccata Plus processor

Dynamic Hearing’s strategies

– Feedback suppression

– Directional microphones

– The ADRO™ processing strategy


What are the effects of a hearing loss?

Higher audibility threshold

Speech perception is reduced in noise

– A higher signal-to-noise ratio is required

– Reduced frequency resolution

– Reduced temporal resolution

Reduced dynamic range

– Elevated thresholds

– Near-normal sensitivity to loud sounds


Hearing impairment reduces the dynamic range

20 30 40 50 60 70 80 90 100

dB HL

Example of a hearing loss

Loudness growth for normal hearing

Uncomfortable

Too loud

Loud

Comfortable

Soft

Very soft

Inaudible


How can DSP help?

Higher audibility threshold– Provide extra gain in the required frequency bands by using

a digital filter

Require a higher signal-to-noise ratio– Directional microphone, noise cancellation algorithms and

matched filters can improve the SNR

Reduced dynamic range– Compression algorithms or the ADRO™ processing

strategy


DSP in Hearing Aids

Digital hearing aids have been available since the mid 1990s

Early versions used ASICs (and some still do)

Most recently general purpose DSP processors optimised for

hearing aids have become available

The most widely used is the Dspfactory’s Toccata Plus

processor


Dspfactory’s Toccata Plus processor

Small size

Power consumption ~ 600 μA at 1.4 V

Low clock rate - 1.28 MHz– Up to 2.56 MIPS

Incorporates a hardware FFT unit and an Input/Output Processor

Also includes EEPROM, ADCs, DACs and drivers


Dspfactory’s Toccata Plus processor

PreAmps, ADCs and sampling

circuitry

IOP (Input Output Processor)

RCORE (DSP Core Processor)

Buffers samples from the input stage and going to the output stage

Output

Stage

DACs and direct drive

circuitry

WOLA (Weighted Overlap-Add) Coprocessor

Input

Stage

Executes assembler code to process time domain samples from the IOP or frequency domain samples from the WOLA

Calculates FFT, applies gains and outputs data to the IOP

EEPROM

Stores programs, fittings and calibrations


Dynamic Hearing has developed a brand new solution to

an old problem

Feedback detection and suppression operates

independently in each of the 64 channels

Stops ringing while preserving the intelligibility of speech

Feedback Suppression


Without Feedback Suppression Activated

Am

plitu

de

Time

With Feedback Suppression Activated

Am

plitu

de

Time

Feedback Suppression


Dynamic Hearing has developed new techniques to increase

the signal-to-noise ratio.

Directional microphones provide a natural way to increase the

signal-to-noise ratio by reducing the amount of noise entering

the hearing aid from directions other than the front.

The majority of sound to be processed will come from in-front

of the hearing aid wearer.

Directional Microphones


In this sample the hearing aid is worn by a listener (not free space)

Signal is directly in front of the wearer

Noise is 135° to the right of the wearer

Both the signal and the noise are presented at

the same level – 67dB (SPL A weighted)

Without Directional Microphone

With Directional Microphone

Directional Microphones

ADRO™


ADRO: Architecture (Toccata Plus)

Microphone

ADC FFT ADRO FFT DAC

Output Device (Receiver)64 Channels

Input StageIOPRCoreWOLAOutput Stage

64 Channels


Perceptual Dynamic Range

UCL

Threshold

Comfort

20

30

40

50

60

70

80

90

100

110

120

250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000

frequency (Hz)

2 c

c co

up

ler

ou

tpu

t le

vel (

dB

)


Perceptual levels and output targets

Comfort Target

Maximum Output Level

Audibility Target

Threshold

Uncomfortable

Listener’s

dynamic

range

Hearing aid output range


Percentiles: 75 dB speech

20

30

40

50

60

70

80

250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000

frequency (Hz)

2 c

c co

up

ler

ou

tpu

t le

vel (

dB

)

Max

98th

90th

Leq

70th

50th

10th


ADRO™ Architecture

An FFT is performed every 4 ms. A statistical analysis is then performed

on each of the 64 channels to calculate the 90th and 30th percentile

estimates.

The 90th percentile is compared to the comfort target value. The gain is

decreased if the target is exceeded. (Comfort Rule)

If the comfort rule did not decrease the gain, then the 30th percentile is

compared to the audibility target value. The gain is increased if the 30 th

percentile is lower than the target. (Audibility Rule)

If the gain is increased, it is limited to the maximum gain value.

(Maximum Gain Rule)

If the comfort rule and the audibility rule do not come into play, then the

gain is not changed.

The instantaneous output power in each bin is not allowed to exceed

the maximum output level (Maximum Output Rule)


ADRO™ Architecture

90th percentile >

comfort?

30th percentile <

audibility?

Gain > maximum gain?

Magnitude > maximum?

Decrease the Gain

Increase the Gain

Limit the Gain

Limit the Output

Comfort Rule

Audibility Rule

Maximum Output Rule

Comfort Rule

Maximum Gain Rule


ADRO™ with no

additional noise reduction.

The graph shows the

ADRO™ gains varying

in real time.

ADRO™ Processing

Example of gain adjustments for a real world situation


ADRO™ (Adaptive Dynamic Range Optimisation)

Compression strategies

– Typically use only a few channels

– Fast attack and release times

– Vary gain based on signal input amplitude

ADRO™ strategy

– Uses 64 channels

– Long attack and release times

– Calculates long term estimates of the output signal’s dynamic range in each

channel

– Varies the gain in each channel so that the dynamic range of the output

signal is matched to the user’s requirements


Technical Comparison

Speech outputs for ADRO™ and compression hearing aids:

ADRO™ CompressionFemale 75 dB Babble 85 dB

200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000

2 cc

Cou

pler

Out

put L

evel

(dB)

20

30

40

50

60

70

80

90

100

110

Female 75 dB Babble 85 dB

200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000

2 cc

Cou

pler

Out

put L

evel

(dB)

20

30

40

50

60

70

80

90

100

110

Female speaker with 4-talker babble at -10 dB signal-to-noise ratio.


ADRO™ Architecture (Toccata Plus)

Apply Gain

64 Channels in

Limit to

Max Output Level

Apply Volume Control Gain

64 Channels out

Adjust Gain

values

Calculate

Percentiles

Calculate

Magnitudes

Slowly adapting values WOLARCore


What do users think of ADRO™?

Hearing Aid Measure of Contrast ScaleADRO preferred 74% of the time in 25 listening situations

16%

10% 0% 12%

31%

31%

ADRO much better

ADRO better

ADRO slightly better

Comp slightly better

Comp better

Comp much better

In a blind clinical trial, the standard Hearing Aid Measure of Contrast

questionnaire (Dillon 1994) showed ADRO was preferred to compression.


Immediate access to sophisticated technology - ADRO

runs in the dspfactory Toccata chipset

Product the result of several years research and

development with proven scientific benefit

Fast path to new product line

Implementing ADRO – Low risk path to new and innovative technology for hearing aid manufacturers

Dynamic Hearing provides

customisation and technology

support

Simple and effective solution for the OEM customers


ADRO awarded the CRC

Association commercialization

award (shared with Cochlear Ltd),

Awards

Federal Government START Grant awarded to accelerate

product development in headset and communication device

research


Dynamic Hearing has successfully licensed the

technology to French company Intrason, where the

hearing aid was enthusiastically received by the French

market – licensed January 2003, launched March 2003

Track record


The Future

COMMUNICATION DEVICES

- Toccata Plus

- Open platform DSPs

- Headsets

HEARING IMPAIRED PEOPLE

- Better hearing

- Cost effective solutions

AUDIOLOGIST

- Product education

- Training

- Support

- Australian Clinic

ADRO

- Clinically Proven

- Customisable

- Embedded software

SIGNAL PROCESSING IN HEARING AIDS October 2003. Signal Processing in Hearing Aids “An Australian...

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