SIGNAL PROCESSING IN HEARING AIDS

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SIGNAL PROCESSING IN HEARING AIDS

• October 2003

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An Australian research and technology company
that develops new solutions for the expanding
digital hearing aid and communications industry.
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Acoustic hearing with Hearing aid
Electric hearing with Cochlear implant
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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.

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Cochlear Implant Hearing Aid
Research 1997-2002 Mipsy (desk-top) Mipsy (desk-top)
Research 1997-2002 Audallion (wearable)
Research 1997-2002 DSP hearing aids
Products SPrint Cochlear Ltd Commercial hearing aids
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Audiologists
Scientists
Research reputation Excellent research
partners Sound processing expertise
Business Team
Engineers
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ADRO is hearing aid software based on a
revolutionary technology that significantly
enhances hearing aid performance for hearing
impaired people.
ADRO
Fitting software for ADRO
ADROFit
ADRO plus Feedback suppression Directional
microphone Noise reduction
Premium hearing aid package
Customisation
Products customised to OEMs needs
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Established with the only independent
manufacture of low power digital signal
processing chips to hearing aid and
communications industry
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ADRO achieves the improvements wearers really want
Recently collected market surveys of hearing aid
users show

• 95 seek better hearing of speech against
  background noise
• 88 seek better sound quality
• 83 would like to hear more soft sounds

from Marke Trak VI - Kochkin, 2002)
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A real life situation
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Multi-band compression hearing aid
Same hearing aid using ADROTM
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In the same clinical trial, ADRO improved
performance for sentences in quiet
4.5
11.9
14.2
plt0.01 plt0.001
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Multi-band compressionhearing aid
Hearing aid using ADROTM
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TECHNICAL ASPECTS
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Introduction

• What are the effects of a hearing loss?
• DSP in hearing aids
• Dspfactorys Toccata Plus processor
• Dynamic Hearings strategies
• Feedback suppression
• Directional microphones
• The ADRO processing strategy

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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

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Hearing impairment reduces the dynamic range
Uncomfortable
Too loud
Loud
Comfortable
Soft
Very soft
Inaudible
Loudness growth for normal hearing
Example of a hearing loss
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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

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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 Dspfactorys Toccata
  Plus processor

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Dspfactorys 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

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Dspfactorys Toccata Plus processor
IOP (Input Output Processor)
RCORE (DSP Core Processor)
WOLA (Weighted Overlap-Add) Coprocessor
Input Stage
EEPROM
PreAmps, ADCs and sampling circuitry
Stores programs, fittings and calibrations
Output Stage
Executes assembler code to process time domain
samples from the IOP or frequency domain samples
from the WOLA
DACs and direct drive circuitry
Buffers samples from the input stage and going to
the output stage
Calculates FFT, applies gains and outputs data to
the IOP
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Feedback Suppression

• 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

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Feedback Suppression
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Directional Microphones

• 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.

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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)

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ADRO
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ADRO Architecture (Toccata Plus)
FFT
ADRO
FFT
DAC
ADC
Microphone
Output Device (Receiver)
64 Channels
64 Channels
Input Stage
IOP
RCore
WOLA
Output Stage
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Perceptual Dynamic Range
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Perceptual levels and output targets
Listeners dynamic range
Maximum Output Level
Hearing aid output range
Comfort Target
Audibility Target
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Percentiles 75 dB speech
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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 30th 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)

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ADRO Architecture

• 90th percentile gt comfort?

Decrease the Gain
Comfort Rule
Comfort Rule
30th percentile lt audibility?
Increase the Gain
Audibility Rule
Gain gt maximum gain?
Limit the Gain
Maximum Gain Rule
Magnitude gt maximum?
Limit the Output
Maximum Output Rule
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ADRO Processing Example of gain adjustments for
a real world situation

• ADRO with noadditional noise reduction.
• The graph shows theADRO gains varyingin real
  time.

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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
  signals dynamic range in each channel
• Varies the gain in each channel so that the
  dynamic range of the output signal is matched to
  the users requirements

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Technical Comparison
ADRO
Compression
Female speaker with 4-talker babble at -10 dB
signal-to-noise ratio.
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ADRO Architecture (Toccata Plus)
Apply Gain
Apply Volume Control Gain
Limit to Max Output Level
64 Channels out
64 Channels in
Adjust Gain values
Calculate Magnitudes
Calculate Percentiles
Slowly adapting values
WOLA
RCore
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What do users think of ADRO?
In a blind clinical trial, the standard Hearing
Aid Measure of Contrast questionnaire (Dillon
1994) showed ADRO was preferred to compression.
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• 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

• Dynamic Hearing provides customisation and
  technology support
• Simple and effective solution for the OEM
  customers

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• ADRO awarded the CRC Association
  commercialization award (shared with Cochlear
  Ltd),

• Federal Government START Grant awarded to
  accelerate product development in headset and
  communication device research

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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
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• COMMUNICATION DEVICES
• Toccata Plus
• Open platform DSPs
• Headsets

ADRO - Clinically Proven - Customisable -
Embedded software
AUDIOLOGIST - Product education - Training -
Support - Australian Clinic
HEARING IMPAIRED PEOPLE - Better hearing - Cost
effective solutions