SIGNAL PROCESSING IN HEARING AIDS October 2003

“An Australian research and technology company that develops new solutions for the expanding digital hearing aid and communications industry.”

A Venture Capital Backed Spin-Off from CRC HEAR Bionic Ear Institute

A new amplification scheme so that cochlear implants and hearing aids can be used more effectively together Electric hearing with Cochlear implant Acoustic hearing with Hearing aid

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.

Commercial hearing aids Digital Signal Processors for ADROTM Cochlear Implant Hearing Aid Research 1997-2002 Mipsy (desk-top) Audallion (wearable) DSP hearing aids Products SPrint™ Cochlear Ltd Commercial hearing aids

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

Excellent research partners Sound processing expertise Dynamic Team Audiologists Scientists Research reputation Excellent research partners Sound processing expertise Business Team Engineers

Advances in Hearing Aid Technology

Products and services ADRO™ ADRO is hearing aid software based on a revolutionary technology that significantly enhances hearing aid performance for hearing impaired people. ADROFit Fitting software for ADRO Premium hearing aid package ADRO plus: Feedback suppression Directional microphone Noise reduction 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

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)

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

A real life situation

Multi-band compression hearing aid Same hearing aid using ADROTM ADRO improves intelligibility in any background noise Multi-band compression hearing aid Same hearing aid using ADROTM

ADRO Significantly Improved Performance for Sentences in Babble

In the same clinical trial, ADRO improved performance for sentences in quiet 4.5% ** 11.9%*** 14.2%*** ** p<0.01 *** p<0.001

Multi-band compression hearing aid Hearing aid using ADROTM ADRO improves intelligibility at a distance. Multi-band compression hearing 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 Uncomfortable Too loud Loud Comfortable Soft Very soft Inaudible Loudness growth for normal hearing Example of a hearing loss

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 Input Stage IOP (Input Output Processor) RCORE (DSP Core Processor) WOLA (Weighted Overlap-Add) Coprocessor 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

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

Feedback Suppression Without Feedback Suppression Activated Amplitude Time With Feedback Suppression Activated Amplitude Time

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.

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

ADRO™

ADRO: Architecture (Toccata Plus) FFT ADRO FFT ADC DAC Microphone Output Device (Receiver) 64 Channels 64 Channels Output Stage WOLA RCore Input Stage IOP

Perceptual Dynamic Range

Perceptual levels and output targets Threshold Uncomfortable Listener’s dynamic range Maximum Output Level Hearing aid output range Comfort Target Audibility Target The perceptual dynamic range is define as the range between the threshold of hearing and the uncomfortable loudness level. For the ADRO strategy, we use only a section of the whole of the dynamic range. This is because the Audibility target is usually set above the threshold level unless the dynamic range is particularly narrow as found with severe and profound hearing losses, then the Audibility target will be placed on or sometimes slightly below the threshold level. At the top end, the Maximum Output Level is set conservatively at a loud level for each individual channel which leads to a greater overall loudness when all channels are working together. The loudness of the combination of channels depends on many factors, such as the level of the input signal, the maximum gain and the severity of the hearing loss. Perceptual levels and output targets

Percentiles: 75 dB speech

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)

ADRO™ Architecture 90th percentile > comfort? Decrease the Gain Comfort Rule Comfort Rule 30th percentile < audibility? Increase the Gain Audibility Rule Gain > maximum gain? Maximum Gain Rule Limit the Gain Magnitude > maximum? Maximum Output Rule Limit the Output

ADRO™ Processing Example of gain adjustments for a real world situation ADRO™ with no additional noise reduction. The graph shows the ADRO™ gains varying in real time.

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 ADRO™ Compression Speech outputs for ADRO™ and compression hearing aids: ADRO™ Compression Female speaker with 4-talker babble at -10 dB signal-to-noise ratio.

ADRO™ Architecture (Toccata Plus) Apply Gain Limit to Max Output Level Apply Volume Control Gain 64 Channels out 64 Channels in Adjust Gain values Calculate Percentiles Calculate Magnitudes Slowly adapting values RCore WOLA

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.

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

Awards 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

Track record 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

The Future ADRO - Clinically Proven - Customisable - Embedded software COMMUNICATION DEVICES Toccata Plus Open platform DSPs Headsets AUDIOLOGIST - Product education - Training - Support - Australian Clinic HEARING IMPAIRED PEOPLE - Better hearing - Cost effective solutions