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SIGNAL PROCESSING IN HEARING AIDS

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SIGNAL PROCESSING IN HEARING AIDS

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    1. SIGNAL PROCESSING IN HEARING AIDS October 2003

    5. 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.

    12. 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

    14. A real life situation

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

    19. TECHNICAL ASPECTS

    20. 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

    21. 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

    22. Hearing impairment reduces the dynamic range

    23. 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

    24. 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

    25. Dspfactory’s Toccata Plus processor Small size Power consumption ~ 600 ľA 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

    26. Dspfactory’s Toccata Plus processor

    27. 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

    28. Feedback Suppression

    29. 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

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

    31. ADRO™

    32. ADRO: Architecture (Toccata Plus)

    33. Perceptual Dynamic Range

    34. Perceptual levels and output targets 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. 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.

    35. Percentiles: 75 dB speech

    36. 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)

    37. ADRO™ Architecture 90th percentile > comfort?

    38. ADRO™ with no additional noise reduction. The graph shows the ADRO™ gains varying in real time.

    39. 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

    40. Technical Comparison

    41. ADRO™ Architecture (Toccata Plus)

    42. What do users think of ADRO™?

    43. 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

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