Signal Processing for Hearing Aids: Challenges and Solutions
This workshop presentation covers speech and hearing topics such as excitation sources, phonemic features, and hearing mechanisms. Explore challenges and solutions in signal processing for hearing aids.
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Presentation Transcript
Workshop Radar and Sonar Signal Processing, NSTL Visakhapatnam, 17-21 Aug 2015 Coordinator: Ms. M. Vijaya < vijaya.m @ nstl.drdo.in > Session: 21 Aug 2015, 1100 to 1230 ============================================================================ Signal Processing for Hearing Aids: Challenges and Some Solutions P. C. Pandey IIT Bombay
Outline A. Speech & Hearing B. Sliding-band Dynamic Range Compression (Ref: N. Tiwari & P. C. Pandey, NCC 2014, Paper No.1569847357) C. Automated Modification of Consonant-Vowel Ratio of Stops (Ref: A. R. Jayan & P. C. Pandey, Int. J. Speech Technology, vol. 18, pp. 113 130, 2015) 2/15
P. C. Pandey, "Signal processing for hearing aids: Challenges and some solutions, Invited talk, Workshop Radar and Sonar Signal Processing, NSTL Visakhapatnam, 17-21 Aug 2015 Workshop Coordinator: Ms. M. Vijaya < vijaya.m @ nstl.drdo.in > Session: 21 Aug 2015, 1100 to 1230 ======================================================================================== Part A Speech & Hearing 3/15
Speech Production Excitation source & filter model Excitation: voiced/unvoiced glottal, frication Filtering: vocal tract filter 4/15
Speech segments Words Syllables Phonemes Sub-phonemic segments Phonemes: basic speech units Vowels: Pure vowels, Diphthongs Consonants: Semivowels, Stops, Fricatives, Affricates, Nasals /ada/ /aba/ /aga/ /apa/ 5/15
Phonemic features Modes of excitation Glottal: Unvoiced (constriction at the glottis), Voiced (glottal vibration) Frication: Unvoiced (constriction in vocal tract), Voiced (constriction in v.t. & glottal vibration) Movement of articulators Continuant (steady-state v.t. configuration): vowels, nasal stops, fricatives Non-continuant (changing v.t.): diphthongs, semivowels, oral stops (plosives) Place of articulation (place of maximum constriction in v.t.) Bilabial, Labio-dental, Linguo-dental, Alveolar, Palatal, Velar, Gluttoral Changes in voicing frequency (Fo) Supra-segmental features: Intonation, Rhythm 6/15
Hearing Mechanism Peripheral auditory system External ear: sound collection Pinna Auditory canal Middle ear: impedance matching Ear drum Middle ear bones Inner ear (cochlea): analysis & transduction Auditory nerve: transmission of neural impulses Central auditory system Information processing & interpretation 7/15
Auditory system Tonotopic map of cochlea 8/15
HearingImpairment Types of hearing losses Conductive Sensorineural Central Functional Sensorineural hearing loss Associated with abnormalities in the cochlear hair cells or the auditory nerve. Causes: aging, excessive noise exposure, infection, adverse effect of medicines, congenital. 9/15
Effects of sensorineural hearing loss Elevated hearing thresholds: inaudibility of low-level sounds Reduced dynamic range & loudness recruitment (abnormal loudness growth): distortion of loudness relationship among speech components Increased temporal masking: poor detection of acoustic landmarks Increased spectral masking (widening of auditory filters): reduced ability to sense spectral shapes >> Poor intelligibility and degraded perception of speech, particularly in noisy environment. 10/15
Signal Processing in Hearing Aids Currently available techniques Frequency selective amplification: improves audibility but not necessarily intelligibility Automatic volume control: not effective in improving intelligibility Multichannel dynamic range compression (with settable attack & release times, compression ratios): effectiveness reduced due to processing artifacts 11/15
Techniques under development Noise suppression Distortion-free dynamic range compression Techniques for reducing the effects of increased spectral masking o Binaural dichotic presentation o Spectral contrast enhancement o Multi-band frequency compression Improvement of consonant-to-vowel ratio (CVR): for reducing the effects of increased temporal masking 12/15
Analog Hearing Aids Pre-amp AVC Freq. Response Amp. Digital Hearing Aids Pre-amp & AVC ADC Existing Problems Poor intelligibility in noisy environment & reverberation Distortions due to multiband amplitude compression Poor speech perception due to increased spectral & temporal masking Visit to audiologist for change of settings Multi-band Amplitude Compr. & Freq. Resp. DAC & Amp. 13/15
Proposed Hearing Aids Distortion-free dynamic range compression & adjustable frequency response Noise suppression & de-reverberation Processing for reducing the effects of increased spectral masking Processing for reducing the effects of increased temporal masking Implementation of signal processing using a low-power DSP chip with acceptable signal delay (< 60 ms) User selectable settings 14/15
Some Solutions for Improving Speech Perception by Listeners with Moderate-to- Severe Sensorineural Loss Sliding-band dynamic range compression as a solution to the problem posed by loudness recruitment Automated modification of consonant-vowel ratio of stop consonants as a solution to the problem posed by increased intraspeech spectral and temporal masking. Implementation using a 16-bit fixed-point DSP processor & testing for satisfactory operation. 15/15
