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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 /aba/ /apa/ /ada/ /aga/ 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 Hearing Impairment Types of hearing losses • Conductive • Central • Sensorineural • 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 → Multi-band Amplitude Compr. & Freq. Resp. → DAC & Amp. 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 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-toSevere 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