Download Signal Processing for Hearing Aids - EE-IITB

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
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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)
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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
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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.
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