09 March 2013

1. Speech Processing for Persons with Moderate Sensorineural Hearing Impairment Prem C. Pandey EE Dept., IIT Bombay pcpandey @ ee.iitb.ac.in www.ee.iitb.ac.in/~pcpandey, www.ee.iitb.ac.in/~spilab Second International Conference on Intelligent Interactive Technologies and Multimedia (IITM 2013), 09-11 March 2013, Allahabad, India 09 March 2013

2. Outline A. Speech & Hearing B. Noise Suppression S. K. Waddi, P. C. Pandey, N. Tiwari Speech Enhancement Using Spectral Subtraction and Cascaded Median Based Noise Estimation for Hearing Impaired Listeners (Proc. NCC 2013, Delhi, 15-17 Feb. 2013, Paper 3.2_2_1569696063) C: Reducing the Effect of Increased Spectral Masking N. Tiwari, P. C. Pandey, P. N. Kulkarni Real-time Implementation of Multi-band Frequency Compression for Listeners with Moderate Sensorineural Impairment (,Proc. Interspeech 2012, Portland, Oregon, 9-13 Sept 2012, Paper 689)

3. Speech Production Mechanism Excitation source & filter model • Excitation: voiced/unvoiced glottal, frication • Filtering: vocal tract filter

4. Speech segments • • Words• Syllables• Phonemes• Sub-phonemic segments • Phonemes: basic speech units • • Vowels: Pure vowels, Diphthongs • • Consonants: Semivowels, Stops, Fricatives, Affricates, Nasals • /aba/ • /ada/ • /aga/ • /apa/

5. Phonemic features • Modes of excitation • Glottal • Unvoiced (aspiration, constriction at the glottis) • Voiced (vibration of vocal chords) • Frication • Unvoiced (constriction in vocal tract) • Voiced (constriction in vocal tract & glottal vibration) • Movement of articulators • Continuant (steady-state vocal tract configuration): vowels, nasal stops, fricatives • Non-continuant (changing vocal tract): diphthongs, semivowels, oral stops (plosives) •  Place of articulation (place of maximum constriction in vocal tract) • Bilabial, Labio-dental, Linguo-dental, Alveolar, Palatal, Velar, Gluttoral •  Changes in voicing frequency (Fo) • Supra-segmental features • • Intonation • Rhythm  

6. Hearing Mechanism • Peripheral auditory system • External ear (sound collection) • Pinna • Auditory canal • Middle ear (impedance matching) • Ear drum • Middle ear bones • Inner ear (analysis and transduction): cochlea • Auditory nerve (transmission of neural impulses) • Central auditory system • Information processing & interpretation

7. Auditory system • Tonotopic map of cochlea

8. Hearing impairment Types of hearing losses • Conductive loss • Sensorineural loss • Central loss • Functional loss Sensorineural hearing loss • Elevated hearing thresholds Reduced intelligibility as speech components are inaudible • Reduced dynamic range & loudness recruitment (abnormal loudness growth) Distortion of loudness relationship among speech components • Increased temporal masking Poor detection of acoustic events • Increased spectral masking (due to widening of auditory filters) • Reduced frequency selectivity • Reduced ability to sense spectral shapes of speech sounds >> Poor intelligibility and degraded perception of speech

9. Signal processing in hearing aids Currently available • Frequency selective amplification Improves audibility but may not improve intelligibility in presence of noise • Automatic volume control • Multichannel dynamic range compression(settable attack time, release time, and compression ratios) Compresses the natural dynamic range into the reduced dynamic range Under Investigation • Improvement of consonant-to-vowel ratio (CVR): for reducing the effects of increased temporal masking • Techniques for reducing the effects of increased spectral masking: Binaural dichotic presentation, Spectral contrast enhancement, Multi-band frequency compression • Noise suppression

10. Analog Hearing Aids Pre-amp → AVC → Selectable Freq. Response → Amp. • Programmable Digital Hearing Aids Pre-amp → AVC → Multi-band Amplitude Compression & Freq. Response → Amp. • Major Problems • 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 • Proposed Hearing Aids (with user selectable settings) Pre-amp → AVC → Noise Suppression → Processing for Reducing the Effects of Increased Spectral Masking → Processing for Reducing the Effects of Increased Temporal Masking → Multi-band Amplitude Compression & Freq. Response → Amp.

11. Our Research Objectives • Developing techniques for improving speech perception by listeners with moderate-to-severe sensorineural loss • Reduction of effects of increased spectral masking Binaural aids: Binaural dichotic presentation using comb filters for spectral splitting Monoaural aids: Mutiband frequency compression • Reduction of spectral masking Enhancement of transient parts (weak & short but perceptually important ) • Noise Suppression • Implementation of the techniques using a low-power DSP chip for real-time operation and with acceptable signal delay (< 60 ms)

12. Our Research Objectives • Developing techniques for improving speech perception by listeners with moderate-to-severe sensorineural loss • Reduction of effects of increased spectral masking Binaural aids: Binaural dichotic presentation using comb filters for spectral splitting Monoaural aids: Mutiband frequency compression • Reduction of spectral masking Enhancement of transient parts (weak & short but perceptually important ) • Noise Suppression • Implementation of the techniques using a low-power DSP chip for real-time operation and with acceptable signal delay (< 60 ms)

13. P. C. Pandey (EE Dept, IIT Bombay): "Speech Processing for Persons with Moderate Sensorineural Hearing Impairment", Plenary talk, Second International Conference on Intelligent Interactive Technologies and Multimedia (IITM 2013), 09-11 March 2013, Allahabad, India Abstract Our objective is to develop techniques for improving speech perception by listeners with moderate-to-severe sensorineural loss and to implement these techniques using a low-power DSP chip for real-time operation and with acceptable signal delay (< 60 ms). Here we present two techniques to reduce the adverse effects of increased spectral masking associated with sensorimeural loss. The first technique reduces the effects of noise in the listening environment and the second one reduces the effects of increased intra-speech spectral masking. A spectral subtraction technique is presented for real-time speech enhancement in the aids used by hearing impaired listeners. For reducing computational complexity and memory requirement, it uses a cascaded-median based estimation of the noise spectrum without voice activity detection. The technique is implemented and tested for satisfactory real-time operation, with sampling frequency of 12 kHz, processing using window length of 30 ms with 50% overlap, and noise estimation by 3-frame 4-stage cascaded-median, on a 16-bit fixed-point DSP processor with on-chip FFT hardware. Enhancement of speech with different types of additive stationary and non-stationary noise resulted in SNR advantage of 4 – 13 dB. Widening of auditory filters in persons with sensorineural hearing impairment leads to increased spectral masking and degraded speech perception. Multi-band frequency compression of the complex spectral samples using pitch-synchronous processing has been reported to increase speech perception by persons with moderate sensorineural loss. It is shown that implementation of multi-band frequency compression using fixed-frame processing along with least-squares error based signal estimation reduces the processing delay and the speech output is perceptually similar to that from pitch-synchronous processing. The processing is implemented on a 16-bit fixed-point DSP processor and real-time operation is achieved using about one-tenth of its computing capacity.