Meena Ramani

04/12/06

EEL 6586 Automatic Speech Processing

Topics to be covered

Lecture 1: The incredible sense of hearing 1The incredible sense of hearing 1

Anatomy

Perception of Sound

Lecture 2: The incredible sense of hearing 2The incredible sense of hearing 2

Psychoacoustics

Hearing aids and cochlear implants

The incredible sense of hearing-2The incredible sense of hearing-2

“Behind these unprepossessing flaps lie structures of such delicacy that they shame the most skillful craftsman"

-Stevens, S.S. [Professor of Psychophysics, Harvard University]

How do we hear?

Threshold of Hearing

Equal loudness curves

The Bass Loss Problem

Rock music

Too lowno bass

Too hightoo much bass

Threshold variation with age

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Thresholds of hearing for normal & HI listeners

Normal hearingHearing impaired

The Audiogram

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Audiogram

Left EarRight Ear

The Audiogram (contd.)

Pure tone audiogram

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<20 dB HL is Normal Hearing

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Loudness Growth Curve

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Input level (dB SPL)

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LGOB loudness growth curve at 250 Hz

Normal hearingHearing impaired

Otoacoustic emissions

• The ear produces some sounds!– OHC-outer hair cell

• Used to test hearing for infants & check if patient is feigning a loss

Monoaural beats

If two tones are presented monaurally with a small frequency difference, a beating pattern can be heard

500 & 502 Hz 500 & 520 Hz

Interaction of the two tones in the same auditory filter

Waveform: 150 Hz + 170 Hz

Beating can also be heard when the tones are presented to different ears!

Beating arises from neural interaction

Only perceived if the tones are sufficiently close in frequency

500 Hz - left 520 Hz - right binaural

Binaural beats

The case of the missing fundamental

Telephone BW: 300-3400 Hz

How do we know the pitch?

Primary Auditory cortex

•Pitch sensitive neurons [Bendor and Wang, Nature 2005]

•Neuron responds to fundamental and harmonics

•What are the I/Ps to these neuron?

How do spikes represent periodic, temporal and spectral information?

Matlab code available

Feed it a wav file

Spits out PSTH

<post stimulus time histogram>

Auditory-periphery model

(Zhang et al. ~2001)

Critical bands

Equally loud, close in frequency

•Same IHCs

•Slightly louder

Equally loud, separated in freq.

•Different IHCs

•Twice as loud

Psychoacoustic experiments

Critical Band (cont.)

• Proposed by Fletcher• How to measure?

– S/N ratio vs noise BW • CB ~= 1.5mm spacing on BM• 24 such band pass filters

• BW of the filters increases with fc

• Logarithmic relationship – Weber’s law example

• Bark scale

Center Freq Critical BW

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Critical bands for HI

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4 kHz tuning curve for normal & HI listeners

MaskerNormal hearingHearing impaired

“You know I can't hear you when the water is running!”

MASKING

Frequency Masking

• Masking occurs because two frequencies lie within a critical band and the higher amplitude one masks the lower amplitude signal

• Masking can be because of broad band, narrowband noise, pure and complex tones

• Low frequency broad band sounds mask the most– Eg. Truck on road, water flowing

• Masking threshold– Amount of dB for test tone to be just audible in presence of noise

Temporal Aspects of Masking

• Simultaneous Masking• Pre-Stimulus/Backward/Premasking

– 1st test tone 2nd Masker

• Poststimulus/Forward/Postmasking– 1st Masker 2nd test tone

Simultaneous masking– Duration >200ms constant test tone threshold– Assume hearing system integrates over a period of 200ms

Postmasking– Decay in effect of masker for 100ms– More dominant

Premasking – Takes place 20ms before masker is on!!– Each sensation is not instantaneous , requires build-up time

• Quick build up for loud maskers• Slower build up for softer maskers

– Less dominant effect

Temporal Aspects of Masking (contd.)

Temporal masking for HI

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Desired-Masker tone separation (ms)

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Temporal resolution at 4 kHz for normal & HI listeners

Normal hearingHearing impaired

Meena Ramani

04/14/06

EEL 6586 Automatic Speech Processing

Normal Hearing

Sensorineural Hearing Loss

Mild to Severe Loss

[10 20 30 60 80 90] dB HL

Time (s)

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Cell phone speech for normal hearing

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Cell phone speech for SNHL

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What do the hearing impaired hear?

Facts on Hearing Loss in Adults

• One in every ten (28 million) Americans has hearing loss. 

• The vast majority of Americans (95% or 26 million) with hearing loss can have their hearing loss treated with hearing aids. 

• Only 6 million use HAs

• Millions of Americans with hearing loss could benefit from hearing aids but avoid them because of the stigma.

Types of Hearing aids

Behind The earIn the Ear

In the Canal Completely in the canal

Anatomy of a Hearing Aid

• Microphone• Tone hook• Volume control• On/off switch

• Battery compartment

Ear Mold Measurements

Hearing Aid Fitting

Acclimatization effect

Auditory cortex brain plasticity

Time for the HI to reuse the HF information: Acclimatization effect

How does this affect HA fitting?– Multiple fitting sessions– Initial fitting should be optimum one

So doc, what is the fitting methodology employed by the hearing aid company to compensate for my hearing loss?

Not-so-average Joe

(PhD EE/Speech person)

CO

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So, do you want your HA to:

1) Always be comfortably loud2) Equalize loudness across

frequencies3) Normalize loudness

…?

?

Which fitting methodology is the bestbest?

Existing HL compensation algorithms

Rationale Adhoc: Half Gain, POGO Make speech comfortable: NAL-R Loudness normalization: IHAFF, Fig 6 Loudness equalization: DSL

Hearing aid fittingalgorithms

Threshold-only Suprathreshold

NAL-R POGO HG Fig 6 IHAFF DSL

Sensorineural hearing loss [10 20 30 60 80 90] dB HLSpeech level= 65 dBA

Spectrograms and sound files

Normal hearing Hearing impaired HI with Linear gain

HI with DSL gain HI with RBC gain

Section Two

Speech Intelligibility

Objective MeasuresAI, STI

Speech Quality

Objective MeasuresPESQ

Subjective MeasuresMOS

Speech Intelligibility (SI): The degree to which speech can be understood

Performance metrics

Subjective MeasuresHINT

Speech Quality: “Does the speech match your expectations?”

Performance metrics (contd.)• Objective speech quality measure

– Perceptual Evaluation of Subjective Quality (PESQ)• Subjective speech quality measure

– Mean Opinion Score (MOS)• Subjective speech intelligibility measure

– Hearing In Noise Test (HINT)

Reference signal

Comparison signal

Score

Hearing In Noise Test (HINT)

Subjective listening experiments

Audiograms of the HI patients

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Left ear audiograms of the HI subjectsLocation:

Shands speech & hearing clinic

(sound proof booth)

Subjects:

15 HI people– PTA: 40-70 dB HL

15 normal hearing people

Tools used:

Matlab HINT and MOS GUIs

Subjective HINT and MOS scores for RBC:hearing impaired, cell phone speech

RBC has a 7 dB improvement in SI when compared to DSL

MOS scores reveal that RBC has a quality rating of ‘Good’

None HPF RBC NALR POGO HG NALRP DSL

1-Bad

2-Poor

3-Fair

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Algorithm

Ave. MOSs of 15 HI subjects

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Ave. HINT scores of 15 HI subjects

Subjective HINT and MOS scores for RBC:normal hearing, cell phone speech

RBC has a 12 dB improvement in SI when compared to DSL

MOS scores reveal that RBC has a quality rating of ‘Good’

Cochlear Implants

The first fully functional Brain Machine The first fully functional Brain Machine Interface (BMI)Interface (BMI)

Definition:

A device that electrically stimulates the auditory nerve of patients with severe-to-profound hearing loss to provide them with sound and speech information

Who is a candidate?

• Severe-to profound sensorineural hearing loss

• Hearing loss did not reach severe-to-profound level until after acquiring oral speech and language skills

• Limited benefit from hearing aids

• Worldwide:– Over 100,000 multi-channel implants

• At Univ of Florida:– Implanted first patient in 1985– Currently follow over 400 cochlear patients

CI statistics

Technical and Safety Issues

• Magnetic Resonance Imaging• Surgical issues

How does the Cochlea encode frequencies?

Example: New Freedom

CI characteristics

1. Electrode design – Number of electrodes, electrode configuration

2. Type of stimulation – Analog or pulsatile

3. Transmission link – Transcutaneous or percutaneous

4. Signal processing – Waveform representation or feature extraction

Signal processing

• Compressed Analog (CA)• Continuous Interleaved Sampling (CIS)• Multiple Peak (MPEAK )• Spectral Maxima Sound Processor (SMSP)• Spectral Peak (SPEAK)

Compressed Analog (CA) approach

CA activation signals

Continuous Interleaved Sampling (CIS)

CIS activation signals

Multiple Peak (MPEAK)

MPEAK activated electrodes

Spectral Maxima Sound Processor (SMSP)

SMSP activated electrodes

Spectral Peak (SPEAK)

SPEAK activated electrodes

Outcomes for Post-lingual Adults

• Wide range of success

• Most score 90-100% on AV sentence materials

• Majority score > 80% on high context

• Performance more varied on single word tests

Auditory Brainstem Implant

• Approved October 20, 2000• Uses the Nucleus 24 system

processors• Plate array with 21 electrodes

Review-1Pinna:

ITDs,IIDs: Horizontal localizationReflections: Vertical localization

Ear canal:¼ wave resonance 1-3 kHz

Middle ear:Amplification by lever action and by areaStapedius reflex

Cochlea:IHCs/OHCs: convert mechanical to electricalPlace theory: frequency analysisMissing fundamental

Review-2

Adaptation: AN firing sensitive to changes

Otoacoustic emissions:Produced by movement of OHCs

Beats:Monaural & binaural

Measurement of hearing:Audiogram: threshold of hearingThreshold variation with ageEqual loudness curves

Bass loss problem: discrimination against LFs

Review-3

Critical bands:used for efficient encodingBark scale

Masking:Frequency: LFs mask moreTemporal: simultaneous, pre and post

Hearing impairment:Hearing aids: external to cochleaCochlear implants: inside cochlea