Senior Design

Fall 06 and Spring 07

Speech Strategy for the Cochlear Implant

Outline

• Sound and Speech

• The Biological Auditory System

• The Electrical System of the Cochlear Implant

• Challenges Facing the Cochlear Implant Users

• Senior Design Task

Fourier Series

Time Domain

100 Hz

200 Hz

300 Hz

400 Hz

100 200 300 400

Time

Frequency

Frequency Domain

Rel

ativ

e A

mpl

itud

e (d

B)

20

500 1500 2500

Transfer FunctionResonance of Acoustic Tube

17 cm

Freq.

Rel

ativ

e A

mpl

itud

e (d

B)

OutputProduct of Source and Transfer Functions

Freq.

Rel

ativ

e A

mpl

itud

e (d

B) Source Function

Glottal Pulses

Freq.1000 2000 3000

fo=100Hz

F1

F2

F3

Sound wave at the lips and at the glottis for the letter “O”.

Speech

Spectrogram

Bab Dad Gag

TympanicMembrane

OssiclesBones

BasilarMembrane

SpiralGanglion Cells

&Cochlear nerve

Auditory Pathways&

Auditory Cortex

Inner Hair Cells

SoundS(t)

Biological Hearing SystemCochlear

NT(t) = Glutamate

Oval Window

Frequency of Sound (Pitch)

Apex

Inner Hair Cells

Base

Basilar MembraneHard bone

Auditory nerves

SpeechProcessor

Microphone

Spiral Ganglion Cells &

Cochlear nerve

Auditory Pathways&

Auditory Cortex

Electrodes

Cochlear ImplantSound S(t)

EP(t)

Electrical and Biological Hearing System

Speech Processor

Speech Strategies

• Continuous Interleaved Sampling (CIS)

• n-of-m and Advanced Combination Encoder (ACE)

• Spectral Peak (SPEAK)

• Simultaneous Analog Stimulation (SAS)

• High Resolution (HiRes)

Sound Simulation

• Increase channels 1 > 2 > 4 > 8 > 16 > 32 > Original• Decrease Channels Original > 38 > 16 > 8 > 4 > 2 > 1• Song 4 > 8 > 16 > 32 > Original• Music > 4 > 8 > 16 > 32 > Original

4000HzfnLf1L f2L f3L f4L f5L

fnCf1C f2C

BPF1

BPF2

BPF n

Rect./LPF1

Rect./LPF2

Rect./LPF n

Sin(2πf1Ct)

Sin(2πf2Ct)

Sin(2πfnCt)

InputSpeechSignal

OutputSpeechSignal

Find the Optimum number of Channels

Senior Design Tasks• Learn the mechanism of human speech production and the

important components of speech for speech intelligibility.• Learn the mechanism of human hearing system.• Search the literature for the current speech strategies and

processors. You may need to contact manufacturers for the speech strategies of their speech processor.

• Design and simulate a speech processor with the optimum number of channels to achieve speech intelligibility in a quite environment.

• Test your design with a speech in a noisy environment. If speech intelligibility deteriorate then modify your design to improve speech intelligibility.

• Build your speech processor and test it with a real speech signal in both quite and noisy environments.

• Investigate the effects of channel interactions.

Skills Needed for the Cochlear Implant Project• Ability to read the literature.• Ability to use Matlab and Simulink to run circuit

simulations.• Ability to use Fourier analysis and software to analyze

the frequency components of speech.• Ability to build electrical filters, multiplier, summer,

and rectifier.• Organization, determination, and team work ethic.