ECE2025_CochlearBackgroundLecture - The Coding of Sound by...

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Unformatted text preview: The Coding of Sound by a Cochlear Prosthesis ECE 2025 Background Lecture Voiced Speech Signal • Voiced speech is periodic • Unvoiced signals are non-periodic and much noisier Propagation of Sound • Longitudinal pressure waves • Transmitted through a medium equally in all direction • Propagated by molecular interactions in the medium – Rarefactions: low pressure regions, particles are pulled apart – Compressions: high pressure regions, particles pushed together Propagation of Sound – Rarefactions (R): low pressure regions, particles are pulled apart – Compressions (C): high pressure regions, particles pushed together C C C R R R R Characteristics of a Sound Wave Amplitude – Corresponds to loudness – Measured in decibels (dB) Frequency – Corresponds to pitch – Middle C is 261.626Hz Amplitude 1/frequency Measure of Loudness Ratio – Ratio of sound pressure level X(t) relative to a standard level X0 – X0 is typically very soft, just on the threshold of hearing – Rather than using an instantaneous measure, an average measure over some small interval of time is used Bels = log X2 2 X0 – Since humans can only hear sound separated by 10Bels, we use a Decibel scale to measure sound Decibels = 10 • log X2 2 X0 The Peripheral Auditory System Auditory nerve Cochlea Outer Ear Middle Ear Inner Ear COCHLEA The Organ of Corti Hair cells mechanical-toelectrical transducer Frequency To Place Mapping: Tonotopic arrangement of the Basilar membrane The Cochlea as a Frequency Analyzer • The cochlea serves to separate the frequencies in the sound signal—the frequency spectrum • The cochlea’s tonotopic mapping • Discovered by Georg von Békésy: 1961 Nobel Prize in Physiology or Medicine • Youtube demonstration: Sensorineural Hearing Loss • Hair cells fail as the mechanical-to-electrical transducers • Causes: – – – – – Congenital defects Prenatal infections Head trauma Excessive noise Antibiotics (aminoglycosides) • Intervention/Treatment: auditory nerves respond to direct electrical stimulation in the cochlea Sensorineural Hearing Loss Statistics • Disorders of the cochlea or cochlea nerve • Most common in adults over 65 years (23%) • Continues to degrade with age, typically lose high frequency hearing first • Sensorineural hearing loss in children can be detected early with screening • Sources: – National Institutes on Deafness and other Communication Disorders: – American Speech Language Hearing Association: Assessing Hearing Loss: Decibels • Logarithmic scale for measuring sound intensity • Relative increase by a factor of 10 • Decibel Ratings Examples – – – – – – Near total silence: 0dB Whisper: 1dB Normal conversation: 15dB Lawn mover: 90dB Rock concert or jet engine: 120dB Gunshot or firecraker 140dB • Source: Assessing Hearing Loss: The Audiogram A Cochlear Prosthesis - Provides the sensation of sound through electrical stimulation of the auditory nerve. - 120,000+ patients implanted worldwide. - Implantation age: 6mos. – 90+ years. - Scores of nearly 100% on speech recognition tests in quiet. - Most successful neural prosthesis to date. Cochlear Prosthesis Components 5. Receiver/stimulator 6. Electrode array 7. Vestibulocochlear nerve Images courtesy of Citizen Airman Cochlear Implant System A Contemporary Array Implanted Receiver Stimulator Chip scale packaging Case: titanium Coil: platinum Electrode: platinum Overmolding: silicone rubber External Components Inductive transmitting coil Microphones Electronic assembly Controls Replaceable Batteries Display The Sound Processor Sound Processing • 16 - 22 band filterbank • Mapping onto electrode array at appropriate level and position in cochlea • Output to receiver-stimulator Example of Filterbank Speech Processing (Cochlear Pty. Limited) Speech waveform ah /a/ Waveform constantly passed through filterbank 22 band filterbank Filterbank output sample 6 largest maxima mapped to electrode array Stimuli sent to electrodes time Sound Processor Chip (Integrated Circuit) •5 x 5 mm chip size • 8,000,000 transistors •Operates at < 1 Volt • Up to 180 million operations per second • Multi-processor architecture •0.18μm CMOS technology Sound Processor Block Diagram LCD, Buttons & Progra mming POD Micro-Controller 8 Mbit Flash Memory DSP 1 Audio Inputs (dual mic, telecoil, accessories) 16-bit ADC 16-bit ADC 16-bit ADC Beamformer ASC Whisper DSP 2 Filterbank (FFT, FIR, IIR, …) DSP 3 Stimulus Control DSP 4 Spare RF Link Encoder To transmit coil Spectrogram “choice” Electrodogram “choice” Cochlear Implant: Speech Coding Diagram internal external Frequency Bands: 16 Bandpass Filters Bandpass Filtering vp(t) is separated into it’s frequency components Frequency Domain: Cosine Wave Spectrum 250 500 750 1000 1500 2000 Frequency (Hz) Full-Wave Rectification The absolution value of the signal is determined Acoustic Simulation: Speech Coding Diagram audio output Cochlear Prosthesis Summary 1. Microphone 2. Speech processor 3. Cable to transmitter 4. Transmitter coil 5. Receiver/ stimulator 6. Electrode Array 7. Cochlear nerve ...
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This note was uploaded on 01/27/2011 for the course ECE 2025 taught by Professor Juang during the Spring '08 term at Georgia Institute of Technology.

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