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Transcript
The Coding of Sound by a Cochlear Prosthesis IEEE Real World Engineering Project Background Lecture 1 Topic Concepts Tasks/Challenge Course time Speech generation from a sound source Voltage representation Periodicity Signal construction 10% Propagation of sound Pressure waves Sound measure Decibels scale construction 5% Auditory system: form & function Peripheral system Central system Amplification Impedance Filtering Perception Spectral decomposition by the basilar membrane Audiogram 35% Cochlear Prosthesis Components Microphone Preemphasis filter Bandpass filter bank Frequency response High pass filter Low pass filter Bandpass filter Bode plot Modification of the pre50% emphasis filter Exercising of bandpass filter bank Explore relationship between speech perception and system complexity, repeating for music 2 Voiced Speech Signal • Voiced speech is periodic • Unvoiced signals are non-periodic and much noisier 3 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 4 Propagation of Sound – Rarefactions (R): low pressure regions, particles are pulled apart – Compressions (C): high pressure regions, particles pushed together C R 5 C R C R R Characteristics of a Sound Wave Frequency Amplitude – Corresponds to loudness – Measured in decibels (dB) Amplitude 1/frequency 6 – Corresponds to pitch – Middle C is 261.626Hz The Peripheral Auditory System Auditory nerve Cochlea Outer Ear 7 Middle Ear Inner Ear COCHLEA The Organ of Corti Hair cells 8 mechanical-toelectrical transducer Frequency To Place Mapping: Tonotopic arrangement of the Basilar membrane 9 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: h-p://www.youtube.com/watch?v=dyenMluFaUw&feature=related 10 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 11 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: http://www.nidcd.nih.gov/ – American Speech Language Hearing Association: http://www.asha.org 12 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: http://www.hyperacusis.net/hyperacusis/decibel+guide 13 Assessing Hearing Loss: The Audiogram 14 A Cochlear Prosthesis - Provides the sensation of sound through electrical stimulation of the auditory nerve. - 100,000+ patients implanted worldwide. - Nearly 1/2 of implantees are children. - Scores of nearly 100% on speech recognition tests in quiet. - Most successful neural prosthesis to date. 15 Cochlear Prosthesis Components 5. Receiver/stimulator 6. Electrode array 7. Vestibulocochlear nerve Images courtesy of Citizen Airman 16 A Contemporary Array Image courtesy of Cochlear Ltd. 17 Cochlear Prosthesis Overview 1. Microphone 2. Speech processor 3. Cable to transmitter 4. Transmitter coil 5. Receiver/ stimulator 6. Electrode Array 7. Cochlear nerve 18 Cochlear Implant: Speech Coding Diagram 19 Gain Gain Basic Ideal Filters: Low & High Pass passband stopband fC cutoff frequency Low Pass Filter 20 stopband Frequency (Hz) passband fC cutoff frequency High Pass Filter Basic Ideal Filters: Bandreject (Notch) Gain passbands stopband fC1 21 fC2 Frequency (Hz) Basic Ideal Filters: Bandpass stopbands Gain passband bandwidth fC1 22 fC2 Frequency (Hz) Gain (dB) Real Filters: Bode Plot (Magnitude) 0dB -3dB bandwidth fC1 23 fC2 Frequency (Hz) log scale Acoustic Simulation: Speech Coding Diagram cos(2πfc1t+θ1) audio output cos(2πfc2t+θ) cos(2πfc16t+θ) 24 Frequency Bands: 16 Bandpass Filters 156‐276 Hz, fc =216 Hz 276‐411 Hz, fc = 343 Hz 411‐562 Hz, fc = 486 Hz 562‐732 Hz, fc = 647 Hz 732‐924 Hz, fc = 828 Hz 924‐1139 Hz, fc = 1031 Hz 1139‐1381 Hz, fc = 1260 Hz 1381‐1655 Hz, fc = 1518 Hz 1655‐1962 Hz, fc = 1808 Hz 1962‐2307 Hz, fc = 2134 Hz 2307‐2696 Hz, fc = 2501 Hz 2696‐3133 Hz, fc = 2914 Hz 3133‐3624 Hz, fc = 3378 Hz 3624‐4178 Hz, fc = 3901 Hz 4178‐4800 Hz, fc = 4489 Hz 4800–5500 Hz, fc = 5150 Hz 25 Bandpass Filtering vp(t) is separated into it’s frequency components vp(t) 1.0cos(2π250t) 0.5cos(π/2‐2π500t) 1.0cos(2π750t) 0.5cos(π/2‐2π1000t) 0.75cos(2π1500t) 1.0cos(π/2‐2π2000t) msec 26 Frequency Domain: Cosine Wave Spectrum 250 500 750 1000 27 1500 2000 Frequency (Hz) Full-Wave Rectification The absolution value of the signal is determined |vp(t)| vp(t) msec 28 Real World Engineering Project Outline • The goal is to take you through the basic components of a cochlear implant signal processor • Filter basics GUI in LabVIEW • Cochlear Implant simulation in LabVIEW • Explore real world engineering design challenges faced by cochlear implant designers 29 select filter type specify cutoffs 30 select filter type specify upper cutoff 31 run specify .wav file select channel count 32 Cochlear Signal Processor Channel Count GUI indicates a linked audio clip (takes a few seconds to load) from http:// users.ece.gatech.edu/~pamelab/filename.wav Audio Clip (source) Fur Elise (instrumental) 2 FurElise.wav Cochlear_Implant2 -music.wav Cats & dogs (voice) 2 catsdogs.wav Row (vocals & music) BOAT.WAV 33 Channels 4 4 Cochlear_Implant2 .wav 2 8 16 Cochlear_Implant8 -music.wav Cochlear_Implant16 -music.wav 8 16 Cochlear_Implant8 .wav 4 8 16 BOAT_4C.WAV BOAT_8C.WAV BOAT_16C.WAV Project Challenges • Investigate in teams of 3-4 students • Compare speech recognition vs. music appreciation • Consider how to improve cochlear implant performance in a variety of environments • Consider the cost and accessibility of cochlear implants • Present team results to class 34
