W6 Sound Processing Flashcards
Define “Psychophysics”
How the perception of a stimulus depends on its physical parameters
Example: Loudness
Compare the Physical and Perceptual attributes of loudness of acoustic pure tones.
Physical attribute:
- Pressure P in Pa
- Reference sound pressure PO = 20 uPa
- Sound Pressure Level (SPL) in dB = 20 log (P / PO)
Perceptual attribute:
- Loudness L in sones
- 1 sone = loudness of a 1000 Hz tone at 40 dB SPL
- Loudness power law: L ∝ P0.6
- 10 dB increase in SPL => doubling of loudness
Compare the Physical and Perceptual attributes of loudness of electrical pulse trains.
Physical attributes:
- Current I in microamps
- Phase width PW in microseconds
- Pulse rate in Hz
Perceptual attributes:
- Typically PW and pulse rate constant; current varies
- Loudness power law: L ∝ I2.7
- Nucleus CI:
- Current I is an exponential function of 8-bit current level c
- Log(loudness) is a linear function of current level
What are Threshold and Comfort levels?
- Threshold (T): the lowest current that is just audible
- Comfort (C): the highest current that is comfortable
- T and C vary between implant recipients and across electrodes
- Depends on:
- Number of surviving neurones
- Distance from electrode to neurones
Compare voiced and unvoiced speech sounds
Voiced
- all vowels, some consonants (e.g. b, d, m)
- periodic waveforms: Pitch (F0)
- intonantion (question/statement)
- tonal languages (e.g. Mandarin)
- singing
- formants (F1, F2): spectral peaks
- identify vowels
- most energy at low frequencies
Unvoiced
- some consonants (e.g. p, t, s, sh)
- unstructured spectrograms
- most energy at high frequencies
- lower amplitude
Compare Normal and Electrical Hearing in terms of:
- Spectral resolution
- Temporal resolution
- Dynamic range
Spectral resolution:
-
Normal
- Tonotopic - place pitch
- Vowel recognition
-
Electrical
- Stimulation site
- Want many channels
Temporal resolution:
-
Normal
- __Detect transients and changes in amplitude
- Consonant recognition
-
Electrical
- __Amplitude pattern on each channel
- Want high pulse rate on each channel
Dynamic range:
-
Normal
- > 100 dB between loudest bearable sounds and softest detectable sounds
-
Electrical
- Typically 10 dB between Comofrt level (C) and Threshold level (T)
Describe the 5 components involved in the Signal Path from the Microphone to the Implant
(microphone)>>
Front End
Filterbank
Sampling and Selection
Amplitude Mapping
RF Encoder
>>(implant)
Compare Omnidirectional, Directional and Dual microphone types
Omnidirectional:
- single port
- equally sensitive to all directions
Directional:
- Two ports
- Most sensitive to front
- Inherent pre-emphasis (boosts high frequencies)
- Used in older processors
Dual:
- DSP combines the two microphone signals
- Can vary the directionality
Describe the components of the Front End stage of the Signal Path
Pre-amplifier
- Microphone: 1 kHz tone at 74 dB SPL gives 3.5 mV peak
- Provides 20 dB gain
Analog-to-Digital Converter
- To allow Digital Signal Processing (DSP)
- Nucleus sound processors: 16 bits at 15 kHz
Automatic Gain Control
- To cope with different sound levels (e.g. user’s own voice 20 dB louder than friend’s)
Describe the components of the Filterbank stage of the Signal Path
Band-Pass Filter (BPF)
- Passes one band of frequencies
- Blocks other frequencies
Filterbank is a set of BPFs
- Divides the sound into N frequency bands
- CIS: typically 6 wide filters
- ACE: typically 22 narrow filters
- One filter allocated to each channel (electrode pair)
- Frequency spacing:
- Linear spacing below 1000 Hz
- Logarithmic spacing above 1000 Hz
Describe the components of the Sampling and Selection stage of the Signal Path
- Filterbank provides N channels output
- Implant performs sequential stimulation
- Processor needs to sample the channels
- Sound coding strategies:
- CIS
- ACE
- MP3000
- HiRes
- FSP
- HiRes120
What is CIS?
Continuous Interleaved Sampling
- First developed for Ineraid implant (Blake Wilson, 1991)
- 6 channels, percutaneous plug
- Samples each filter envelope at fixed rate
- Pulses interleaved across channels
- Typical channel pulse rates: 500 - 3500 pps
- total pulse rate - channel pulse rate * number of channels
What is ACE?
- Based on Spectral Maxima Sound Processor (SMSP) (Hugh McDermott, Uni Melbourne)
- Selects M largest filter bands (maxima) out of N channels at rate R (peak rate per channel)
- total rate = M*R
- CIS has M = N
Two benefits:
- Reduces total pulse rate –> saves power
- e.g. CIS vs ACE:
- CIS: 1000 pps * 20 channels = 20000 pps
- -> 50 us pulse period –> 20 us phase width
- ACE: 1000 pps * 10 maxima from 20 channels = 10000 pps
- -> 100 us pulse period –> 40 us phase width
- Double phase width –> half the current –> half the voltage rail
- Simple form of noise reduction
* If SNR is positive, omitting the smallest pulse improves the SNR
Describe the components of the Amplitude Mapping stage of the Signal Path
Each channel has:
- Channel gain
- Static: allows the spectral shape to be adjusted
- Automatic: vary gain based on signal (e.g. ADRO, ClearVoice, SNR-NR)
- Loudness Growth Function (LGF)
- Match electrical loudness to acoustic loudness
- Compresses the filter output
- T and C mapping
- Maps all sound between Threshold level (T) and Maxmimum Comfortable level (C)
(see image)
The ProTouch prosthetic hand (Motion Control Inc) is a neural-enabled hand prosthesis collaboration between Florida International University, Motion Control Inc and Cochlear Ltd.
Discuss the facts:
- who for?
- how controlled?
- limitations?
- project goals?
- For people with a hand amputation (below elbow)
- < 100,000 arm amputees in the US
- Presently controlled by a myoelectric system
- electrode pads in an elastic sleeve, worn on forearm, sense muscle tension
- Limitations:
- only one degree of freedom (open/close)
- awkward to control
- cannot feel how hard you’re gripping
Project Goals:
- Feel how hard you’re gripping
- sensor signals control stimulation of sensory (afferent) nerves
- Control prosthetic hand by thinking about it
- measure motor control (efferent) nerve firing using Neural Response Telemetry (NRT)
- Integrate with Nucleus implant system! (see image)
