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Sound Coding Strategies for Cochlear-
Implants
Waldo Nogueira Auditory Prosthetic Group, Department of Otolaryngology, Medical University Hannover
Many Slides from Prof. Andreas Büchner and Tobias Rottmann
Slides and Materials available under: http://auditoryprostheticgroup.weebly.com/blog
Auditory Prosthetic Group
APG
Waldo Nogueira
Benjamin Krüger
Marina Frye
Florian Langner
APG, http://auditoryprostheticgroup.weebly.com
• Goals APG
– Improve speech intelligibility in noise and music perception with
cochlelar implants
• How? Two possibilities
– Technology: Modify the Cochlear Implant Sound Processor
– Art: Create contents specific for Cochlear Implant users
Auditory Prosthetic Group (APG)
APG, http://auditoryprostheticgroup.weebly.com
Characterize
the Electrode-
Nerve-Interface
Subjective
Responses
Psychophysical
Measures
Individualize
Parameteres
(Noise Reduction)
Goal: Optimize Individually
Speech Intelligibility and
Music Perception for CI users
Technology: Personalization
State-Of-the Art
Signal Processing
(Noise Reduction)
APG, http://auditoryprostheticgroup.weebly.com
APG:
Auditory Prosthetic Group
5
Research (H4A, Auditory Prosthetic Group) The group started in September 2013 as part of Excellence
Cluster Hearing4all
Covering fields Signal processing
Auditory modelling applied to implanted hearing protheses Cochlear implants and electroacoustic stimulation implants
Psychophysics
Electrophysiology
Department Otoloringology/German Hearing Center 30 years of cochlear implant experience
World largest cochlear implant programm (7000 Impl. 2013)
500 patients are implanted every year
Large spectrum of Hearing Systems Cochlear-Implants, Middle Ear Implants, Heraing Aids
ABI (auditory brainstem implant), AMI (auditory midbrain implant)
0
50
100
150
200
250
300
350
400
1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013
Patientenzuwachs an der MHH
Verlauf von 1984 bis 2013
(Erw=2858 Kinder=2294)
Erwachsene Kinder
Number of CI-Implantations at MHH
0
100
200
300
400
500
600
1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013
Implantzahlen nach Implanttyp
Verlauf von 1984 bis 2013
Nucleus=4107 AB=2081 Medel=726 Digisonic=3
N = 6944
Nucleus AdvBionics MedEl Digisonic Reimplantationen
-300 -200 -100 0 100 200 300
1
4
7
10
13
16
19
22
25
28
31
34
37
40
43
46
49
52
55
58
61
64
67
70
73
76
79
82
85
88
91
Age distribution at implantation time
Woman Man
Medizinische Hochschule
Hannover
8
Audiology
Surgery
(HNO)
DHZ (German Hearing
Center) APG
Contents
1. Introduction
2. Fundamentals of electric stimulation
3. Historical overview of sound coding strategies
4. Current sound coding strategies
5. Summary
The Cochlear Implant
Hybrid Hearing Aid
and
Cochlear Implant
The Cochlear Implant
The Cochlear-Implant – An electronic innear ear prosthesis
Implant Speech processor
Fa. Medel
Advanced
Bionics
USA
Neurelec/Oticon
France
MEDEL
Austria
Cochlear
Australia
CI-Manufacturers and current models at the MHH
Other CI-Manufactuters
Cochlear-Implantat-System: IES (= „I Enjoy Sound“)
Cochlear-Implant-System from a chinese manufacturer: Nurotron
Hearing performance with a cochlear-implant
0
10
20
30
40
50
60
70
80
90
100
FES HSM in quiet HSM im noise
% c
orr
ect
n=480
Duration of deafness < 10 Years
Newest generation implant (all
manufacturers)
Column of Volta (around 1800)
Alessandro Volta (1745 – 1824)
- Inserted in the ear canal a solution of salt water and
inserted a voltage through two electrodes of its own
developed battery
- Discovered the battery.
- Hearing perception described as: „Dense soup
being cooked“.
Alessandro Volta
Charles Eyriès
Charles Eyriès & André Djourno (1957)
First “Cochlear-Implantat”
im Humans
André Djourno
The otologist Eyriès and der physicist Djourno from Paris
placed electrodes on the trunk of the auditory nerve using
primitive electrodes with a primitive speech stimulator.
They were able to create hearing sensations.
Simple
induction coil
for implants
The implanted coil
William House (1923 – 2012), in 1970 (3M Implant)
William House Jack Urban (right) 3M Single-
Electrode Implant
Single Electrode Cochlear-Implantate
Amplifier
340 bis 2700 Hz
Bandpass Microphone Modulation Electrode
AM Carrier 16kHz
Single Electrode Cochlear-Implantate
Zeng 2004
Speech I
nte
lligib
ility
in Q
uie
t [%
] Speech Intelligibility
Multichannel Cochlear-Implant
Graham Clark (right) The first multichannel cochlear implant
Multichannel Cochlear-Implant
The goal of a speech processing strategy consists of decomposing the frequency,
time and loudness information of the acoustic sound to create electric signals that
stimulate the auditory nerve.
Clark Feature Extraction 1978
Multichannel Cochlear-Implant
The first speech processor for the multichannel cochlear implant system
Multichannel Cochlear-Implant
Multichannel Cochlear-Implant
The first body worn speech processor for the multichannel cochlear implant
system
Contents
1. Introduction
2. Fundamentals of electric stimulation
3. Historical overview of sound coding strategies
4. Current sound coding strategies
5. Summary
Sound capturing and transmission
Boenninghaus, Lenarz:
Hals-Nasen-Ohren-Heilkunde, 2007
Scala vestibuli
Scala tympani Ductus cochlearis
Elektrode
Standard (31 mm)
FLEXSOFT (31 mm)
FLEX28 (28 mm)
FLEX24 (24 mm)
FLEX20 (20 mm)
Medium (24 mm)
Compressed (15 mm)
Contour Advance
Slim Straight
Straight
Hybrid L24
Cochlear (22 Contacts) MEDEL (12 Contacts)
Advanced Bionics (16 Contacts)
Electrode arrays
Stimulation modes
Büchner (2004)
Ball
Electrode
Case
Electrode
Electrode Carrier
Stimulation mode
Monopolar Stimulation
- One intracochlear electrode is connected to an extracochlear reference
electrode such that the corrent flows between both electrodes.
- A simultaneous stimulation of multiple electrodes at the same time is not
desirable because undesirable channel interaction between the individual
electrodes would arise.
Büchner (2004)
Stimulation modes
Bipolar Stimulation
- Here the active electrode and the reference electrode are both
intracochlear.
- If the implants has multiple current sources it is possible to stimulate
multiple channels with moderate channel interaction.
- The more distant are the stimulating and the reference electrodes, the
louder is the hearing perception because a larger number of nerve fibers
is stimulated.
Büchner (2004)
Stimulation modes
Common Ground Stimulation
- Here one intracochlear electrode is stimulated against all other
intracochlear electrodes, which are used as reference electrodes.
- This stimulation mode has been substituted by the simple monopolar
mode which offers power savings without decreasing speech
performance.
Büchner (2004)
Stimulation with biphasic pulses
+
-
Pulse width - The amount of charge defines the
loudness (Pulse width and amplitude)
Am
plit
ude
- Stimulation rate (or. Pulse rate)
- The charge density has to be limited (it
depends on the injected charge and the
electrode dimensions)
Pulse distance Stimulationsrate
- Charge balanced pulse
Stimulation with biphasich pulses
+
Pulse width - The amount of charge defines the
loudness (Pulse width and amplitude)
Am
plit
ude
- Stimulation rate (or. Pulse rate)
- The charge density has to be
limited (it depends on the injected
charge and the electrode
dimensions)
- Charge balanced pulse
This can cause an irreversible reaction in the surrounding of the electrode,
including changes in the pH-value, release of part of the electrode material and
forming of metal-protein complexes.
-
Pulse distance Stimulation rate
Electrode-Impedance Interface
ρ : resistivity
: Dielectric constant
: Electric field constant
Current flow inside the cochlea
Klinke & Hartmann (1979)
Stromfluss innerhalb der Cochlea
Tang (2011)
Simulation of the voltage distribution in the cohlea
Nogueira (2014)
Monopolar Stimulation:
Acoustic vs. electric stimulation
Normal Hearing
Hearing with
CI
high
reduced high Synchronicity
Spontaneous activity
120 dB 12 dB Dynamic range
3500 IHC ~10 channels Frequency resolution
simultaneous coding Temporal stimulation pattern
Processing of new signals through the auditory system required
missing
narrow Spatial stimulation specificy wide
Contents
1. Introduction
2. Fundamentals of electric stimulation
3. Historical overview of sound coding strategies
4. Current sound coding strategies
5. Summary
t
Envelope
Hilbert 1912: Signal decomposition
in fast + slow oscilating components
Fine structure
Speech
Pitch & Localization
Acoustic Phonetic – Vocal formants
Jussen et al., 1994
F3
F2
F1
F0F2 (beginning of 1980‘s)
AGC
Pulse-
generator
1000 4000 Hz
F2 Zero-cross
Envelopes A2
270 Hz
F0 Zero-cross Pulse-
rate
/a/
F0 = 140 Hz
F0F2 (beginning of 1980‘s)
- The fundamental frequency of the speaker (F0 < 280 Hz) is used as stimulation
rate
- Selection of the stimulating electrodes depends on the frequency of the second
formant (between 800 und 4000 Hz)
- The stimulation current is proportional to the estimated amplitude of the 2.
Formant
- Only one electrode is stimulated for each stimulating cycle
- Cleary, this strategy is designed to encode speech signals
Characteristics of the F0F2-Strategy:
F0F1F2 (1985)
AGC
Pulse-
generator
280 1000 Hz
F1 Zero-cross
Envelope A1
1000 4000 Hz
F2 Zero-cross
Envelope A2
F0 Zero-cross Pulse-
rate
/a/
270 Hz
F0F1F2 (1985)
Characteristics of the F0F1F2-Strategy:
- The fundamental frequency of the speaker (F0 < 280 Hz) is used as stimulation
rate
- Electrodes 1 to 5 are used to transmit the 1. Formant (between 280 und 1000
Hz)
- Electrodes 6 to 20 are used to transmit the 2. Formant (between 1000 und 4000
Hz)
- Two electrodes are stimulated in each processing cycle
- The stimulation current is proportional to the 1. und 2. Formants
- Cleary, this strategy is designed to encode speech signals
MPEAK (End 1980‘s)
AGC
Pulse-
generator
300 1000 Hz
F1 Zero-cross
Envelope A1
800 4000 Hz
F2 Zero-cross
Envelope A2
270 Hz
F0 Zero-cross Pulse-
rate
2 2,8 kHz
Envelope
2,8 4 kHz
Envelope
4 6 kHz
Envelope Electrode 1
Electrode 4
Electrode 7
1 4
7
F1
F2
MPEAK (Ende 1980er)
Characteristics of the MPEAK-Strategy (Multipeak-Strategy):
- The strategy is based on the F0F1F2-Strategy (Useful to recognize vowels)
- Introduces three high frequency filters:
- The problem of erroneous feature extraction persists
Stimulation electorde Frequency region
7 2-2,8 kHz
4 2,8-4 kHz
1 4-4,7 kHz
Compressed Analog (CA)
Amplifier
100 700 Hz
Bandpass Michrophone Electrodes
700 1400 Hz
1400 2300 Hz
2300 5000 Hz
G1
G2
G3
G4
AGC
Characteristics of the CA-Strategy:
- Four band-pass filters between around 300 until 5000 Hz
- Simultaneous stimulation of four intracochlear electrodes
- Monopolar stimulation (later improved through bipolar stimulation)
- Better than a single electrode system
- Better performance than the F0F2 Strategy
- Problem: Strong channel interaction => Addition of neighbouring
channels
Compressed Analog (CA)
Zeng 2004
Speech inte
lligib
ility
in q
uie
t [%
] Speech intelligibility
CIS (Beginning of 1990‘s)
AGC
n
A1
A2
A3
1
2
3
An
Continuous Interleaved Sampling
CIS (Anfang 1990er)
Characteristics of the CIS-Strategy as implemented by the Advanced Bionics
Systems (Clarion):
- 8 Bandpassfilters from 250 unitl 5500 Hz
- 60 dB dynamic range => It is compressed/mapped into the patient specific
electric dynamic range
- Stimulation with biphasic squared pulses with a pulse width of 75µs/Phase
- The stimulation rate for each electrode is around 833 Hz
- It is based on a similar principle as the Vocoder from Dudley (see next slide)
The Vocoder from the Bell Laboratories (H. Dudley, 1939)
Bell Labs Records, Dec. 1939
10 Channel– Vocoder (H. Dudley 1939)
Example with fundamental frequency Example without fundamental frequency
Frequency bands as used by the MEDEL-Systems
Compression example as used by the MEDEL-Systems
120
0
Sound pressure
level [dB SPL]
Normal hearing / Acoustic Cochlear-Implantat / Electric
Current
ampltiude [µA]
THL: threshold level
MCL: most
comfortable level
100
400
Pre-processing
12 dB 120 dB 40-65 dB
Signal input in the pre-processing
SPEAK (Begining of 1990‘s)
AGC
20
A1
A2
A3
1
2
3
250 Hz
A20
10000 Hz
Selection
of the 5-
10 largest
A
Compression
Spectral Peak
„n-of-m“
Maxima selection
500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 5500 Frequenz [Hz]
500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 5500 Frequenz [Hz]
-5
0
10
20
26
20
10
/eh/
Inte
nsity [d
B]
Inte
nsity [d
B]
SPEAK (Begining of 1990‘s)
- Larger frequency range (250 until 10000 Hz)
- 20 channels
- Only the 5 to 10 channels with the largests energies (maxima) are
selected for stimulation.
(The amount of information to be transmitted to all electrodes would require too much time
reducing the stimulation rate)
- Stimulation rates up to 250 Hz (limited by the technology of the Mini-22
implant)
Characteristics of the Speak-Strategie:
Zeng 2004
Speech inte
lligib
ility
in q
uie
t [%
] Speech intelligibility
ACE (Mitte 1990er)
AGC
22
A1
A2
A3
1
2
3
250 Hz
A22
8000 Hz
Auswahl
der 1-16
größten A
Kompression
Advanced Combination Encoder
Modified SPEAK strategy
- more channels
- higher rates
Speech intelligibility
Zeng, Trends In Amplification 2004:8:1-34
Speech inte
lligib
ility
in q
uie
t [%
]
Er hat heute einen Arzttermin.
Ist das Flugzeug gestartet?
F0/F2 ACE
Example: F0/F2 and „ACE“
F0F2 (in noise)
Osciloscope picture taken with and 8
Channel CIS (AB)
833 pps
Detector box for the MEDEL-System
Output of implant circuits:
Example: CIS and „n of m“
Lower frequencies
Higher frequencies
How is the CI configured in the clinical routine?
Basis Parameter:
- Comfortable (C) Level: The maximum current level that is not
perceived as unpleasant.
- Threshold (T) Level: The lowest current level, that is just perceived.
- Dynamic Range: Difference between the C- and T- level
Low frequency High frequency
Electrode Array
Implant
Dynamic Range
C-Levels
T-Levels
DR = 19-40 CL
120
0
Sound pressure
level [dB SPL]
Normal hearing/ Acoustic Cochlear-Implantat / Electric
Current level
[µA]
THL: threshold level
MCL: most
comfortable level
100
400
Pre-processing
12 dB 120 dB 40-65 dB
Signal input and Pre-processing
Contents
1. Introduction
2. Fundamentals of electric stimulation
3. Historical overview of sound coding strategies
4. Current sound coding strategies
5. Summary
-ACE & PACE/MP3000 (Cochlear)
-HiRes, HiRes 120 & HiResOptima (Advanced Bionics)
-FSP & FS4 (Med-El)
Current Speech coding strategies
Problem of low bandwidth in a Cochlear-Implant System
Cochlear-Implantat System : 10 - 60 kbit/s
Electrode / Nerve
Interface
Audio-Signal
auditorisches
System ??
700 kBit
One possible solution: Relevant data reduction
Relevant
Signal components
Audio-Signal
Redundant
Auditory
System
Elektrode / Nerve
Interface
The multimedia industry has successfuly implemented psychoacoustic models
for many years.
Motivation for the use of a psychoacoustic masking model
MP3 Player Apple‘s iPod
Reduction of the data volume by 1/10 !
Other channel selection strategies
Simultaner Maskierungeffekt
Hörschwelle
in Ruhe
80
60
40
20
0
2 0 4 6 8 10 12 14
Frequenz [kHz]
Lauts
tärk
e [
dB
]
Hörschwelle bei Präsentation
eines Sinustons 4 kHz 60dB
ACE PACE MP3000 (2005)
AGC
22
A1
A2
A3
1
2
3
250 Hz
A22
8000 Hz
Selection
of 1-6
non-
masked
A
Compression
Nogueira et al., Eurasip, 2005
Speech intelligibility
37 CI-Centers, more than 200 Patients
Büchner et al., 2011
• MP3000 not better than ACE
• Significantly less current consumption than MP3000
„Spectral Contrast Enhancement“ in CI sound coding strategies:
Improvement of the spectral contrast
Nogueira et al., JASA, (2014)
Ma
gn
itu
de
[d
B]
Electrode
original enhanced
22 20 18 16 14 12 10 8 6 4 2
-130
-120
-110
-100
-90
Improvement of the spectral contrast
„Spectral Contrast Enhancement“ in CI Stimulation strategies:
Nogueira et al., JASA, (2014)
Original ACE SCE
-ACE & MP3000 (Cochlear)
-HiRes, HiRes 120 & HiResOptima (Advanced Bionics)
-FSP & FS4 (Med-El)
Current speech coding strategies
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Development stages for an 8 channel sytem of Advanced Bionics
Continuous Interleaved
Sampler – CIS Simultaneous Analogue
Stimulation – SAS
Multiple Pulsatile
Stimulation – MPS o. PPS
from CIS to HiRes
Stimulation rate [pps]
1970 1980 1990 2000 2010 2020
100 250 800 1500 3000 5000
t
A
Compliance-Limit
PW Charge Q = A0 PW
A0
Refractoriness: 1 ms = 1000 pps
0
10
20
30
40
50
60
70
80
90
100
FreiburgerMonosyllabics
FreiburgerNumbers
HSM in quiet HSM in noise
% c
orr
ec
t
SAS user (n=23)
standard mode High Resolution Mode
0
20
40
60
80
100
120
FreiburgerMonosyllabics
FreiburgerNumbers
HSM in quiet HSM in noise
% c
orr
ec
t
CIS user (n=21)
standard mode High Resolution Mode
0,00
20,00
40,00
60,00
80,00
100,00
120,00
FreiburgerMonosyllabics
FreiburgerNumbers
HSM in quiet HSM in noise
% c
orr
ec
t
PPS user (n=2)
standard mode High Resolution Mode
Improved Hearing Performance with HiRes
HiRes120 with „Current Steering“
16 Electrode-Contacts 15 current steering-Fields
16 Channels 120 Channels
1 16 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
HiRes120: HiRes:
HiRes120 with „Current Steering“
Current Steering Music examples
Original 16 Channels 64 Channels
Speech test results
HiRes vs. HiRes 120 => only minimally better than HiRes
Brendel et al., 2008
From HiRes120 to HiRes Optima
The current goal is to develop low power consumption sound coding
strategies keeping hearing performance:
RF Link
Reduction of the supply voltge of the implant from
around. 8V to 4V
Fitting of the pulse parameter to the new supply
voltage
Always use a virtual electrode (Current Steering)
HiRes Optima – Reduced Supply Voltage
Current Steering with HiRes Optima
HiRes 120 HiRes Optima
HiRes Optima
-ACE & MP3000 (Cochlear)
-HiRes, HiRes 120 & HiResOptima (Advanced Bionics)
-FSP & FS4 (Med-El)
Current speech coding strategies
Frequency analysis in the cochlea
Place principle: Periodicity principle:
Boenninghaus, Lenarz:
Hals-Nasen-Ohren-Heilkunde (2007) Hellbrück, Ellermeier: Hören (2004)
FSP
Channel-Specific Sampling
Sequences CSSS
Fine Structure Processing
Fa. Medel
CIS
MEDEL Fine structure-Strategy FSP
CIS-channels with
transmission of the envelepe
Stimulation pattern with individual electrode channels:
2 to 4 fine structure
channels (information up
to around 1000 Hz)
Fa. Medel
Speech test results
Müller et al., 2012
With FSP only minimally better than with CIS+/HDCIS
Hearing performance with a Cochlear Implant
0
10
20
30
40
50
60
70
80
90
100
FES HSM in Quiet HSM in Noise
% c
orr
ect
n=480
Duration of deafness < 10 years
Implant with the newest generation (all
manufacturers)
“An ocean of sounds…”
Alex Katz, The Cocktail Party, 1965.
Signal input and preprocessing
1. Signal input /
Mikrophone
technik
2. Signal pre-
processing
Example: Noise reduction
- Clear Voice (AB)
- Noise Canceller (AB & Phonak)
- Noise-Function (Cochlear)
Single channel noise reduction:
Clear Voice und Noise Canceller
• Single-microphone-system (omnidirectional)
• Channel specific analysis of the amplitude modeluation of the signals
• Assumption: Speech (Target signal) has an amplitude modulation of around 2 until 6 Hz.
• Reduces amplification (Gain) in each channel were less modulation is estimated, because this part is considered to not be speech.
=> Effectiv method in stationary noise
Adaptive directional microphone from Phonak
=> Effective in fluctuating background noises
Directional dependent amplification/reduction
Independent directional pattern in each band (N=20, Phonak VoiceZoom)
SNR Improvement
Adaptive System: The noise source with the largest energy
is reduced
CI-SP:Harmony Hearing Aid:
Phonak Exilia
Method: Experimental Design
Methode: Measurement System
OlSA Noise
OlSA Noise
OlSA Noise
60 dB SPL
Speech (OLSA)
Variable Level
-10
-8
-6
-4
-2
0
2
4
6
8
10
12
14
Omni DirMic Noise Reduction DirMic + Noise
Reduction
SRT
in d
B
**
**
**
*
Besser
Results
Average Results:
1970 1980 1990 2000
F0F2
F0F1F2
MPEAK ACE CIS
SAS
HiRes
single channel multi channel
Feature extraction Vocoder
SPEAK
2010
FSP
FS4
HiRes120
CA
SMSP MP3000
Optima
2020
single
channel
CIS+
HDCIS
Summary
Thank you very much!
Sound Coding Strategies for Cochlear-
Implants
Waldo Nogueira Auditory Prosthetic Group, Department of Otolaryngology, Medical University Hannover
Many Slides from Prof. Andreas Büchner and Tobias Rottmann
Slides and Materials available under: http://auditoryprostheticgroup.weebly.com/blog
Contents
• Cochlear Implant Research Interfaces
• VoCoder Sync Modular
• VoCoder Matlab
• DHZ-Simulator
Cochlear Implant Research Interfaces
• Why do we need them?
– Direct Stimulation of the electrodes vs
acoustic processing
Cochlear Implant Research Interfaces
• Cochlear
– NIC: API that allows stimulation and recording
for the nucleus implant
• Psychophysics
• Speech Coding
• Matlab/Python interface
– xPC: Real-Time streaming
• Simulink real time interface
– RF generator: Bilateral streaming
- Streaming of pre-defined stimulation sequences
- Possibility of trigger - Capable for bilateral stimulation
• MedEl Research Interface Box (RIB)
User
Matlab
GUI
RIB II Isolation Box
PC
Cochlear Implant Research Interfaces
- GUI for implementation of complete psycho-acoustic experiments
- Presentation of acoustic tones also possible - Can be controlled out of matlab via COM
Bionic Ear Data Collection System (BEDCS)
HRstream
Cochlear Implant Research Interfaces
• Advanced Bionics
VoCoder
Speech Production Process
Source Filter Model
VoCoder
• Sync-Modular
– http://www.sync-modular.org/
VoCoder
• Matlab Code
The advanced combinational
encoder (ACE)
31.0
8.20
16
Fußzeile 135
The advanced combinational
encoder (ACE)
31.0
8.20
16
Fußzeile 136
Windowing:
Raised Cosine Window - Hanning
31.0
8.20
16
Fußzeile 137
Hamming Window
• This is a raised cosine window function.
otherwise
MnM
n
nw
,0
10,1
2cos15.0
)(
Hamming Window:
otherwise
MnM
n
nw
,0
10,1
2cos46.054.0
)(
Filterbank • FFT is applied to each frame
• Windowed signal is shifted over a time interval equal to the
stimulation period (1/CSR) Ns
• The center frequencies of the FFT bins (fc) are linearly
spaced
• Resolution of the FFT is 125 Hz
• Bin 0 (fc = 0 Hz) and bin 64 (fc = 8000 Hz) are real, bins 1
to 63 are complex
• x(l) real X(n) Hermitian symmetry bins 65 to 127 not
required
• Exercise: Apply FFT on each frame
31.0
8.20
16
Fußzeile 139
Filterbank
• (1) Sampling frequency (fs) of a CI is 16000!
– Resample incoming sound to 16000
• (2) Channel stimulation rate (CSR) is set to
values between 500 and 1500 Block shift
in your analysis should be similar to this.
round(Fs/CSR)
31.0
8.20
16
Fußzeile 140
Envelope Detection (1)
• The linearly-spaced L/2 coefficients obtained
at the output of the FFT are groupped into M
bands
31.0
8.20
16
Fußzeile 141
Envleope (2)
Sampling and Selection
31.0
8.20
16
Fußzeile 143
Mapping • The „Mapping“ block determines the current
level li from the envelope magnitude and the
channel characteristics.
• This is done by using the loudenss growth
function (LGF)
31.0
8.20
16
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Mapping (2)
Mapping (3)
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Exercise
• Take the audio signal ASA.WAV
• Plot the electrodogram
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VoCoder: Sound Simulations
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VoCoder
• DEMO: DeHoCI Simulator
Be careful with a VoCoder …
• A VoCoder does not simulate sound
perception with a Cochlear Implant
• However, it is possible to configure a
VoCoder such that similar performance
(e.g. speech) is obtained by Cochlear
Implant listeners and normal hearing
listeners