Digital amplification strategies
for hearing aids and cochlear implants
Guido F. Smoorenburg
Three important advantages of digital amplification
1. High flexibility in amplifier design;
complicated strategies can be implemented in a small volume
2. The hearing aid can be reprogrammed
(upgraded) when a better strategy becomes available, without changing the hardware
3. The strategy can be optimized with respect to the individual's hearing impairment
An unusual strategy
• Impaired hearing implies reduced capacity to process all information contained in the speech signal
• Thus, extract certain important features from the speech and present only these features to the hearing impaired
• This strategy was first implemented by Cochlear in cochlear implants
Important features: F0, F1 & F2
Approach abandoned
This approach has become obsolete although:
1. it provides F0 (pitch) information
2. it provides maximum spectral sharpening re the first (F1) and (F2) formants
3. Too much emphasis on vowels??
F0, F1, F2 coding in acoustic hearing aids
Ampl. = linear amplification F1: 1st formant
F2: 2nd formant
2 spectral components in each formant
N=12
Presentation:
Auditory + Visual 0
10 20 30 40 50 60 70 80 90 100
Ampl. F1 F1+F2
discrimination score in %
Van Son
Conclusion: no clear benefit beyond straightforward amplification
Current approach in acoustic hearing aids originates with physiology
Sonic Innovation Natura
Current approach in cochlear implants originates with physiology
Banfai - Hartmann
Physiology shows three important factors in hearing impairment
1. Threshold increase
2. Loss of amplitude compression 3. Loss of frequency resolution
(4.) Loss of temporal resolution
The three factors originate at level of cochlear mechanics
Mario Ruggero
Compensating hearing loss in acoustic hearing aids
1. Increase in threshold:
amplify signal
2. Loss of amplitude compression:
include compression in amplification Note: loss of compression implies enhancement of contrast
3. Loss of frequency resolution:
Enhance spectral contrast ?
Compression in acoustic hearing aids
• Compression is frequently measured by loudness scaling
• Compression results in combination tones (distortion product oto-acoustic emissions) and two-tone suppression
• Combination tones and two-tone
suppression suggest that the compression factor is about 2
Compression modelling
Two-tone suppression DPOAEs
Giguere
Subjective compression preference
Optimizing number of channels,
compression ratio and attack/release times
Survey of publications
release time
compression ratio
Experimental Design – Rolph Houben
Number of Channels (NC)
Compression Ratio (CR, ∆Lin/∆Lout)
CRlow (<1000 Hz) and CRhigh (>1000 Hz)
Time Constants (T) Tattack and Trelease in ms
(time to reach 1/e of over/undershoot)
CRlow CRhigh
1 2
2 2
2 3
3 3
51 conditions (for each subject) 1 2 6
Tattack Trelease 4 4
4 40
40 40
4 400
40 400
Results for stationary noise
2 channels
6 channels
2.6 2.4 2.2 2 1.8
1.6
Standard Deviation (dB)
2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8
Equal ∆SRT-contours (dB)
best
-0.7 ± 1.7 dB
1.4 1 0.6 0.2 -0.2
-0.6
1/2 2/2 2/3 3/3 CRlow/CRhigh
2.5 2 1.5 1
0.5
TA/TR
40/400 4/400 40/40 4/40 4/4 TA/TR 40/400 4/400 40/40 4/40 4/4
p<0.05
Tukey’s HSD
worst
+2.1± 1.7 dB
best
-0.4 ± 1.1 dB worst
+1.8± 1.8 dB
Tukey’s HSD 1/2 2/2 2/3 3/3
CRlow/CRhigh
∆SRT
∆SRT
stdev
stdev
Conclusions for compression with moderate impairment
• The optimum choice of parameters yields only a very small improvement re linear amplification in speech perception in noise (0.7 dB ~ 12 %)
• Compression ratio and attack / release times do not interact
• Increase of number of channels >> increase of (attack) / release times
• Optimization will be very difficult in everyday practice
• In the moderately hearing impaired compression can improve listening comfort without reducing speech intelligibility
Results for severely impaired subjects
Drullman
Spectral resolution
Temporal resolution
Amplitude / loudness scaling
Bob Shannon, Qien-Jie FU, Fan-Gang Zeng, John Galvin
Mean T and C levels in conventional and ECAP based fitting (n = 18)
100 120 140 160 180 200 220
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
electrode number
current level (CU)
ECAP 40 uV C conv T conv C ECAP T ECAP
CVC results of 2* 6 weeks cross-over study
0 10 20 30 40 50 60 70 80 90 100
EE JC JK WM GE CB JB JM JD EB KT AG CD WR M L AH M H mean
subject
phonemescore (%)
conv 65 dB conv 65 + 55 conv 55 dB
ECAP 65 dB ECAP 65 + 55 ECAP 55 dB
Frequency analysis
Mario Ruggero
Frequency selectivity as a function of level
experiment model
/a e / 5 5 d B S P L
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0
0 4 0 8 0 1 2 0 1 6 0 2 0 0 2 4 0 2 8 0 3 2 0 B M p l a c e ( c h a n n e l n u m b e r )
BM velocity (dB)
F1 F2 F3
normal cochlea
damaged cochlea (50% OHC loss)
+S
+S: Spectral enhancement = 8 harmonics retained (F0, 2F0, 3F0, 4F0, 5F0, 6F0, 14F0, 19F0)
Forward masking & ECAP spread
A U S T R A L I A
Conclusions 1
• Digital amplification offers high flexibility in signal processing strategies. However, the physiology of hearing impairment imposes considerable
limits on the extent to which hearing impairment can be compensated for by digital amplification
• Compression contributes hardly to speech intelligibility (even in the severely hearing impaired) but it enlarges the acoustic input
window without loss of speech intelligibility when the compression parameters are well chosen
Conclusions 2
• Loss of spectral and temporal resolution are important limiting factors. They cannot be compensated for by (even complicated) amplification strategies
• Loss of spectral resolution affects pitch perception.
• Better pitch perception in cochlear implants depends critically on reducing spread of
excitation by improving the electrode-neuron interface.
Conclusions 3
• Nowadays cochlear implants tend to give better results in the severely hearing
impaired than acoustic hearing aids. The combination of high-frequency electric and low-frequency acoustic stimulation may
improve their pitch perception by
presenting F0 information acoustically.
