Companding to improve cochlear implants’ speech processing in noise
Aparajita Bhattacharya and Fan-Gang Zeng
Hearing and Speech Research Laboratory, Department of Otolaryngology, University of California, Irvine
(b): Temporal waveform of the consonant /aPa/. FIGURE 4: Phoneme recognition and Sentence recognition scores for cochlear Methods
The performance of the present day cochlear implants in noise is severely Phoneme recognition and sentence recognition tests were conducted in quiet and
compromised. with steady state speech-shaped noise at varying signal to noise ratios. Stimuli
C1 were presented at 70 dB SPL and the signal to noise ratios (SNR) varied
Two tone suppression is thought to be one of the reasons for the outstanding 60
from -10 dB to 10 dB at 5 dB interval.
performance of human ears in noise. Two tone suppression is characterized by a Without companding With companding
decrease in response to a tone in the presence of a second tone. Companding changes the temporal waveform.
0 Subjects : Normal : 9 subjects were tested for the vowel recognition tests, 7
subjects for the consonant recognition tests and 10 subjects for the sentence
Turicchia and Sarpeshkar have shown that companding i.e., compression followed recognition tests. Out of these 5 subjects participated in both phoneme
FIGURE 3: Phoneme recognition and Sentence recognition scores for 40 0
by expansion, simulates two tone suppression. Companding enhances the spectral 20 recognition and sentence recognition tests. Cochlear implant users : 5 Nucleus
Normal subjects. -20
peaks and therefore reduces the background noise keeping the signal level the 40 24 and 2 Clarion cochlear-implant users also participated in this research.
same. Their architecture uses a broad filter bank, a compression block, followed 100 Vowels 30 Vowels 60
by a narrowly tuned post filter bank and an expansion block. Both the compression 80
20 40 0 Tests on normal subjects included two types of stimuli, stimuli processed with the
increase in percent score
and expansion blocks consist of an envelope detector and a non-linear unit
companding strategy alone and stimuli processed with companding strategy
followed by a multiplier manipulating the filter output by the non-linear block 20 C4
0 5 10 quiet 5 10 quiet
followed by acoustic simulation of an 8-channel cochlear implant. The phoneme
-10 80 20 SNR
60 materials included 12 /hvd/ vowels and 20 /aCa/ consonants spoken by a male
increase in percent score
-10 -5 0 5 10 Quiet
talker and a female talker. The target sentence material consisted of 250 HINT
FIGURE 1: Single channel of the companding architecture.
sentences spoken by a male talker.
60 10 100 C5
-20 20 -20
100 Hint sentences 30 Hint sentences
-10 -5 0 5 10 Quiet 0
40 Companding enhances spectral peaks but also changes the temporal waveforms.
80 20 C6 C6
ED C ED C 80 20
F, G – filters, G is narrowly tuned ED – envelop detector 60
C – non-linear compressor unit 40 0 40 Companding does not improve speech performance in normal-hearing subjects
0 but it does improve performance in normal subjects listening to an 8 channel
The purpose of the present study is to investigate the effect of companding 0 -20 40
C7 0 5 10 quiet C7 0 5 10 quiet
cochlear implant simulation, particularly for the vowels.
architecture on cochlear-implant performance in quiet and with noise. -10 -5 0 5 10 quiet -10 -5 0 5 10 quiet 80
X Data 20 SNR
SNR SNR 60
without CI simulation, with companding 20
Companding improves speech performance in cochlear-implant users, particularly
without CI simulation
Results without CI simulation, without companding
with CI simulation, with companding
with CI simulation 0
for users with better hearing.
with CI simulation, without companding
0 5 10 quiet 0 5 10 quiet
FIGURE 2 (a): FFT of the consonant /aPa/. SNR SNR
vowels with companding
Maximum benefit from companding is seen for the recognition of vowels. vowels without companding
Future work: We will investigate the performance of companding using a
consonants with companding consonants
Improvement in performance is much more prominent with stimuli processed with consonants without conpanding sentences competing talker as a masker. We will analyze the error patterns in consonant
sentences with conpanding
the companding strategy followed by 8 channel cochlear implant simulation sentences without companding recognition to find out whether and how companding effects recognition of
compared to stimuli processed with the companding strategy alone. different types of consonants.
Subjects derived relatively small benefits for recognition of the consonants when
C1 to C5 are Nucleus 24 users while C6 and C7 are Clarion II users.
stimuli were processed with companding followed by CI simulation. The scores
The left panel shows the initial vowel part while the right panel shows that during Five of the seven subjects showed significant improvement in scores with Acknowledgements
went down for some SNRs when the consonant stimuli were processed with the
the sudden release of airflow after the stop i.e., the consonant part. companding.
We would like to thank our CI listeners and normal hearing subjects for their time and dedication. This
companding strategy alone. experiment was supported by NIH.
The green curve shows the data with companding while the red curve shows that C6 did not show any improvement with companding. The percent scores for this
Significant differences between increase in percent scores with and without CI
without companding. subject are much lower than the rest of the subjects.
simulation were seen in case of recognition of vowels and sentences. • Turicchia L and Sarpeshkar R (2005). “A bio-inspired companding strategy for spectral enhancement.” IEEE
Significant spectral enhancement seen in case of the initial vowel part. Not much Most of the subjects gained little or no improvement in quiet.
Subjects showed little or no improvement in quiet. transactions on speech and audio processing 2005 Vol 13(2): 243-253
spectral enhancement seen with companding for the consonant part.