Desirable Properties in Modern
Compression Schemes
Challenges to Get the Best Out of
Today’s Technology
Agenda
1. WDRC – its Relevance to the
Present ……………………………………. 1
2. Phenomena and Comments …….. 4
3. The ChannelFree Processing
Scheme ………………………………….. 20
4. List of Sources ………………………... 33
1. WDRC – its Relevance to
the Present
Insights from Recent Publications
1/34
WDRC – a Topic of Current Interest?
Most modern hearing aids incorporate some form of
compression or automatic gain control (AGC) …
However, controversy continues about the “best” way
to implement AGC, and particularly whether it should
be fast acting or slow acting.
2/34
WDRC – a Topic of Current Interest?
It has been more than 40 years since initial
applications of multiple compression channels (MCC)
were implemented in hearing aids (Caraway & Carhart,
1967); however, the appropriate number of channels
remains an unanswered question.
3/34
… and a Side-Effect of Compression
Open canal (OC) fittings deliver sound to the meatus
either via a thin tube or via a receiver in the ear canal.
…With the advent of digital feedback cancellation,
open fittings have become much more widely used,
but the effect of time delays for OC fittings has
received relatively little attention.
2. Phenomena and Comments
Compression Speed, Channels,
and Throughput Delay
4/34
Phenomena with Compression Speed
STEP FUNCTION INPUT AND SYSTEM RESPONSES
SPL
INPUT SIGNAL
TIME
SPL
SLOW COMPRESSION
TIME
SPL
FAST COMPRESSION
TIME
5/34
The Expert’s Comment
Fast compression can restore the audibility of
weak sounds rapidly following intense sounds.
This at least provides the potential for listening in
the dips.
It also improves the ability to detect a weak
consonant following a relatively intense vowel.
Fast compression can give good results when
two voices alternate with markedly different
levels.
6/34
Phenomena with Compression Speed
SPL
RESPONSE TO STEP FUNCTION WITHOUT TIME ALIGNMENT
TIME
SPL
RESPONSE TO STEP FUNCTION WITH TIME ALIGNMENT
TIME
7/34
The Expert’s Comment
It [fast compression] can introduce spurious
changes in the shape of the temporal envelope
of sounds (e.g., overshoot and undershoot
effects; …).
Delaying the audio signal by a small amount
relative to the gain-control signal can reduce
such effects (…).
8/34
Phenomena with Compression Speed
NUMBER OF PERSONS
LISTENING COMFORT WINNERS (N = 50)
25
20
15
10
5
0
NAL-RP
LINEAR
SLOW-SLOW
FAST-FAST
FAST-SLOW
NUMBER OF PERSONS
SPEECH TEST WINNERS (N = 50)
25
20
15
10
5
0
NAL-RP
LINEAR
SLOW-SLOW
FAST-FAST
FAST-SLOW
9/34
The Expert‘s Comment
We evaluated the benefits of fast-acting WDRC, slowacting AVC, and linear reference fittings for speech
intelligibility and reported disability.
Slow-acting AVC outperformed the fast-acting WDRC
fittings for listening comfort, while for reported and
measured speech intelligibility the converse was true.
10/34
Phenomena with Compression Speed
SOUND PRESSURE
MODULATED PURE TONE FOR THE SOUND EAR
20 dB
16 ms
0
10
20
30
40
50
60
70
SOUND PRESSURE
COMPRESSED MODULATED PURE TONE FOR THE IMPAIRED EAR
10 dB
0
10
20
30
40
TIME [ms]
50
60
70
11/34
The Expert’s Comment
However, …, compression would still need to be fast
acting to ensure that weak sounds are audible when they
occur just after strong sounds.
Most commercially available hearing aids with ‘‘fastacting compression’’ have release times in the range 50
to 200 ms. For such release times, the compression
would not be very effective for modulation rates above 5
to 10 Hz.
Shorter release times than this are generally avoided
because they can lead to significant harmonic and
intermodulation distortion.
12/34
Phenomena with Channels
EFFECTIVE GAIN CURVES IN FAST TWO-CHANNEL COMPRESSION
GAIN [dB]
40
30
SIBILANT /SH/
VOWEL /OE/
NAL-NL1
20
10
0
250
500
750 1000
1500 2000
3000 4000
6000 8000
EFFECTIVE GAIN CURVES IN FAST CHANNELFREE PROCESSING
GAIN [dB]
40
30
SIBILANT /SH/
VOWEL /OE/
NAL-NL1
20
10
0
250
500
750 1000
1500 2000
FREQUENCY[Hz]
3000 4000
6000 8000
13/34
The Expert’s Comment
In a multichannel hearing aid with fast-acting
compression …, short-term changes in the
spectral pattern of sounds may be distorted
because the pattern of gains across frequency
changes rapidly with time.
14/34
Phenomena with Channels
SOUND PRESSURE LEVEL [dB SPL]
SPECTRAL FLATTENING DUE TO FAST COMPRESSION IN MANY CHANNELS
70
60
50
40
30
BEFORE
AFTER MULTICHANNEL COMPRESSION
20
100
250
500
1000
2000
FREQUENCY [Hz]
4000
8000
15/34
The Experts’ Comments
In a multichannel hearing aid with fast-acting
compression in many channels, the spectrum is
flattened, reducing spectral contrasts.
Overall spectral contrasts of vowels are
significantly reduced as the number of
compression channels increases.
Listeners with mild sloping to moderately severe
hearing loss demonstrated poorer vowel
identification.
16/34
Phenomena with Throughput Delay
EFFECTIVE GAIN CURVES WITH OPEN CANAL FITTING
15
GAIN [dB]
10
DELAY: 4 ms
5
0
-5
-10
-15
500
1000
2000
4000
8000
1000
2000
FREQUENCY [Hz]
4000
8000
15
GAIN [dB]
10
DELAY: 0 ms
5
0
-5
-10
-15
500
17/34
The Expert’s Comment
In principle, the mixing of direct airborne sound
and delayed sound produced by an OC hearing aid can
have several undesired perceptual effects. First, there is a
comb-filtering effect (ripples in the spectrum), which
leads to an alteration of the timbre of the sound
(coloration).
A second effect occurs for longer delays; the delayed
sound may be perceived as an echo.
Across-frequency delay can itself have disturbing effects.
For example, sounds like clicks may appear smeared in
time, or may be perceived as rapid frequency glides.
18/34
5000
5000
4000
4500
3000
90
4000
6 ms
80
2000
3500
1000
0.22
5000
FREQUENCY [Hz]
Gehörgang
(offene Anpassung)
SOUND INIm
THE
FREE FIELD
100
Frequenz [Hz]
FREQUENCY [Hz]
Phenomena with Throughput Delay
70
60
3000
0.225
0.23
0.235
0.24
50
2500
SOUND IN THE EAR CANAL (OPEN FITTING)
40
2000
4000
30
6 ms
1500
20
2000
1000
10
1000
500
3000
0.22
0.225
0.22
0
0.23
0.225
TIME [s]
0.23 0.235
Zeit [s]
0.235
0.24
0.24
19/34
The Expert’s Comment
As expected, disturbance increased significantly
with increasing delay: the main increase occurred
between 4 and 12 msec.
Following presentation of a sample of the processed
speech, the participant was asked to rate the effect of the
delay, using the same seven-point scale as in our earlier
studies (…). For this scale, “1” corresponds to “Not at all
disturbing,” “4” corresponds to “disturbing,” and “7”
corresponds to “highly disturbing.”
A disturbance rating of 3 was reached for
a delay of about 5.3 msec.
3. The ChannelFree
Processing Scheme
Block Diagram, Operation,
and Evaluation
20/34
An implementation of WDRC
Level
Measurement
Input
Synchronization
Filter Control
Controllable
Filter
ChannelFree™ – an elegant WDRC design
Output
21/34
Level Measurement
Level
Measurement
Input
Synchronization
Filter Control
Controllable
Filter
Continuously measures the Sound Pressure Level
Output
22/34
Level Measurement
Waveform of
acoustic signal
Traditional SPL
measurement
ChannelFree™
SPL
measurement
23/34
Filter Control
Level
Measurement
Input
Synchronization
Filter Control
Controllable
Filter
Determines appropriate gain from measured SPL
Output
24/34
Filter Control
Frequency
response
Varying gain in ChannelFree™ processing
Almost constant gain in traditional processing
25/34
Controllable Filter
Level
Measurement
Input
Synchronization
Applies time-varying gain
Filter Control
Controllable
Filter
Output
26/34
Controllable Filter
Waveform
of acoustic
signal
Less gain for loud vowels
More gain for soft consonants
27/34
Synchronization
Level
Measurement
Input
Synchronization
Filter Control
Controllable
Filter
Keeps the acoustic signal time-aligned with the gain
Output
28/34
Synchronization
Compensation for time delay in level measurement and filter
control
t = control signal from level to gain
t = acoustic signal (with added delay)
t = acoustic signal – synchronized and amplified
29/34
Flexibility in Gain Shaping
CONTINUOUSLY VARIABLE COMPRESSION RATIO
40
35
30
50 dB SPL
60 dB SPL
70 dB SPL
80 dB SPL
90 dB SPL
INTERPOLATED
GAIN [dB]
25
20
15
10
5
0
0.25
0.5
0.75 1
1.5
2
FREQUENCY [kHz]
3
4
6
8
30/34
The Expert‘s Comment
ChannelFree is completely different from the outdated singlechannel wideband compression scheme that produces the
frequency responses [with] the same shape, irrespective of
input level.
ChannelFree … directly implements continuous frequency
responses that coincide with the fitting targets to within 1 dB
thus providing a continuously variable compression ratio across
frequency.
31/34
Verification of Benefit
8
6
4
2
ChannelFree
AVERAGE PREFERENCE SCORES
SPEECH AND MUSIC RATED BY HEARING IMPAIRED LISTENERS
10
0
-2
-4
-6
-8
OTHER IMPLEMENTATIONS
VARIOUS IMPLEMENTATIONS OF WDRC
32/34
The Expert’s Comment
The aim of our project was to compare the perceived
sound quality of several current advanced hearing aids
while they are amplifying a range of different signals.
For the hearing-impaired listeners, Symbio received the
highest average scores for male and female voices and
piano music.
5. List of Sources
Literature – Further Reading
33/34
Literature
Compression Speed
Moore, B.C.J. (2008). The Choice of Compression Speed in Hearing Aids: Theoretical and Practical
Considerations and the Role of Individual Differences. Trends in Amplification, Vol. 12, No. 2,
103-112.
Gatehouse S, Naylor G, Elberling C. (2006). Linear and nonlinear hearing aid fittings –1. Patterns
of benefit. Int J Audiol, 45(3):130-52.
Moore, B.C.J., Wojtczak, M., Vickers, D.A. (1996). Effect of loudness recruitment on the
perception of amplitude modulation. Journal of the Acoustical Society of America, 100, 481-489.
Spectral Contrast
Bor S, Souza P, Wright R. (2008). Multichannel compression: Effects of reduced spectral contrast
on vowel identification. Journal of Speech Language and Hearing Research, 51, 1315-27.
Throughput Delay
Stone M.A., Moore B.C.J., Meisenbacher K., & Derleth R.P. (2008). Tolerable Hearing-Aid Delays.
V. Estimation of Limits for Open Canal Fittings. Ear Hear. 29:601-617.
Gain Shaping
Schaub A. (2009). Enhancing Temporal Resolution and Sound Quality: A Novel Approach to
Compression. Hearing Review (August 2009): 28, 30, 32-33.
Best Scores
Dillon H, Keidser G, O’Brien A, Silberstein H. (2003). Sound quality comparisons of advanced
hearing aids. Hearing Journal 56(4): 30-40.
34/34
Further Reading