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Desirable Properties in Modern Compression Schemes. Challenges to Get the Best Out of Today’s Technology. Agenda. WDRC – its Relevance to the Present ……………………………………. 1 Phenomena and Comments …….. 4 The ChannelFree Processing Scheme ………………………………….. 20 List of Sources ………………………... 33.

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desirable properties in modern compression schemes

Desirable Properties in Modern Compression Schemes

Challenges to Get the Best Out of Today’s Technology

  • WDRC – its Relevance to the Present ……………………………………. 1
  • Phenomena and Comments …….. 4
  • The ChannelFree Processing Scheme ………………………………….. 20
  • List of Sources ………………………... 33
1 wdrc its relevance to the present

1. WDRC – its Relevance to the Present

Insights from Recent Publications

wdrc a topic of current interest


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.

wdrc a topic of current interest1


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.

and a side effect of compression


… 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

2. Phenomena and Comments

Compression Speed, Channels, and Throughput Delay

the expert s comment


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.

the expert s comment1


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 (…).

the expert s comment2


The Expert‘s Comment

We evaluated the benefits of fast-acting WDRC, slow-acting 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.

the expert s comment3


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 ‘‘fast-acting compression’’ have release times in the range 50 to 200 ms. For such release times, the compressionwould 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.

the expert s comment4


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.

the experts comments


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.

the expert s comment5


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.

the expert s comment6


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

3. The ChannelFree Processing Scheme

Block Diagram, Operation, and Evaluation



  • An implementation of WDRC

Level Measurement

Filter Control





ChannelFree™ – an elegant WDRC design



  • Level Measurement

Level Measurement

Level Measurement

Filter Control





Continuously measures the Sound Pressure Level



Level Measurement

Waveform of acoustic signal

Traditional SPL measurement

ChannelFree™ SPL measurement



  • Filter Control

Level Measurement

Filter Control

Filter Control





Determines appropriate gain from measured SPL



  • Filter Control

Frequency response

Varying gain in ChannelFree™ processing

Almost constant gain in traditional processing



  • Controllable Filter

Level Measurement

Filter Control






Applies time-varying gain



  • Controllable Filter

Waveform of acoustic signal

Less gain for loud vowels

More gain for soft consonants




Level Measurement

Filter Control






Keeps the acoustic signal time-aligned with the gain



  • 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

the expert s comment7


The Expert‘s Comment

ChannelFree is completely different from the outdated single-channel 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.

the expert s comment8


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

5. List of Sources

Literature – Further Reading



  • 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.