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.
Desirable Properties in Modern Compression Schemes
Challenges to Get the Best Out of Today’s Technology
1. WDRC – its Relevance to the Present
Insights from Recent Publications
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.
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.
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
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.
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 (…).
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.
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.
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.
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.
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.
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
ChannelFree™ – an elegant WDRC design
Continuously measures the Sound Pressure Level
Waveform of acoustic signal
Traditional SPL measurement
ChannelFree™ SPL measurement
Determines appropriate gain from measured SPL
Varying gain in ChannelFree™ processing
Almost constant gain in traditional processing
Applies time-varying gain
Waveform of acoustic signal
Less gain for loud vowels
More gain for soft consonants
Keeps the acoustic signal time-aligned with the gain
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
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 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