HIS 140 - HI Compression Strategies Overview

HI Compression Strategies

Compression’s major role is to

decrease sound levels to fit the

restricted dynamic range of the

patient/client.


Due to hearing loss, the

patient/client’s dynamic range (the

level difference between discomfort

and the threshold of audibility) is

reduced compared to a “normal

hearing” dynamic range.


The patient/client with sensorineural

hearing loss will have difficulty with

reduced dynamic range and also,

reduced frequency resolution.


Reduced dynamic range and

frequency resolution are the primary

considerations in the development of

various compression strategies and

digital algorithms.


Multi-channel Compression

With today’s digital hearing

instruments, there are often more

than one type of compressor written

into various frequency channels of a

digital algorithm.


To meet these frequency by channel signal challenges, three fundamental compression strategies are in use.

They are:

1. Low level compression (input compression)

2. Compression limiting (output compression)

3. Wide dynamic range compression (combination)


Compression Characteristics

These three compressor strategies are basically controlled by either the input signal (input compression) or the output signal (output compression) or a combination of the two.


Compressor Characteristics

1. Attack and release times

2. Spectral envelope

3. Compression threshold

4. Compression ratio


Attack and Release Time

Attack time is the time it takes for a compressor to react to an increased input level.

Release time it the time it takes for a compressor to react to a decreased input level.


Spectral Envelope

Different syllables of speech have

characteristic spectral information (peaks

and valleys over a time period).

The components of this spectral

information (peaks and valleys) are

placed inside an envelope (time period).


Spectral Envelope

Envelopes are used to define the

effects of various attack and release

times upon spectral information.


Compression Threshold

This is defined as the point when compression commences.

It is also the threshold where definition begins for static compression characteristics


Static Compression Characteristics

Attack and release times provide

information regarding how quickly a

compressor operates.

Input-output (static characteristics) tell us

how much a compressor decreases gain

as the input level rises.


Compression Ratio

The static characteristics of input vs.

output are described with

compression ratios.

For example, the compression ratio

for a linear amplifier would be 1:1.


Compression Ratios

There are compression ratios which

are less than 1:1. This is then

termed expansion instead of

compression.


Compression Use Rationale

There are primarily six patient/client performance strategies addressed by compressors. Any or all, may be written into various digital algorithms.



The six strategies are:

1. Avoiding distortion, discomfort, and damage

2. Reduce inter-syllabic & inter-phonemic intensity

differences

3. “Normalize” loudness

4. Increase sound comfort

5. Maximize intelligibility

6. Reduce background noise



These six rationale use various

configurations of attack and release

times, spectral envelope, compression

ratios, and compression thresholds to

modify the gain and output of the

frequency response in each channel.


Compressor Use Rationale

Various combinations of these

compressors may be written into

digital hearing instrument algorithms

and appropriately applied for best

patient/client hearing outcomes.



A good reference summary may be

found in Dillon page #183.

HIS 140 - HI Compression Strategies Overview

Technology

Transcript of HIS 140 - HI Compression Strategies Overview