Description
Compression:
Normal hearing persons have a useful dynamic range from 0 to
**0dB.Hearing impaired people have
often both, a reduced sensitivity to sounds (hearing loss) and are
equal or more sensitive to loud
noises than normal hearing people. When using compression a change
of the waveform takes place.
In order to minimize the negative effects Swisstone hearing aids
allow different ways to proceed:
- Peak clipping, cuts the extreme loudness peaks, creates
distortions, efficient for extreme peak
noises. Waveforms below the peak levels are fully undistorted
- AGC, which prevents the output to exceed a certain level
(compression ratio very high).There
is a wide range available. It is recommended to stay with –*0dB. If
the effect is too low, select
very short attack time constant and long decay time constant (eg
1ms attack and **8ms
decay). Regulation on peak take place. Regulation on peak has the
advantage, that the
dynamics of the amplitude just below the peak are fully maintained.
If the compression
ratio is too high, Amplitude information is lost and perception in
background noise is worse
- Adaptive AGC: long-term loud levels create more effective AGC
action than short peaks
- WDRC (Wide dynamic Range Control): As compression starts at low
intensities, the
compression ration can be kept small, so the distortions of the
waveform are kept low
- Dual Compression: Compression effective at low levels for long
time constants average value
and compression effective at peak levels with shorter time
constants
Noise reduction:
In each filter of a filter bank the levels are measured. If the
levels are modulated, they are considered
to be speech. If the levels stay stable without modulation, the
levels are considered to be unwanted
noise and are attenuated.
