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.