Time constants of compression schemes – Less is more?
Resumé
In recent years, Wide Dynamic Range Compression (WDRC) has been established in the listening device industry so that it became the most utilized tool in modern hearing instruments. However, although the general system has become status quo, the question of a correct tting of the necessary parameters is still open. The developed tting rules only calculate the target gain and perhaps the compression ratio, the number of channels and/or the compression kneepoints. Unfortunately, the time constants are generally not considered. The present work presents investigations into this area. Two compression systems operating with completely different time constants were compared. One system works instantaneously with very short time constants. Additionally, the applied gain and compression depends on the distance of the instantaneous frequency from the centre frequency of the particular frequency band. The sec- ond system is a compression system which is already available in commercial hearing devices and which allows limited access to the time constants. For the evaluation, both subjective and objective speech tests were used. In addition, a static and a dynamic loudness scaling method were integrated to provide information regarding how the loudness is normalized. In the third part of the evaluation, several sound samples were presented in different level ranges to be judged (i) absolutely using questionnaires and (ii) relatively in complete paired comparisons. In all cases, the compression systems were evaluated in level ranges relevant for real life listening situations. The results found with the instantaneous compression system do not con rm the assumption of improved speech intelligibility in modulated noise but they show a better restoration of loudness than in conventional compression systems without deteriorating the sound quality.
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