The continuous frequency dynamic range compressor

Abstract

The typical multiband audio compressor (TMC), such as that used in many modern hearing aids, consists of a bandpass filter bank coupled to a compression circuit which applies gain to each frequency band as a function of power in that band. Generally the filter bank is designed so that the sum of magnitude responses of the filters is unity with the band edges as steep as the implementation will allow, to minimize overlap between bands. There are a number of problems with this approach. Difficult decisions must be made regarding placement of band edges. While the composite response for broad band signals may be flat, the narrow band-e.g. swept sine-response exhibits bumps near the band edges. In other words, the system is non-shift invariant with respect to frequency. We show that these problems can be eliminated by increasing the number of bands, and by extending the overlap region between bands. The problem is examined in terms of frequency domain sampling of the power spectrum. If the sampling rate is sufficiently high then artifacts disappear, and the system can be viewed as continuous in frequency with no band edges.