Compression circuit of a multiband analog system for hearing aid

This paper describes the design and evaluation of a circuit which performs the compression of narrow-band signals within a multiband analog system for a hearing aid. The system has twelve narrow-band modules. Each module is formed by four stages. The first stage is a band-pass filter which selects the bandwidth of the module. The second stage, the object of this paper, is a compression circuit which performs a nonlinear operation for removing the attack and release times typical of automatic gain control systems. The third stage is another band-pass filter like the first, the function of which is to reduce the distortion produced by the compression stage. The fourth stage is a controlled linear gain stage. The simulation and experimental results obtained show that the compression circuit has good accuracy within the dynamic range of speech signals. The output narrow-band filter reduces to a great extent the harmonic and intermodulation distortion inherent in all compression systems when the input signal has some important formants very close.