A multichannel compression strategy for a digital hearing aid

Multi-channel compression schemes are a practical methods of mapping the wide dynamic range of speech signals into the reduced dynamic range of hearing impaired listeners. These systems address two of the shortcomings of single-channel compression systems: (1) the reduction of gain as a result of narrow-band non-speech stimuli and (2) the reduction of gain that often occurs when high-frequency speech components are followed by intense low-frequency speech components. They also provide frequency-dependent compression ratios that are needed by many newer supra-threshold fitting strategies (e.g., DSL I/O). This paper presents a multichannel compression scheme that employs an oversampled, polyphase DFT filterbank. In each compressor channel, the gain is controlled by an adjustable combination of a overall, dual time-constant input signal level and the individual channel signal level that is measured with a short time-constant RMS detector. Informal listening tests have demonstrated that the design has very good audio quality and performs well in real-world listening situations. The design is suited for low-power, real-time operation.