The restoring of correct loudness functions for hearing impaired requires inputloutput functions which depend on both, amplitude and frequency. The described gain controller does so by splitting the input spectrum in three bands and correcting the loudness functions in each separately. The inputloutput static characteristics are fully programmable and the gain is piloted by an envelope detection based on speech properties.
Bibliographic reference. Ventura, J. C. (1989): "Multiband digital gain controller", In EUROSPEECH-1989, 1582-1585.