Improvements in speech processors

A signal processing system for converting a speech signal into a data signal for controlling a hearing prosthesis including an implanted electrode array adapted to stimulate the auditory nerve fibres of a patient by the application of electrical currents to selected electrodes in said array, said processing system comprising means (20, 21) for generating an input signal corresponding to a received speech signal, means (30-36) for estimating the amplitude and frequency of the fundamental voicing component of said speech signal, means (22-26, 27, 28) for estimating the amplitude and frequency of at least one formant of said speech signal, programmable means (40) for producing instruction data which in use causes the application of said electrical currents to selected groups of electrodes in said array with or without delays between the stimulation of each electrode in said groups, said programmable means being programmable with data defining a predetermined relationship between each group of electrodes and a selected range of at least one formant frequency and with data defining a predetermined relationship between another formant frequency and the delay between stimulation of each electrode in each said group based on psychophysical testing of the patient and causing selection of said electrodes based on the estimated frequency of said formants such as to produce the desired percepts in the auditory-like sensations generated in the patient, said programmable means (40) further being programmable to produce data which determines the level of stimulation of each selected group of electrodes and determines the delay between stimulation of electrodes in each said group dependent on said estimated amplitude of said formants of said speech signal as well as on predetermined data relating to the sensitivity of each electrode implanted in the patient.