In a speech processor for controlling a hearing prosthesis, an implanted electrode array stimulates the auditory nerve fibres of a patient by the application of electrical currents to selected electrodes in the array. The processing system comprises means for generating an input signal corresponding to a received speech signal, means for estimating the amplitude and frequency of the fundamental voicing and second formant components of the speech signal, and means for determining whether the speech signal is voiced or unvoiced. Programmable means produce instruction data which in use causes the application of the electrical currents to selected electrodes in the array. The programmable means is programmed with data defining a predetermined relationship between each electrode and a selected range of second formant frequencies based on psychophysical testing of the patient and causing selection of the electrodes based on the estimated-frequency of said second formant component such as to produce the desired percepts in the auditorylike sensations generated in the patient. The selected electrode is stimulated at a frequency dependent on the estimated frequency of said fundamental voicing component for voiced speech signals and at a lower substantially constant frequency for unvoiced speech signals. The programmable means also produces data which determines the level of stimulation of each selected electrode dependent on the estimated amplitude of the second formant component of the speech signal as well as on predetermined data relating to the sensitivity of each electrode implanted in the patient. By providing an output signal in which both formant and prosodic information is present, the production of confusing precepts is avoided.
SPEECH PROCESSOR FOR CONTROLLING HEARING PROSTHESIS