DSP-implementations of speech coding for multielectrode cochlear implants and multiband loudness correction for digital hearing aids

Abstract

SUMMARY Hearing sensations can be restored for profoundly deaf patients via artificial electrical stimulation of the auditory nerve. Present electrode technology and electrophysiological constraints however allow at best a very crude and limited approximation of the normal neural excitation pattern. Signal processing for cochlear implants therefore is confronted with the problem of a severely restricted channel capacity and the necessity to select and encode a subset of the information contained in the sound signal reaching the listeners ear. With single chip digital signal processors (DSPs) incorporated in personal computers different speech coding strategies can be evaluated in relatively short laboratory experiments. In addition to the well known strategies realized with filters, amplifiers and logic circuits a DSP approach allows the implementation of much more complex algorithms such as nonlinear multiband loudness correction, speech feature contrast enhancement, adaptive noise reduction. Although many aspects of speech encoding can be efficiently studied using a laboratory digital signal processor it would be desirable to allow subjects more time for adjustment to a new coding strategy. Several days or weeks of habituation are sometimes required until a new mapping can be fully exploited. Thus for scientific as well as practical purposes the miniaturization of wearable DSPs will be of great importance. A cochlear implant digital speech processor (CIDSP) for the Nucleus 22-channel cochlear prosthesis has been implemented using a single chip digital signal processor (TMS320C25, Texas Instruments). For laboratory experiments the CIDSP is incorporated in a general purpose computer (PDP11/73) which provides interactive parameter control, graphical display of input/output and intermediate buffers and offline speech file processing facilities. In addition to the generation of stimulus parameters for the cochlear implant an acoustic signal based on a perceptive model of auditory nerve stimulation is output simultaneously. For field studies and as a take-home device for patients a wearable battery-operated unit has been built. Advantages of a DSP-implementation of speech encoding algorithms as opposed to offline prepared tcst lists are increased flexibility, controlled, reproducible and fast modifications of processing parameters, use of running speech for training and familiarization. Disadvantages are the more complex programming and numerical problems with integer arithmetic.