Full Phase Filter In The In Cochlear Cis Program

Through stimulating remainder hearing nerve by electrical signal, cochlear implants are the only medical tools that can make the deaf get part audition. 20 years ago, it is a simple single electrode setting in order to strengthen patient lip reading and apperceive sound. Today cochlear implants have been developed to be a complex multi-electrode setting that can support 60,000 people in the world to communicate by phone successfully. Nearly, including 3 months children and adults, especially remainder in the low frequency, the scope using cochlear implants has been extended. Although the difference in different individual is very large, on the average, most of the users can communicate by phone successfully through cochlear implants in the quiet environment. The number of users and scientific literature is exponential increasing, which proves that cochlear implants have been developed to be a ripe field.First of all, the research background and the development of cochlear implants are summarized. Through describing the speech signal synthesizing method, the operating principle of normal hearing people's cochlea and the pathology foundation of deafness, the principle of cochlear implants and the function of every part in cochlear implants are introduced.In the development of cochlear implants, signal processing plays an important role. According to the differences in speech processing, the speech processing method can be divided into feature extracting strategy and wave strategy. Feature extracting strategy extracts the important feature at first, then transmits the feature to different electrodes to stimulate hearing nerve and the representational method includes F₀/F₂,F₀/F₁/F₂; Wave strategy transmits the wave of speech signal to electrodes in different means and the representational method includes ACE,CIS,SPEAK. We take CIS strategy for an example and present an improved CIS strategy. In the improved CIS strategy, a new type filter—All Phase DFT filter replaces the old filter. In the former CIS strategy, Butterworth filter is adopted in most of cochlear implants, which is a kind of non-linear filter. The disadvantage of non-linear bandpass filters is that when speech signal is transmitted to the non-linear bandpass filters, the speech signal is divided into several channels, but at the same time the phase of output signal is changed. The APDFT is a new type of zero-phase linear filter, which is a new FIR filter design possessing concurrently the merits of the conventional windowing and the frequency sampling. The phase of original signal can be reserved more integrally in the improved CIS strategy, so the synthesized speech will have better sound quality.The principle of APDFT is introduced detailedly in this paper. Compared with FIR filter design, the APDFT has the advantage of the frequency sampling in overall filter performance and can be used to realize strictly and power complementiuy sub-band filtering. Compared with 8 channels Butterworth bandpass filter, different characteristics of 8 channels APDFT bandpass filter can be seen in the comparing picture. CIS strategy simulation is done on the computer and gets the last synthesized speech signal. The speech signal wave, spectra and actual hearing experiment shows that we can get better sound quality in the improved strategy.Above all, there are three points in this paper:1. The APDFT filter is used into cochlear implants CIS strategy at first;2. CIS strategy simulation is done on the computer with Matlab and we get the last synthesized speech signal;3. We have done the actual hearing experiment with the synthesized speech signal. The result of experiment proves that the improved strategy has more advantages than old strategy.