The sound received by the microphone is then converted into an electrical signal and is inputted to the chip. The implantable microphone receives the sound that the user wishes to hear coming from outside the body. The F-IMEHD comprises an implantable microphone, a transducer, and a wireless communication module. Experiments were performed on a cadaver to verify the performance of the fabricated chip.
The chip was fabricated using a 0.18 μm CMOS process (SMIC, China). The chip was designed by Verilog-HDL and the designed HDL codes were verified by Modelsim-PE 10.3 (Mentor graphics, USA). The communication control part is designed to perform the wireless fitting and control. The PDM signal is then interpolated four times to obtain a high-quality analog signal. The output signal of the WDRC is converted to a pulse density modulation (PDM) signal using the designed pulse density modulator. The FFT algorithm was improved to reduce the computational complexity and the size of the memory. Each channel is separated using a 64-point fast Fourier transform (FFT) method and the gain value is matched using the fitting table in the fitting memory. The designed signal-processing chip comprises a 4-channel WDRC, a fitting memory, a communication control part, and a pulse density modulator. This algorithm was developed focusing on eliminating time delay and system complexity in the transform process. In this study, we designed and fabricated a signal-processing chip using the modified FFT algorithm. So, the WDRC system that has a new signal processing method for low time delay and minimum complexity for low power consumption is needed. And, the complexity of the system increases by logic circuit for IFFT processing and memories for IFFT processing. A time delay occurs corresponding to the window size, because it has to store and process the signal as much as the window size for the FFT and IFFT at one time. The process of FFT and IFFT cause time delay and increase the system complexity. In this case, the FFT transmitted signal is needed inverse fast Fourier transform (IFFT) for conversion to time domain signal. The flow diagram of signal processing of WDRC that use the FFT is shown as Fig. The filter bank that consists of narrow band-pass filter has signal delay problems . Moreover, the narrow band-pass filter that consists of FIR or IIR filter needs a lot of taps. The filterbank needs many band-pass filters for multi-channel WDRC. Generally, the filterbank or fast Fourier transform (FFT) is used for WDRC algorithm . Hence, because the F-IMEHD is implanted in the human body, a wireless fitting function is employed. The fitting algorithms are used to compensate for the hearing loss of a user. The WDRC algorithm is employed because non-linear fitting algorithms are required for digital hearing aids .
The function of the WDRC is to incorporate an additional input range of 100 dB into the existing dynamic range of the user. The WDRC algorithm is a widely used compensation algorithm in digital hearing aids.
Hence, the signal processor for the F-IMEHD must comprise a wide dynamic range compression (WDRC) algorithm and a wireless fitting function to effectively compensate for the hearing loss. The main function of the signal processor is to compensate for the hearing loss via amplification, filtering, and compression. Furthermore, the signal processor should have a small size and low power consumption for implantation in a human body. In the design of the signal processor, the input and output characteristics, such as the output signal level of the microphone and the resonance frequency of the vibration transducer, must be considered. The F-IMEHD system comprises an implantable microphone, a vibration transducer, and a signal processor . Otologics MET Carina™ and Envoy Esteem are examples of F-IMEHDs . In particular, a fully implantable middle ear hearing device (F-IMEHD) is developed for people with sensorineural hearing loss. To deal with this problem, various types of hearing aids are being rapidly developed . Recently, the population of hearing impaired people has increased, with many people suffering from hearing problems .
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