Sophisticated Adaptive Signal Processing Systems for Howling Canceller

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This project investigates digital signal processing systems for canceling howling efficiently. A fundamental idea to cancel howling is to use adaptive notch filters to track and reject howling a signal because it is approximately a sinusoid whose frequency is unknown. Main results of this project are as follows ;(1)An actual howling signal does not have a line spectrum but a bandpass one due to phase jitter. In order to reject such a howling signal efficiently, it is required to detect its width and to determine the bandwidth of an adaptive notch filter. Then a method for the detection of passband edge frequencies of a bandpass signal by using a second-order adaptive notch filter has been proposed. This method was presented IEICE Technical Group Workshop on Circuits and Systems on March 2006. Now it is in preparation to submit IEICE Transactions.(2)By integrating the studies in early three years it became possible to cancel howling signals in real time in last year, which is the end goal of this project. The howling canceller is realized as a software system on a personal computer, whose sound chip is used as an I/O device. Since the convergence property of an adaptive coefficient is not still perfect, very small residual howling exists. However, when this canceller is not applied, microphone amplifiers cannot work well due to large howling. Therefore, the designed howling canceller is effective. Now more studies to enhance the convergence property is doing toward patent filings.