Howling canceller

A howling canceller mounted in an apparatus including a digital-to-analog converter that converts a digital speech signal into an analog speech signal, a power amplifier that amplifies the analog speech signal output from the digital-to-analog converter, a speaker that plays back the analog speech signal amplified by the power amplifier and outputs sound, a microphone that converts sound including playback sound output from the speaker into a second analog speech signal, a microphone amplifier that amplifies the second analog speech signal output from the microphone, and an analog-to-digital converter that converts the second analog speech signal amplified by the microphone amplifier into a second digital speech signal, the howling canceller comprising:

• a finite impulse response filter that operates the digital speech signal with a tap coefficient generated by an adaptive filter, thereby generating replica of a playback sound component;
• a subtractor that subtracts the replica of the playback sound component from the second digital speech signal, thereby generating a residual signal;
• a predictor that predicts a prediction value which is a signal component having a time difference corresponding to one sample with respect to the digital speech signal, and subtracts the prediction value from the digital speech signal to generate a second residual signal;
• a filter circuit that subtracts a second prediction value, which has a time difference corresponding to one sample with respect to the second digital speech signal, from the second digital speech signal, thereby generating a desired signal;
• the adaptive filter that operates the second residual signal with the tap coefficient to generate a pseudo echo, and updates the tap coefficient such that a third residual signal is an optical value;
• a second subtractor that subtracts the pseudo echo from the desired signal to generate the third residual signal;
• and
• an amplitude limiting circuit that limits an absolute value of an amplitude of the residual signal to be equal to or smaller than a predetermined threshold value, and outputs the amplitude-limited residual signal to the digital-to-analog converter, the finite impulse response filter, and the adaptive filter as the digital speech signal,
• wherein the threshold value is a minimum value of a first threshold value set in a linear region of the digital-to-analog converter, a second threshold value set in a linear region of the power amplifier, a third threshold value set in a linear region of the speaker, a fourth threshold value set in a linear region of the microphone, a fifth threshold value set in a linear region of the microphone amplifier, and a sixth threshold value set in a linear region of the analog-to-digital converter.