Patent ID: 8036376

Claim:
An echo prevention circuit for an earphone microphone comprising: an input terminal into which a first input signal corresponding to voice output from an external communication device is received; a first FIR filter into which the first input signal is input through the input terminal; a second FIR filter into which the first input signal is input at the same time as into the first FIR filter; an input/output terminal to which an output signal of the first FIR filter is output and a second input signal output from the earphone microphone is input; a first subtracter that subtracts an output signal of the second FIR filter from a combined signal of the output signal of the first FIR filter and the second input signal; an adaptive digital filter into which the output signal of the first FIR filter is input; a second subtracter that subtracts an output signal of the adaptive digital filter from an output signal of the first subtracter; and an output terminal to which an output signal of the second subtracter is output, wherein the earphone microphone is connected to the input/output terminal, and converts voice into the second input signal, the first FIR filter performs a convolution operation on the first input signal based upon first FIR filter coefficients, the second FIR filter performs a convolution operation on the first input signal based upon second FIR filter coefficients, the second FIR filter coefficients differing from the first FIR filter coefficients, the adaptive digital filter adaptively updates filter coefficients based on the output signal from the first FIR filter while the first FIR filter is receiving as input the first input signal from the external communication device and the output signal from the second subtracter, such that the output signal from the output terminal is the output signal from the first subtracter that has had the other than the second input signal removed or reduced; and the first subtracter includes a differential amplifier having the combined signal of the output signal of the first FIR filter and the second input and the output signal of the second FIR filter inputted thereto.