A method includes: generating a first signal according to a digital signal; filtering the first signal according to first filter coefficients of first filter to generate a second signal; adding a first reference signal with the second signal to generate a third signal; performing digital-to-analog conversion according to the first and third signals to generate and output an echo signal; performing analog-to-digital conversion according to the echo signal to generate a fourth signal; generating a fifth signal according to the digital signal and the fourth signal; and updating the first filter coefficients according to the fifth signal. Generating the fifth signal includes: generating second filter coefficients of second filter; updating the second filter coefficients according to a second reference signal and the fourth signal; obtaining a first and second representative coefficients of the second filter coefficients; and generating the fifth signal according to the first and second representative coefficients.