Quick real-time identification and estimation of a time-non-varying or time-varying system. A new H evaluation criterion is determined, a fast algorithm for a modified H filter based on the criterion is developed, and a quick time-varying system identifying method according to the fast H filtering algorithm is provided. By the fast H filtering algorithm, a time-varying system sharply varying can be traced with an amount of calculation O(N) per unit time step. The algorithm completely agrees with a fast Kalman filtering algorithm at the extreme of the upper limit value. If the estimate of impulse response is determined, a pseudo-echo is sequentially determined from the estimate and subtracted from the real echo to cancel the echo. Thus an echo chancellor is realized.