说明：  The Window Design Method The basic idea behind the design of linear-phase FIR filters using the window method is to choose a proper ideal frequency-selective filter [which always has a noncausal, infinite duration impulse response] and then truncate its impulse response hd[n] to obtain a linear-phase and causal FIR filter h[n]. To truncate the impulse response of the ideal filter a time window w[n] is used. Available windows in Matlab are rectangular [or boxcar in Matlab], bartlett, hamming, hanning