7.2. 1 Windows function method o Using parseval s relation we can further obtained ∑h团-h[ ∑h团小-h[可+∑h[小+∑h[小 n=-M n=M+1 It is evident that the integral-squared error is minimum when h[小]=h[n]for-M≤n≤M,In other words the best finite-length approximation to ideal infinite-length impulse response in the mean-square error sense is simply obtained by truncation7.2.1 Windows function method  Using Parseval’s relation, we can further obtained             2 1 2 2 2 1 R t d n M M t d d d n M n n M h n h n h n h n h n h n  =− − −  =− =− = +  = − = − + +     h n h n t d   =   It is evident that the integral-squared error is minimum when for . In other words, the best finite-length approximation to ideal infinite-length impulse response in the mean-square error sense is simply obtained by truncation.−   M n M