5.1.1 Frequency Response Phase and Group Delay Ideal lowpass filter with linear phase delay Hp(e) Hp(em）=e®h,ldko The impulse response(delayed by time n) ,（m）-s ina(n-na) -oo<n<oo π(n-na) noncausal 0.6 na=5 hp(n),O<ns10 0.4 causal 复 0.2 H(e).M=19 0.2 windowed hn can be causal, 10 approximations to ideal filters. 19◆Ideal lowpass filter with linear phase delay ( ) ( ) ( ) sin , c d lp d n h n n n n n   − = −  − ◆The impulse response (delayed by time nd ) 5.1.1 Frequency Response Phase and Group Delay 19 nd =5 ( ) , j lp c d j n H e e     − =  noncausal causal approximations to ideal filters .h n lp( ), 0 1  n 0 windowed hlp [n] can be causal