high frequency is changed into low frequency in time domain: w-l. l and w=0are the same 0.5 0 -0.5 10 trigonometric function property co?(r]s)=c02(0)cos(1.1n) = cos(0.9n) high frequency is changed into low frequency in time domain： w=1.1π and w=0.9πare the same trigonometric function property