2/28/08
DTFS is for periodic signals only
aperiodic signals must use a discrete time fourier transform DTFT (Fig1)
need inverse DTFT (Fig2)
properties of DTF spectra
	continuous in OMEGA
	periodic in OMEGA with period of 2pi rad/s
	only 2pi radians of unique frequencies in DT
	if f[n] is real, then F(OMEGA) has conjugate symmetry
		abs(F(-OMEGA))=abs(F(OMEGA))
		angle(F(-OMEGA)) = -angle(OMEGA)
	linearity
	existence of DTFT
		DTFT exists if sum of mag of terms in signal is less than inf (absolutely summable)
	
properties of DTFT
	time and freq inversion
	multiply by n -> derivative of F
	time shift -> exponential
	frequency shift -> exponential
	convolution

example (Fig3)