Unless mentioned otherwise, a low-pass filter has real
coefficients, so that IH(e/")l is symmetric and +(U) is antisymmetric
with respect t o o = 0. Usually (H(e/")( is plotted
for 0 I f 5 0.5 (i.e., for 0 5 w 5 s). If up and us denote the
passband and stopband edges of a low-pass filter, the quantity
wc = (ap + wJ2 is said to be the cutoff frequency