Initialization of a baseband down-converter.
dn = downconvert_init(nu) dn = downconvert_init(nu, opt)
Normalized RF or IF frequency (between -0.5 and 0.5)
'r' (default) for real input signal, 'c' for complex input signal
If the input signal is complex, no image filtering is down.
If the input signal is real, image is removed with a low-pass filter
with cut-off frequency set to if
, and
otherwise
(in either case, half-way between baseband signal and image signal).