heterodyne

Shift a signal in frequency using heterodyning, with appropriate filtering

Given a signal x, its sample rate sRate, and passband of a two-element vector [loFreq highFreq], transform the signal by shifting the passband in frequency down to 0 Hz and removing the signal at other frequencies. This is done by (1) filtering the signal to remove energy outside the passband, (2) heterodyning to frequency-shift the passband down to 0 Hz (the 'difference frequencies'), and (3) filtering again to remove the second copy of the signal that heterodyning creates (the 'sum frequencies').