Making a Bandpass Filter from a Lowpass Filter
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Figure 9.13 shows how a bandpass filter can be made using a lowpass
filter together with modulation. The input spectrum is
frequency-shifted by
, lowpass filtered, then
frequency-shifted by
, thereby creating a bandpass filter
centered at frequency
. From our experience with
rectangular-window transforms (Fig.9.11 being one example), we
can say that the bandpass-filter bandwidth is equal to the main-lobe
width of the aliased sinc function, or
radians per sample
(measured from zero-crossing to zero-crossing).
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Uniform Running-Sum Filter Banks
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Modulation by a Complex Sinusoid