On Fri, 23 Jun 2000 10:27:59 +0100, Talbot Andrew
<[email protected]> wrote:

1)      Take the off air signal recording at (say) 8000 Hz sampling rate

2) Digitally mix down to some arbitrarily low frequency

3)      Filter to the necessary sub-Hz bandwidth centred on the signal

4)      Decimate this filtered data to a much lower sampling rate by

just taking one sample out of every (say) 256 .

For anyone contemplating writing software to do this, note that stages
3) and 4) can be taken together - if using an FIR filter, the filtering
process needs to be done for each decimated sample only - this saves
considerable processing overhead for the long filter tap lengths
necessary

If you plan to combine steps 3) and 4) by doing first decimating and
then narrow band (sub-Hz) filtering, shouldn't you low pass filter the
signal produced by the mixdown at 2) to have a bandwidth less than 15
Hz for the final 31.25 Hz sample rate to meet the Nyquist criteria ?

Paul OH3LWR