From:                   Paul Keinanen <[email protected]>
To:                     [email protected]
Subject:                LF: Re: Shrinking sounds - fiasco
Date sent:              Fri, 23 Jun 2000 15:48:12 +0300
Send reply to:          [email protected]

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

73 de Klaus, DJ5HG