Hello,

a good abstract!

Imre


On Tue, 31 Aug 1999, Steve Thackery wrote:

> Thanks for that, Imre.
>
> I'm a mathematical dunderhead so have to see these things in very simple
> terms. I can summarise the two key points of FFT very briefly, and do so in
> order to help other mathematically-challenged people.
>
> Firstly, the maximum frequency you can extract is half the sampling
> frequency.  In other words, if you sample at 10kHz then the highest
> frequency you can pull out is 5kHz (this is known as the Nyquist frequency).
>
> Secondly, the maximum *resolution* is 1 / duration of sound bite.  In other
> words, if your sound bite is 100ms long, then the output of the FFT is in
> steps of 10Hz.  If it's 2 seconds long, the steps are 0.5Hz apart.  Note
> that this is *independent* of the sampling frequency.
>
> There is one last thing to emphasise: filter out everything above the
> Nyquist frequency *before* you do the sampling.  In the example 1/ above
> with a 10kHz sampling frequency, filter out everything above 5kHz.  The
> reason for this is that frequencies above 5kHz get "mirrored" back into the
> 0 to 5kHz range and give you invalid results.
>
> Hope this is a useful summary.
>
> Steve Thackery
> Suffolk, England.
> Web Site: http://www.btinternet.com/~stevethack/
>
>