Re: Large FFT vs Many FFTs

On Apr 5, 9:42 am, "Edison" <bell...@xxxxxxxxxxxxxx> wrote:
Edison,
It seems it would be easier to use a bank of five band-pass filters.
This requires much less memory than the FFT approach.

Because of the very large number of samples to be averaged, to find the
average signal levels you could use a fairly crude approach of averaging
the absolute values of the filtered signal samples, for each of the five
filters. As long as the sampling rate is not locked to the fundamental
this should be OK.

Also, as the fundamental will have the same scaling error as the
harmonics, you could probably safely leave out applying a scaling factor
to convert 'average value' to RMS.

If you really wanted the maximum possible precision you would convert
the signal to analytic, use a bank of complex filters, calculate the
magnitude of each of the five complex signals at each sample period, and
compute the five average magnitudes over a time of one second.

Regards,
John

Thanks John,

Just to complicate things there is a requirement for a second fundamental
at 90kHz. with its associated 5 harmonics. Is the solution you suggest
still feasable or am I back to a big FFT?

Ed

_____________________________________
Do you know a company who employs DSP engineers?
Is it already listed athttp://dsprelated.com/employers.php?

The method that John suggested would be good for one where you just
need to observe a few known harmonics. However, as you asked in your
first message, your idea of averaging a number of small FFTs is a
reasonable solution. In fact, Welch's method of spectral estimation is
very similar to that. It involves splitting a long signal into a
number of smaller chunks, perhaps with overlap. Each chunk is windowed
and FFTed, and the spectrum estimate is taken as the average of all of
the FFT outputs. Some other parametric methods may be more optimal in
some sense, but Welch's method is simple and it works relatively well
if you need an estimate of the entire spectrum. In your case, it
sounds like you don't, so the bandpass filter bank solution would
probably work well, but if the number of frequencies that you want to
observe increases, you might look at a different solution.

Jason

.

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