"BobW" <nimby_GIMME_SOME_SPAM@roadrunner.com> wrote in message
news:Rsidnd6P067YldXXnZ2dnUVZ_u-dnZ2d@giganews.com...
>
> I'm using a microcontroller (dsPIC30F3013) to do an FFT on some "real"
> data (from a microphone) and I don't have enough internal RAM to get the
> frequency resolution I need.
>
> I found a forum entry by Rick Lyons as follows:
>
> -quote-
> Does the dsPIC33 allow you to perform
> 512-point FFT on complex-valued input
> samples? If so, there's a way to perform
> a 1024-point *real-input* FFT using
> a 512-point complex FFT routine.
> [-Rick-]
> -end quote-
>
> The FFT routine I have available does take a complex input (two sixteen
> bit words), and the existing code merely stuffs zeroes into the imaginary
> part of each complex input.
>
> So, can anyone tell me what the trick is for allowing this complex-input
> FFT to be more efficient with real input data such that I can effectively
> cut in half the input record length I need to use.
>
> Thanks in advance.
>
> Bob

If anybody is interested, I found the answer. It was in the last place I
looked.

Rick Lyon's book "Understanding Digital Signal Processing" has an excellent
and (relatively) simple description of the technique. It involves a decent
amount of processing beyond the complex FFT, but it's doable.

So, with Rick's help, I think that I can do what I need to do with the
limited RAM that I have available.

Bob
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