FFT_LENGTH and FFT.h

pfournier · Joined: 30 Sep 2003 Posts: 89

In FFT.h (Using compiler 5.076 on the DSP Analog Demo Kit.)

There is this line:

#error FFT LENGTH must be a power of two and not more than 512!

I am curious as to the limitation for not being able to do 1024 or 2048.

I was able to compile and run 1024 (pushing the limit on memory), but the output was missing a freq peak (expecting 2 freq peaks which I get on 512 and lower). So while I will trust that it does not work, I am kind of hoping that someone can tell me if it is beyond hope or what is the source of the limitation?
_________________
-Pete

newguy · Joined: 24 Jun 2004 Posts: 1907

I don't know the precise reason as I've never used the FFT functionality, but perhaps it's to prevent an overflow of a variable(s)?

Ttelmah · Joined: 11 Mar 2010 Posts: 19495

Historically in the old version of this code they put the xdata array at 0x800. Positioned here this limits the maximum supported number of points to 1024. Latter include files have this relocated to 0x1000 which should support up to 2048 points, but the validity test from the older code was not updated. With enough RAM, the code ought to accept your 1024 entries, if you add this to the error test. With the right values it ought to work.