wrmoy / avr-fft-display Goto Github PK

Hi William,

I ran across your code looking for a AVR Goertzel implementation, and having a closer look at it and trying to understand your optimizations, I'm afraid I have found some flaws in it:

By only updating the n-th frequency bin every 2^n-th time and discarding all the samples in between, you are actually performing a downsampling by a factor of 2^n. But you need to perform lowpass filtering with a cutoff frequency of 2pi/2^n in order to avoid aliasing.

For the 0th frequency bin, everything is fine and you get the spectrum at 0.94 f_samp, but the 1st frequency bin contains not only the spectrum at 0.94/2 f_samp, but also at (1+0.94)/2 f_samp. For the 2nd bin, it's the superposition of the spectrum at 0.94/2 f_samp, (1+0.94)/2 f_samp, (2+0.94)/2 f_samp and (3+0.94)/2 f_samp, and so on for the following bins.

Even though you haven't been working on this project for 3 years now, maybe this info is helpful for you.

best regards
Henning