6/14/10 Daily Journal of AT

Another day where things go wrong. They seem to come in cycles.

I basically had two things I wanted to do today: figure out a better peak finder, and find what peaks correspond to what frequencies. Since the correspondence was easier, I decided to get that out of the way first. It started out fairly easily: I created a number of files of frequencies ranging from 10 to 200, going up by 10 each time, then wrote a shell program to run an FFT and a peak finder on each of them. (Since there was only one peak, I ran the halfing function six times on each, which got the major peak, or occasionally peaks.) I plotted the data and found the ratio. To be on the safe side, I did so for a variety of buffer sizes, from 2^15 to 2^10. I decided to use the largest, as it gave the most exact data. With 2^15, or 32768, the ratio of array placement to Hz was 28.6. So far so good.

The problem came when I decided to run some more frequencies through; namely, frequencies 50 through 2000, going up by 50. That's when the problems began. For 50, 100, 200, and 250, the peaks corresponded perfectly with their respective Hz. (150 had some problems with resonance, I think. I don't know why.) However, for 300 onward, there were no peaks in their respective frequencies. Every peak was less than 300. I thought it might be the multiplier, but changing it didn't help with the higher frequencies, and made the lower ones wrong as well. The only conclusion I could come to was that the FFT was cutting off data. Which made sense, given the data.

However, this flies in the face of the testing data done on one of our files, a440AndOnePartial.wav. When graphed, it clearly gave two peaks, the nearer one much larger. To try and figure out what was going on, I graphed the data from 50, 100, 150, 200, 250, and the a440 file. A little blurry, but I'll explain.
The five graphs on the left are 50 through 250, with the yellow boxes pointing out the peaks. 150 is hard to see, but looking at 140 and 160 (immediately to the right) it is clear where 150 should be. However, to the top right is the a440 and partial graph, which, fairly clearly, shows those peaks. In the completely wrong positions!

I haven't had much time to mess with the FFT. I tried taking out the mirroring function, which gives a few peaks close to accurate for 300 to 500. However, beyond that, the peaks do not exist. I'm going to do some reading tonight to try and figure out what's wrong, but unless this is fixed, the FFT function is useless for any frequencies above middle C.