FFT not working

[btb4198]

ok I am using this site to test :
http://onlinetonegenerator.com/

and I am using 261Hz and I am doing a sawtooth wave.
but the fft give me this back :
the highest point is 1291.992188 hz that is not close to 261 hz
please see attachments

[steve]

The problem is probably the online generator.

This picture shows the online generator waveform (downloaded as a WAV file) in the top track, and an accurate 261 Hz saw wave below it.
Note that the waveform from the online generator is offset vertically from the centre line:

tracks004.png (17.18 KiB) Viewed 2047 times

[btb4198]

ok if you download the file from the site ( and not just recording) and increase the Size to 4096 the fft seem to work.
also but how about this one
I did the Generate Pluck
Pluck MIDI pitch: 60 hz
and
when I do the fft I get
the peaks are 1832 hz and 3401 hz

that is not close at all

[billw58]

MIDI Pitch 60 is not 60 Hz, it is middle C: 261.63 Hz.

In order to accurately show lower frequencies in the spectrogram you need to increase the Size, but I can see the 261 Hz peak with a size of 256 (although it reports as 253 Hz). Increasing the size to 4096 (not unreasonable) and changing to log frequency (not strictly necessary, but it makes it easier to see the low frequency peaks), Plot Spectrum reports a peak at 262 Hz.

-- Bill

[steve]

the peaks are 1832 hz and 3401 hz

Where do those numbers come from?

[btb4198]

steve,

I got those numbers from the plot
please see the pic1.png and spectrum4.txt

[billw58]

the peaks are 1832 hz and 3401 hz
I got those numbers from the plot
please see the pic1.png and spectrum4.txt

Your plot shows a fundamental at about 262 Hz, which is correct for a pluck at middle C.

The frequencies you've identified are the 7th and 13th harmonics.

Nobody said the fundamental would be the highest peak (although it often is).

The FFT (Plot Spectrum) is working fine.

-- Bill

[btb4198]

Bill
"Nobody said the fundamental would be the highest peak (although it often is)."

It is always...

how are you getting 261.63???

looking at this graphic i would not get that the fundamental F is 261.63
how are you getting that ?

[steve]

how are you getting 261.63???

261.63 is the standard tuning for C4 (middle C) based on A440 tuning. Other standard frequencies can be found here: http://www.phy.mtu.edu/~suits/notefreqs.html

[btb4198]

I know that 261.63 is C4 or middle C
but how did you get that from looking that the graphic?

If I did not say that I did a MIDI Pitch 60 Pluck , would you know that the fundamental frequency 261.63 Hz from the graphic ?

that is what I am asking.