low freq FFT accuracy?

rsmrecords · 15th December 2009, 03:11 PM

Hi,

I'm looking for information on the accuracy of FFT frequency spectra for low end information. ie. below 100 hz.

In my system, using VST analysers like Nugen's Visualizer, or Bias Reveal, I consistently see low end out put that I don't think is actually there. My monitors extend down to 30 hz at least, and there are high pass filters on most recorded tracks. So it seems that there should only rarely be any energy in the 20-60hz range.

I have read that the accuracy of FFT is tied to the sample rate and FFT window size. Maybe this is an issue with typical 44.1 and 88.2 working session rates?

Any comments would be appreciated.

thanks

r

myrtlebacker · 15th December 2009, 08:33 PM

Quote:

Originally Posted by rsmrecords

I have read that the accuracy of FFT is tied to the sample rate and FFT window size.

AFAIK it's just tied to the window size and the window shape (like f.e. Hanning or Blackman). I don't have either plugins so I don't know, what you can set there. Try to go for 2048 or better and plot logarithmically.

DrFrankencopter · 15th December 2009, 08:51 PM

Larger window sizes will give you better low frequency resolution, but will have more variability in the high frequency range.

Small window sizes will average out the low frequency information because each window gets de-trended (has it's average, and often linear slope removed). Instead of thinking of your window size in number of points, think of it in the time domain. @ 44.1 kHz, 2048 samples is 0.046 seconds, which corresponds to a frequency of 21.5 Hz (1/0.046-21.5). This is approximately the lowest frequency you can resolve with this window size.

Another problem with smaller window sizes is that they have coarse low frequency resolution (not a whole lot of points where the FFT is calculated)...instead, most of the points are distributed in the high frequency area.

A better way to approach this would be for plug-in developers to use a Chirp-Z transform instead of FFT. It would take more processing time, but would give greatly improved low frequency accuracy. Probably this would be overkill for real-time spectral analysis plugins, but would be nice for acoustic analysis.

Cheers

Kris

15th December 2009, 03:11 PM	#1
rsmrecords Gear interested Join Date: May 2009 Posts: 2	low freq FFT accuracy? Hi, I'm looking for information on the accuracy of FFT frequency spectra for low end information. ie. below 100 hz. In my system, using VST analysers like Nugen's Visualizer, or Bias Reveal, I consistently see low end out put that I don't think is actually there. My monitors extend down to 30 hz at least, and there are high pass filters on most recorded tracks. So it seems that there should only rarely be any energy in the 20-60hz range. I have read that the accuracy of FFT is tied to the sample rate and FFT window size. Maybe this is an issue with typical 44.1 and 88.2 working session rates? Any comments would be appreciated. thanks r

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15th December 2009, 08:51 PM	#3
DrFrankencopter Gear addict Join Date: Sep 2003 Location: Ottawa Posts: 433	Larger window sizes will give you better low frequency resolution, but will have more variability in the high frequency range. Small window sizes will average out the low frequency information because each window gets de-trended (has it's average, and often linear slope removed). Instead of thinking of your window size in number of points, think of it in the time domain. @ 44.1 kHz, 2048 samples is 0.046 seconds, which corresponds to a frequency of 21.5 Hz (1/0.046-21.5). This is approximately the lowest frequency you can resolve with this window size. Another problem with smaller window sizes is that they have coarse low frequency resolution (not a whole lot of points where the FFT is calculated)...instead, most of the points are distributed in the high frequency area. A better way to approach this would be for plug-in developers to use a Chirp-Z transform instead of FFT. It would take more processing time, but would give greatly improved low frequency accuracy. Probably this would be overkill for real-time spectral analysis plugins, but would be nice for acoustic analysis. Cheers Kris