Marcus,
Thanks, that would explain the slow updates that I was seeing. Perhaps an
overlapped FFT would be useful for interactive scenarios. Has one been
implemented?
This, however, does not explain why my windows are not responding. After about
8 seconds, the window turns to grey and the GUI of the FFT becomes frozen.
Regards,
Mark McCarron
Date: Thu, 16 May 2013 09:49:59 -0400
From: mle...@ripnet.com
To: discuss-gnuradio@gnu.org
Subject: Re: [Discuss-gnuradio] WX GUI FFT Sink Performance
Marcus,
Sorry for the late reply on this, I've been upgrading my
hardware and I'm just catching up. Here is my issue, in
Spectrum lab if I provide a FFT Input length of 65536 on a
192Ksps stream, I get the following characteristics:
Effect of FFT settings with fs= 192.000 kHz:
Width of one FFT-bin: 2.92969 Hz
Equiv. noise bandwidth: 4.39453 Hz
Max freq range: 0.00000 Hz .. 96.0000 kHz
FFT window time: 0.341 s
Overlap from scroll interval: 98.4 %
It runs quite fast. If I provide the same FFT size to WX GUI
FFT sink, it basically hangs. Do you know why?
Regards,
Mark McCarron
Because apparently SpectrumLab is using an overlapped FFT
implementation. The one in wXGUI doesn't. Further, the wxGUI
implmentation has far
too much Python involved in processing samples, so trying to
process 65536 samples at a time is likely sluggish, to the point
that it can't keep
up in real time.
The underlying FFT implementation itself is very fast--Gnu Radio
uses FFTW.
I've regularly built FFT filters with 250e3 taps, and they are able
to run in real time with sample rates into the many Msps.
So, if you do the math, a non-overlapped FFT implementation of 65536
bins at 192Ksps means 2.92 FFTs/second. If the display update rate
is
more than that, there's no way to actually produce an update rate
faster than 2 per second under those circumstances, with a
non-overlapped
FFT.
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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