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[Discuss-gnuradio] OpenCL FFT synchronization question
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Subject: |
[Discuss-gnuradio] OpenCL FFT synchronization question |
Date: |
Fri, 16 Aug 2019 12:27:29 +0000 |
As I am investigating GPU and OpenCL for acceerating correlation computations,
I thought
I'd give OpenCL-Enabled FFT block in GNU Radio Companion (still version
3.7.13.4) a try to
see if I can see any performance change.
When I run signal source -> throttle -> OpenCL FFT -> time sink, all goes well,
the spectrum
*looks* similar to the GNU Radio FFT (CPU) output. So far so good.
If I now run the correlation graph found at
http://jmfriedt.org/opencl/opencl_xcorr_grc.png
the correlation surprisingly is non existent as seen at
http://jmfriedt.org/opencl/nosubtract.png
(top chart is CPU computation of the correlation as
iFFT(FFT(signal).FFT(delayed signal)^* with
a fine correlation peak on the right, and bottom chart is the same through the
OpenCL FFT).
Running the same chart but fed with a periodic signal (square wave) does show a
set of correlation
peaks spaced by the square wave period as shown at
http://jmfriedt.org/opencl/nosubstract_square.png
hinting at a synchronization issue with the two OpenCL blocks being fed random
sequences sampled at
different time.
Trying to display the difference between the two correlations (output of the
complex conjucate
multiplication) actually seems to solve the issue as seen at
http://jmfriedt.org/opencl/withsubtract.png
where the correlation peak suddenly appears after a fraction of a second of
running the flowchart,
so the GNU Radio scheduler seems to be affected by this new subtraction chain.
Yet, notice that
the subtraction between the two correlation processing chains (top chart) is
not 0, unlike running
the same flowchart with subtraction on the periodic square wave
(http://jmfriedt.org/opencl/withsubstract_square.png) which does show a null
error.
I don't have an explanation, not even sure there is an obvious one, but at
least wanted to share
these observations with the community.
JM
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