# Can I use uhd\_fft or gnu-radio to display the spectrum with adjustable frequency resolution **URL:** \ **Category:** GNU Radio\ **Created:** [June 4, 2014, 12:49am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183 "2014-06-04T00:49:28Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![jason\_sam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7993a0/32.png) [@jason\_sam](https://www.ruby-forum.com/u/jason_sam)\ **Post date:** [June 4, 2014, 12:49am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/1 "2014-06-04T00:49:28Z")
I am trying to use the uhd\_fft or gnu-radio scope in a way that it can show a real-time spectrum but with adjustable resolution. The uhd\_fft spectrum resolution appears to be fixed and not changeable. The gnu-radio scope block only puts out time series but not real-time spectrum. What is my way out of this situation? Thanks, LD ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [June 4, 2014, 1:07am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/2 "2014-06-04T01:07:44Z")
Hi LD, you’re right the QT GUI sink does not seem to care about changing FFT sizes. I remember the set\_fft\_size function being implemented, I think it’s mainly a matter of having a callback defined for changes of the fft size parameter. As I’m not too familiar with that, my mileage when playing with the QT sinks may vary. Could you try adding set\_frequency\_range($fc, $bw) to gr-qtqui/grc/qtgui\_freq\_sink\_x.xml where the other callbacks are defined and try if it works for you? Greetings, Marcus ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [June 4, 2014, 1:16am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/3 "2014-06-04T01:16:34Z")
Hi! On 04.06.2014 01:05, Marcus M. wrote: > set\_frequency\_range($fc, $bw) > to gr-qtqui/grc/qtgui\_freq\_sink\_x.xml where the other callbacks are > defined and try if it works for you? > Sorry, copied the wrong line. > What I wanted to paste was: > set\_fft\_size($fftsize) Greetings, Marcus ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [June 4, 2014, 1:36am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/4 "2014-06-04T01:36:36Z")
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 So I couldn’t resist, and the line that would actually call the set\_fft\_size callback function correctly would’ve been set\_fft\_size(min(int($fftsize),4)) However, QT Gui spectacularly SEGFAULTs on me when doing that. That actually happens in line 123 of gr-qtgui/lib/freqdisplayform.cc : d\_sizemenu-\>getActionFromSize(newsize)-\>setChecked(true); which _might_ be some relict from older times, but I can’t really tell. I hope someone more familiar with the qtgui infrastructure might chime in. Greetings, Marcus On 04.06.2014 01:14, Marcus M. wrote: > \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Discuss-gnuradio > mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 Comment: Using GnuPG with Thunderbird - [http://www.enigmail.net/](http://www.enigmail.net/) iQEcBAEBAgAGBQJTjlvFAAoJEBQ6EdjyzlHtIN4IAMHXlNAi86/E+Y3oSc8WkjEQ Hyh370ab55EaFrbvHYb9yLEXjC7hS1ccBLIpbMSKIRrHL/Z4rfIfMzlhEF4LA2cW TSJ0ydXb+KktRJ7bQ+V62RIufcyUXZc4OWZjfYvjT42GIzgUZAp3kI18n6Ohfeta FfoNWUnFJMxywYlgIE7QUgUYWIwMq4FTMGqNjGbBL8Jn6/RE7X8kJbnErjqNUprH Rx96MSj1MBiZrDPuPKYCgj8LR1c00jALorgA8lXtpYK1gufjSmYO0Bjol179kegb nQFPhMy6cfZChy8cTP0oBxQmC9aPrPtpCmHBcBCoDCIZxcUBU/x4TEEejgFM64g= =s2I+ -----END PGP SIGNATURE----- ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [June 4, 2014, 1:44am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/5 "2014-06-04T01:44:51Z")
On Tue, Jun 3, 2014 at 4:35 PM, Marcus Müller [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > I hope someone more familiar with the qtgui infrastructure might chime in. Hey Marcus, Did you try debugging the SEGFAULT using gdb? This is a good reference to try: > **[RMS's gdb Tutorial: Segmentation Fault Example](http://www.unknownroad.com/rtfm/gdbtut/gdbsegfault.html)** > > A Question-And-Answer tutorial on how to use GDB (the GNU Debugger) ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [June 4, 2014, 9:23am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/6 "2014-06-04T09:23:58Z")
Hi! Alfredo: Jep, did that. GDB was how I figured out things went wrong in FreqDisplayForm::setFFTSize() in the first place :). J?rg: Interesting; but the error is quite reproducibly at the same place; I’ll try to set a breakpoint at the kernel call and see if that happens again. But that will require some gdb magic (so that GDB only interrupts program flow when the fft size changes), and that’s definitively something that would be worthwhile to learn, however I’m postponing this for now for reasons of time available. Greetings, Marcus ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [June 4, 2014, 10:14am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/7 "2014-06-04T10:14:03Z")
I think it’s going wrong in getActionFromSize, which easy as it looks[1], I don’t get the hang of. Why do we need a specific QAction based on the FFT length we’re setting? And why does an FFT size of 4 lead to negative indices in a QList? Anyway, the actual bug was on my side, as I was constantly setting an FFT size of 4. Which seemingly doesn’t work. @LD: try set\_fft\_size(max(int($fftsize),32)) instead. I was then able to change the displayed FFT on the fly using QT Gui sliders [2]. Greetings, Marcus [1] gr-qtgui/include/gnuradio/qtgui/form\_menus.h [2] [Adjustable QT FFT · GitHub](https://gist.github.com/marcusmueller/edfef568a92821261f9f) , the adjustable\_qtgui\_fft.grc file should load nicely in a 3.7.4 GNU Radio, I think it should work with 3.7.x ---
**Author:** ![LD\_Zhang](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/90ced4/32.png) [@LD\_Zhang](https://www.ruby-forum.com/u/LD_Zhang)\ **Post date:** [June 4, 2014, 8:49am UTC](https://www.ruby-forum.com/t/can-i-use-uhd-fft-or-gnu-radio-to-display-the-spectrum-with-adjustable-frequency-resolution/230183/8 "2014-06-04T08:49:12Z")
Hey, I know another GSoC Student has problems with the a GT GUI sink as well. It would randomly SEGFAULT in a volk multiply kernel. I think it is the volk\_32\_fc\_32f\_multiply\_32fc kernel used in waterfall\_sink\_X\_impl::fft(). This kernel takes the FFT size as an input argument, so it could be related to your problem. I just had a quick glance at it so I am not sure though. I’ll ask him to post his exact problem when he arrives at the lab. Maybe he can shed a bit more light on this problem -Jan