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Re: qt waterfall sink time axis

From: Jeff Long
Subject: Re: qt waterfall sink time axis
Date: Fri, 2 Jul 2021 08:47:54 -0400

If there is not enough data to feed the FFT at the selected display rate, the sink does appear to run slow and the time axis will not be accurate. Try either a smaller FFT, or lengthen the interval even more.

On Fri, Jul 2, 2021 at 8:34 AM John Murphy <mr.john.joseph.murphy@gmail.com> wrote:
Hey all,

I have been flailing away, and failing, at getting the qt waterfall sink time axis to be scaled properly.
Using gnuradio on Manjaro

One thing I found looking at the source code for the waterfall sink is that despite the option to set an update period the block always uses 10 seconds as an update period. The setting only affects the scale of the time axis.
So I would have expected that setting the update period to 10 seconds (the default is I think 0.1 seconds) would make everything come out correctly. But this does not seem to be the case at all.
I have a message probe sending out some modulation every 60 sec and I also have a stopwatch to see how far the waterfall display fills up over several minutes (ie 8 mins 20 sec for the 5e2 sec mark with 10 sec update rate set).
I've tried changing fft size etc (looking at the code that would only make a difference if the FFT were so large that accumulating the samples took more than 10 sec) but can't seem to find the correct time scale.

Searching the mail archives all I see vaguely associated with this issue is an old (2014) post on waterfall time axis that does not seem to relate to the issue with the qt waterfall which I see now.

I'd expect that this is an oft-used block. So I am hoping that asking maybe someone knows how to do this.

Thanks folks.

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