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Re: [Discuss-gnuradio] GRC 3.7: QT GUI Sink: Frequency display: Center F

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] GRC 3.7: QT GUI Sink: Frequency display: Center Frequency bug?
Date: Thu, 22 Aug 2013 09:38:20 -0400

On Tue, Aug 13, 2013 at 6:34 PM, Tom McDermott <address@hidden> wrote:
> Hi,
> Recently upgraded to GRC 3.7 (from 3.6.5). The QT GUI Sink Frequency Display: 
> Center
> Frequency (Hz) control and the 'Display RF Frequencies' checkbox do not seem 
> to be
> operating correctly in GRC 3.7.
> In GRC 3.6, Center Frequency (on the block properties) had no effect on the 
> actual display.
> If a value was entered in Center Frequency (Hz), there was no change to the 
> display at all.
> If the 'Display RF Frequencies' checkbox was checked and a non-zero value was 
> in the
> Center Frequency (Hz) block, then the display did not change, but the X-Axis 
> labels were
> changed to the sum of (display + Center Frequency).  I think this agrees with 
> the
> documentation. This was very useful for example as the NCO frequency of a 
> down-converting
> receiver could be entered in the Center Frequency box, and when 'Display RF 
> Frequencies'
> box was checked, the frequency display would show the actual RF frequency of 
> the signal prior
> to downconversion. Searching the  discuss-gnuradio archives on the topic 
> revealed the last
> post on this topic in 2011, and that this seemed to be the intended operation 
> of the controls.
> In GRC 3.7 however it does not work this way. Placing a value in Center 
> Frequency (Hz) shifts
> the graphical display. If Center Frequency has a value greater than the 
> display width, then
> in fact nothing at all is displayed on the graph (it would all be shifted 
> past the right side). The X-Axis
> labels are altered whether or not the Display RF Frequencies checkbox is 
> selected. For
> example: a 10 KHz complex waveform shows a single peak at 10 KHz when Center
> Frequency = 0.  But if Center Frequency = 10000 is selected (Display RF = 
> off), then the single
> peak is shifted to the right on the display, and appears over the 20000 Hz 
> indicator.  The display
> range is labeled -6 to +26 KHz. For the above, when Display RF Frequencies is 
> selected, the
> X-Axis displays  +5 to +15 KHz, and the single peak shows at a frequency of 
> 11.56 KHz.
> I've tried to think of the ways that these controls might have been 
> repurposed, but can't come up
> with a combination that would give this behavior, so assume that it may be a 
> bug. The way
> it worked in GRC 3.6 would be desired behavior.
> Below is the simple GRC 3.7 flowgraph to demonstrate:
>     (Signal Source (complex) -> throttle (complex) -> QT GUI Sink (complex)).
> Thanks.
> -- Tom, N5EG

I just pushed a fix for this to our git repo yesterday. Silly bug
between Qwt 5.2 and 6 for handling the zoomer object.

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