[Discuss-gnuradio] GRC 3.7: QT GUI Sink: Frequency display: Center Frequency bug?

[Tom McDermott]

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


<?xml version='1.0' encoding='ASCII'?>
<flow_graph>
  <timestamp>Tue Aug 13 05:53:25 2013</timestamp>
  <block>
    <key>options</key>

<param>
      <key>id</key>
      <value>top_block</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>title</key>
      <value></value>
    </param>
    <param>

<key>author</key>
      <value></value>
    </param>
    <param>
      <key>description</key>
      <value></value>
    </param>
    <param>
      <key>window_size</key>
      <value>1280, 1024</value>
    </param>
    <param>
      <key>generate_options</key>
      <value>qt_gui</value>
    </param>
    <param>
      <key>category</key>
      <value>Custom</value>

</param>
    <param>
      <key>run_options</key>
      <value>prompt</value>
    </param>
    <param>
      <key>run</key>
      <value>True</value>
    </param>
    <param>
      <key>max_nouts</key>
      <value>0</value>
    </param>
    <param>
      <key>realtime_scheduling</key>
      <value></value>
    </param>
    <param>

<key>_coordinate</key>
      <value>(10, 10)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>variable</key>
    <param>
      <key>id</key>
      <value>samp_rate</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>

<key>value</key>
      <value>32000</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(10, 170)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>blocks_throttle</key>
    <param>
      <key>id</key>
      <value>blocks_throttle_0</value>
    </param>
    <param>

<key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>type</key>
      <value>complex</value>
    </param>
    <param>
      <key>samples_per_second</key>
      <value>samp_rate</value>
    </param>
    <param>
      <key>vlen</key>
      <value>1</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(481, 479)</value>

</param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>analog_sig_source_x</key>
    <param>
      <key>id</key>
      <value>analog_sig_source_x_0</value>
    </param>
    <param>
      <key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>type</key>
      <value>complex</value>

</param>
    <param>
      <key>samp_rate</key>
      <value>samp_rate</value>
    </param>
    <param>
      <key>waveform</key>
      <value>analog.GR_COS_WAVE</value>
    </param>
    <param>
      <key>freq</key>
      <value>5000</value>
    </param>
    <param>
      <key>amp</key>
      <value>1</value>
    </param>
    <param>

<key>offset</key>
      <value>0</value>
    </param>
    <param>
      <key>_coordinate</key>
      <value>(247, 450)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <block>
    <key>qtgui_sink_x</key>
    <param>
      <key>id</key>
      <value>qtgui_sink_x_0</value>
    </param>
    <param>

<key>_enabled</key>
      <value>True</value>
    </param>
    <param>
      <key>type</key>
      <value>complex</value>
    </param>
    <param>
      <key>name</key>
      <value>QT GUI Plot</value>
    </param>
    <param>
      <key>fftsize</key>
      <value>1024</value>
    </param>
    <param>
      <key>wintype</key>
      <value>firdes.WIN_BLACKMAN_hARRIS</value>

</param>
    <param>
      <key>fc</key>
      <value>10000</value>
    </param>
    <param>
      <key>bw</key>
      <value>samp_rate</value>
    </param>
    <param>
      <key>rate</key>
      <value>10</value>
    </param>
    <param>
      <key>plotfreq</key>
      <value>True</value>
    </param>
    <param>
      <key>plotwaterfall</key>

<value>True</value>
    </param>
    <param>
      <key>plottime</key>
      <value>True</value>
    </param>
    <param>
      <key>plotconst</key>
      <value>True</value>
    </param>
    <param>
      <key>gui_hint</key>
      <value></value>
    </param>
    <param>
      <key>freqchangevar</key>
      <value>None</value>
    </param>
    <param>

<key>_coordinate</key>
      <value>(757, 446)</value>
    </param>
    <param>
      <key>_rotation</key>
      <value>0</value>
    </param>
  </block>
  <connection>
    <source_block_id>analog_sig_source_x_0</source_block_id>
    <sink_block_id>blocks_throttle_0</sink_block_id>
    <source_key>0</source_key>
    <sink_key>0</sink_key>
  </connection>
  <connection>
    <source_block_id>blocks_throttle_0</source_block_id>
    <sink_block_id>qtgui_sink_x_0</sink_block_id>

<source_key>0</source_key>
    <sink_key>0</sink_key>
  </connection>
</flow_graph>