# QT Freq Sink Display not centering correctly?

**URL:** <https://www.ruby-forum.com/t/qt-freq-sink-display-not-centering-correctly/241962>\
**Category:** GNU Radio\
**Created:** [May 15, 2015, 9:08pm UTC](https://www.ruby-forum.com/t/qt-freq-sink-display-not-centering-correctly/241962 "2015-05-15T21:08:25Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\
**Post date:** [May 15, 2015, 9:08pm UTC](https://www.ruby-forum.com/t/qt-freq-sink-display-not-centering-correctly/241962/1 "2015-05-15T21:08:25Z")

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So, I first noticed this problem with my main GNURadio install (3.7.5 on  
Debian Jessie) but I’ve reproduced it on a brand new 3.7.7.1 live USB.

Basically I’m looking at two frequency sinks. One of them centered at 0  
and  
one centered at 16kHz. I have a single signal source input to both  
sinks,  
starting at 4kHz but adjustable up to 32kHz. My expectation is that the  
initial 4kHz signal will display at the far left of the 16kHz-centered  
sink…but it doesn’t. It displays as a 20kHz signal instead, to right  
of  
center.

I’m pretty sure that’s not WAD…and if it is, can someone explain why?

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**Author:** ![quangn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/q/e47774/32.png) [@quangn](https://www.ruby-forum.com/u/quangn)\
**Post date:** [May 18, 2015, 5:38pm UTC](https://www.ruby-forum.com/t/qt-freq-sink-display-not-centering-correctly/241962/2 "2015-05-18T17:38:37Z")

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On May 15, 2015, at 12:07, Jason N. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> So, I first noticed this problem with my main GNURadio install (3.7.5 on Debian  
> Jessie) but I’ve reproduced it on a brand new 3.7.7.1 live USB.
> 
> Basically I’m looking at two frequency sinks. One of them centered at 0 and one  
> centered at 16kHz. I have a single signal source input to both sinks, starting at  
> 4kHz but adjustable up to 32kHz. My expectation is that the initial 4kHz signal  
> will display at the far left of the 16kHz-centered sink…but it doesn’t. It  
> displays as a 20kHz signal instead, to right of center.
> 
> I’m pretty sure that’s not WAD…and if it is, can someone explain why?

The sink always displays the content of the signal you give it. The  
purpose of the center frequency setting is to adjust the frequency scale  
displayed in the widget to match the original frequencies of a  
downconverted signal.

The simplest use of this is to set it to the same frequency as your  
receiving hardware is tuned to, so that the frequency scale displays  
true RF-spectrum frequencies rather than 0 bandwidth.

If you want to actually shift the way the frequency content of the  
signal is displayed, you could use the rotator\_cc block before the sink.

–  
Kevin R. [http://switchb.org/kpreid/](http://switchb.org/kpreid/)