Re: [Discuss-gnuradio] Click-to-tune in Qt GUI Frequency SInk

[Martin Braun]

On 22.03.2015 19:26, Marcus D. Leech wrote:
> Just looking at the async message interface for Qt GUI Frequency Sink.
> The "freq" output (is that a PMT?) is always, it appears, in terms of
>    "display" frequency.  Which is cool if you're using the click-to-tune
> output to modify (for example) a hardware source, but if you're using it
>    to tune (for example) a freq-xlating FIR filter, there's  a
> disagreement on semantics.
>
> One could run the Frequency Sink in relative mode in this case.  But it
> seems like there should be more flexibility in dealing with the contents
>    of async messages, in situations where the message tag ("freq" in
> this case) could have semantics that not everyone who takes such
>    a tag might agree on.

There've been long discussions on this subject, at least one of them at 
a dev call. In general, the message format is of the (index, value) 
format. For the case of the xlating FIR, all you need to do is change 
your x-axis to make the center freq 0, and you're good. Getting tags and 
PMTs right is still a learning process for all of us, but we didn't want 
to add loads of extra settings into the QT sink just so we could get the 
tags into any format we liked.

> It should, I think, also be possible to turn such tags into variable
> settings in GRC, but I couldn't find any way to do so.

Hmmmm... maybe some block that would work with with the probes could do 
that. Would have to be written, though.

M