Indeed, if one uses relative, rather than absolute frequencies in the Qt
Frequency sink, everything is wonderful.
But I think there are, as you observe, deeper questions about the
desired semantics of async messages of this type.
An advantage to the "sets a variable" implementation on the WX GUI side,
is that said variable can be manipulated and scaled as appropriate,
depending on who is consuming it.
It's early days for the async-message subsystem within GR, and we are
learning as we're going. No surprises....
On 2015-03-23 02:12, Martin Braun wrote:
> 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
>
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