Re: [Discuss-gnuradio] Msg passing + tag streaming: Is this flowgraph possible?

[Martin Braun]

On 02/20/2015 09:33 AM, Jorge Gallo wrote:
> Hello,
> 
> I would like to run a flowgraph similar to the one I attach in a
> picture. It would work as follows:
> 
> USRP source would send frames with tags indicating the "rx_freq" of
> these samples. Then the power of those samples will be calculated.
> 
> Will the "rx_freq tag" still be present at the input of the Tagged File
> Sink or does any block in the middle get rid of them?

These blocks will keep the tags in there.

> If still present in the Tagged File Sink, this block would store centain
> number of vectors. Lets say 5 vectors of 2048 lenght (this would give me
> 5 power estimations of the current band).
> 
> When the number of stored vectors reaches a threshold, the Tagged File
> sink would stop storing samples and would send a "center_freq" message
> to the USRP in order to tune it to a new frequency. Then Tagged File
> Sink would wait for the new "rx_freq" tag to store samples (this way I
> would discard the old frequency samples).
> 
> Is this flowgraph feasible?

Yes, it's possible. However, I'm not sure it'll do exactly what you want
(or maybe I'm misunderstanding), for two reasons:

- Say you send a msg to the USRP source after you've received 5 vectors
of spectral estimate. The USRP source will already be way ahead of your
downstream block, so you could potentially be getting hundreds of
vectors to process. They will be tagged, so you can discard them if you
like.
- The Vector IIR will not know that you've retuned, so you will be
"smearing" together vectors that don't belong together. What you need is
a form of integrate-and-dump -- maybe the gr-specest toolbox can help
you with that.

> In case it is, I think I only have to program my custom "Tagged File
> Sink" with a vector data input port and a message output port. Is that
> correct?

Apart from what I mentioned above, yes. You might want to choose a
different approach, e.g. an open loop approach where you simply send a
retune message every N milliseconds.

M