Re: [Discuss-gnuradio] Stream Tags

[Josh Blum]

> I have not yet used stream tags, and have a number of
> questions:
> 
> 1. Is it possible to get timetags for every sample?  My
>    readings on time tags seemed to imply that only one
>    is created at the beginning of a stream.
> 

Your block will need to grab a hold of the last time tag that was
produced. If you know the sample count of this tag, and the sample count
of the sample you are interested in, its a simple interpolation.

> 2. What is the accuracy/precision of the time tags?  Jitter
>    is going to be the make-or-break factor on this project.
> 

The fractional seconds are stored as double precision floating point.
This gives the fractional seconds enough precision to unambiguously
specify a clock-tick/sample-count up to rates of several petahertz.

:-)

> 3. It looks like the Start-Of-Burst (SOB) tag is the only
>    way to control the time of transmission.  Is there any
>    limitation on the length of a burst?  In my case, I would
>    want to start the transmitter in response to receiving
>    the uplink, and not stop it until the uplink ceases,
>    about 4 minutes later.
> 

The tx_time tag is the way to control the time of transmission.

Bursts can be any length. Its important to end a burst or an underflow
is produced if you stop producing TX samples.

> 4. Is there any good tutorial or example information on
>    the use of stream tags? (I'm using GR 3.6.1)

Yes:
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/examples/c++/tags_demo.cc

> 
> 5. Does all this sound like I'm headed in the right direction,
>    or is there something fundamental that I'm missing?
> 

I think you have got it!
http://code.ettus.com/redmine/ettus/projects/uhd/wiki/GNU_Radio_UHD#Using-UHD-with-GNU-Radio

-josh