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Re: [Discuss-gnuradio] uhd running parallel tx/rx flowgraphs

From: George Nychis
Subject: Re: [Discuss-gnuradio] uhd running parallel tx/rx flowgraphs
Date: Thu, 1 Mar 2012 14:14:37 -0500

A quick question on how the trigger works in UHD.  I have some basic code integrated with the help of your suggestions.

Burst tagger port 0 is the raw complex sample stream.  Burst tagger port 1 is the trigger.  Let's say the raw complex sample stream has an outgoing packet.  I'm assume that I change the value on port 1 that corresponds to the first sample of the preamble to a value of '1': which is the trigger like "hey, here is the start of the burst!"  My question is, for *all* indexes on port 1 that correspond to samples in the packet on port 1, should those also be of value 1?  Or only just the first sample?  What about the last sample?

Just trying to understand exactly how the trigger works.  

On Mon, Feb 27, 2012 at 9:12 PM, Josh Blum <address@hidden> wrote:

On 02/27/2012 05:30 PM, George Nychis wrote:
> It's be good if you can chime in here, Josh :)
> It seems like this is something that should be fixed about tunnel.py in
> future GNU Radio releases for use with UHD.

Like removing it altogether :-)

> That is clearly documented as control of samples to the host to be
> continuous or not.

Basically, RX is intended to work on a continuous streaming model, which
is why stream command inst swigged up. The start()/stop() methods are
actually the ones issuing the command.

When and if the pmt based message passing gets merged, i was going to
implement control messages to deal with streaming, possibly other
things. This lets you tie the uhd source block into a control plane.

As is stands now, i guess someone could just forward the stream command
stuff, so long as the work() function knew to block when there is
definitely not supposed to be samples. That way you avoid the scheduler
marking the block done on a timeout.

> However, I don't see that same control for the TX stream. Tx_metadata_t and
> t_streamer control the bursts, but don't seem to control the overall
> stream? Maybe I am missing something.

You can use stream tags to control start/stop of burst and transmit
times. See the usrp sink header or the tags demo in gr-uhd.

Now that being said, the framer blocks in tunnel.py could be more
intelligent and properly shutoff streaming (aka end a burst) when there
is no data. That way you avoid underflow when there isnt a continuous
supply of data to modulate.


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