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Re: [Discuss-gnuradio] inband timestamp issues

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] inband timestamp issues
Date: Mon, 18 Aug 2008 19:28:44 -0700
User-agent: Mutt/1.5.17 (2007-11-01)

On Mon, Aug 18, 2008 at 04:54:20PM -0700, George Nychis wrote:
> Okay, more on topic with the original problem: the timestamp issues. The 
> problem that Steve noticed was that the timestamps jump.
> Brian and I wrote a testbench for this and found that the timestamp 
> actually does not really correspond to the first sample (per the design 
> doc), and not quite the last sample either... but closer to the last 
> sample.  So... this is also an issue.  But I think that fixing both will be 
> along the same lines.


The timestamp semantics that we are using on the USRP2 is that for Rx,
the timestamp refers to the time that we clock the sample out of the
DSP Rx pipeline (n.b., not the Rx FIFO).  For transmit, it's the time
we clock the sample into the TX pipeline (not the Tx FIFO).  In both
cases, the reference point is at the edge of the DSP pipeline.  This
seems to be the only place that makes sense, since the group delay
through the pipeline varies depending on the configuration.  In all
cases, the timestamp is in ticks of the master clock.

Does this make sense?


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