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Re: [Discuss-gnuradio] inband timestamp issues

From: George Nychis
Subject: Re: [Discuss-gnuradio] inband timestamp issues
Date: Mon, 18 Aug 2008 16:54:20 -0700
User-agent: Thunderbird (X11/20080724)

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.

Brian, can you verify all of this? I wanted to mail it to the list for documentation, and so when Leo gets back he can read it. I've been slammed with traveling in August and wont be back for another week.

So what we suspect is that the FX2 will keep up for a while, and then get suspended/busy/something for a while which causes it to stop building packets. The samples are still being buffered, but the actual packets are not being built (causing a jump).

In this time, nothing becomes full and dropped, which is why an underrun is not reported. All samples make it through to the host.

Brian's solution was the following:
- connect the input of packet_builder to be the output of the halfband filter (for each channel)
 - output the previous to an asynchronous FIFO driven by rxclk
 - have a FIFO arbiter on the output running off usbclk
 - FIFO arbiter cycles through the FIFOs for a packet
 - if packet is available, connect FIFO to FX2 and wait for FX2 to parse

Does that sound about right, Brian?

Two major issues we're looking at solving: timestamping the first sample (for real), and a non-jumpy timestamp.


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