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Re: [Discuss-gnuradio] Question about metadata filesink

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Question about metadata filesink
Date: Wed, 10 Jul 2019 18:04:54 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 07/10/2019 04:19 PM, Ellie White wrote:
Hi Marcus,

Thanks for getting back to me! I really appreciate your suggestion --
why didn't I think of that! I have done similar calculations before to
determine the amount of time from the beginning of a run, but for a
much less precise application.

This brings me to another question, though -- I notice that when I
read the metadata header, the time ("Seconds" field) always says zero
when I get the file from the TCP client flowgraph, but when I save a
metadata header file on the machine that the Ettus is connected to, it
gives me some fractional answer (always different, never zero). Not
sure why that is, or what time standard the UHD is using (this is the
one I have, by the way:
https://files.ettus.com/manual/page_usrp1.html)? Maybe the Ettus just
needs to be connected to an external clock source -- the final system
will have a 10 MHz clock source connected, but the temporary system
that is set up now doesn't have an external clock. If you have any
thoughts on this, I would be interested to hear them.

Thanks again, and have a good afternoon!
Also, other random questions:

What type of USRP (There are a plethora these days!)

What type of computer is direct-connected to the USRP?

Can you share the flow-graph for the computer that is direct-connected to the USRP?


On Wed, Jul 10, 2019 at 3:07 PM Marcus D Leech <address@hidden> wrote:
The thing to note is that the UHD sends a time stamp only on start of streaming 
and whenever there’s an overrun.

You can know the time of any given sample by knowing the sample rate and offset 
from the beginning.

In your case you will have to throw in some more factors to account for FFT 
size and decimation etc.

More later when I get to a real computer.

Sent from my iPhone

On Jul 10, 2019, at 2:54 PM, Ellie White <address@hidden> wrote:


I am working on a radio astronomy project with the Green Bank
Observatory / NRAO Central Development Lab this summer, utilizing GNU
Radio and an Ettus Research SDR, and I've got a question regarding how
to collect metadata information from a filesink.

I have attached the flowgraph I am using to this email. The project
requires that I use two computers in tandem for data collection -- one
is connected to the Ettus -- it is the TCP server, and sends an
interleaved data stream to the TCP client flowgraph (attached) on the
machine which will be storing the data. As you can see, I am saving
integrated spectra to a file. My question is simply, how do I retrieve
a time stamp corresponding to each spectra using the metadata time
sink? I have been fiddling with this all afternoon attempting to get
it to work properly, and I have been able to save data to a file, read
out spectra (using attached Python program), and display header
information using the command gr_read_file_metadata in the terminal,
but this is just showing me a timestamp for the beginning of the data
collection run, rather than showing me timestamps for each spectrum
which is saved to file, and I am not sure how to implement this.

Any advice would be much appreciated! If I can provide any more info
about my system or what steps I have tried, please let me know. Thank
you so much for your time -- have a good afternoon!

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