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Re: [Discuss-gnuradio] How to get precise time tag from blocks_meta_data

From: Ben Hilburn
Subject: Re: [Discuss-gnuradio] How to get precise time tag from blocks_meta_data_sink
Date: Tue, 7 May 2019 15:52:27 -0400

Hi Ziang -

I'm not sure I completely follow your system set-up, so I'm not sure how to answer all of your questions, exactly. There are almost certainly a couple of things at play here, including the source of your time (is it on the radio? your PC?), and the data representation of the time tag itself.

The `rx_time` field of the file meta sink block uses UHD's tuple representation for time, with an integer number of full seconds and then a fractional second component. See more here: https://github.com/EttusResearch/uhd/blob/master/host/lib/types/time_spec.cpp

So, concatenating them as `full.frac seconds` is correct, but it's important to keep in mind the limitations of data representation in terms of the fractional portion. Having 20 digits of "value" in your float doesn't mean they are significant - you need to be aware of the limitations and accuracy of your hardware to whatever degree it matters in your application.

Does that help?


On Tue, Apr 30, 2019 at 4:13 PM Ziang Gao <address@hidden> wrote:

I'm trying to use blocks_file_meta_sink to record data with tags, and then use gr_read_file_metadata.py to read .hdr file, and I got this output:
" " "
Version Number: 0
Sample Rate: 20000000.00 sps
Seconds: 1556652948.179670
Item size: 8
Data Type: float (5)
Complex? True
Header Length: 171 bytes
Extra Length:  22
Extra Header?  True
Size of Data: 8000000 bytes
              1000000 items

Extra Header:
rx_freq: 2.4205e+09
" " "
It looks like the time in seconds only has 6 floating points, however, it supposes to be double. 
I thought it was caused by string formatting so I went to parse_file_metadata.py and modified line 94 from "print "Seconds: {0:6f}".format(t)" to "print "Seconds: {0:20f}".format(t)", then I got: 
" " "
Seconds: 1556652948.17966961860656738281
" " "
Is this the correct way to solve this problem? Is GNUradio's time tag actually precise to nanosecond? I want to use a PC with GPS time module and sync a B200mini with it, can I actually get the correct GPS time tag in this way?


Best regards,
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