Re: [Discuss-gnuradio] Question about metadata filesink

On 07/10/2019 04:45 PM, Derek Kozel wrote:
> Hi Ellie!
>
> The USRP has an internal time register which starts counting up from 0
> when the power is applied. You can use the external 1 PPS input and the
> set_time_next_pps command to snap that register into alignment with
> UTC/TAI as long as you have a GPSDO or similar that outputs a 1PPS signal.
>
> If you don't have a GPSDO then the timing will be arbitrary and
> un-related to any global timestamping. You can roughly align it to your
> PC's time as long as +- ~1 second accuracy is ok.
>
> Checkout the documentation and code samples here.
> https://files.ettus.com/manual/page_sync.html
>
> Now, all of these methods require modifying the generated Python to add
> in the commands to do the synchronization. Ettus Research used to
> include some functions in GNU Radio Companion to do this synchronization
> but there are some issues with handling situations where the GPSDO is
> not locked or the 1PPS or 10 MHz signals are not present that led to
> them being removed in favor of more documentation.
>
> The timing resolution of the UHD time stamps is very very fine. I can't
> remember it right now but it's good to over 10s of Giga samples/second
> if I remember correctly. The fundamental timestamp is an unsigned
> integer counter that increments at the master clock rate (the same as
> the sample rate on the USRP1). The host computer then converts that into
> time by doing counter_value*master_clock_rate_period_in_seconds =
> time_in_seconds. There's going to be some floating point errors in doing
> that so there's noise in the timestamps, but it's far far below the
> clock rate of the USRP so rounding it to integer multiples of the clock
> period is always valid.
>
> Regards,
> Derek
>
There's still a "sync" field in the UHD Source in UHD 3.13 with GR
3.7.13 -- but I think it just uses the PC clock regardless. However, if
your PC
clock is setup with NTP, things should be reasonably close--I think
better than the +/- 1second that Derek mentioned.

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