Re: [Discuss-gnuradio] Time syncing between SDRs on different computers

[b . shubenok]

Say we need +-10ms accuracy in timestamps. Precision Time Protocol in LAN 
should sync PC's clocks with enough accuracy. 
And then using USRP-like methods - set current/start time - get first sample in 
known time? Of course USB will add
undetermined delays but no external GPS required.

On Wed, 2017-08-16 at 16:57 +0100, Derek Kozel wrote:
> It should be pointed out that the hardware based timestamping is only needed 
> if you need time alignment better than a
> half second or so. With USB transfers, various buffers, NTP based alignment 
> of the host computer's time, and some
> extra code on the host side you could do a coarse time alignment, probably 
> with less than a half second of error.
> 
> You could also time align the streams if both radios receive the same signal 
> and you know the distance (and other
> details depending on the precision needed) between the transmitter and your 
> two receivers. This is done by many
> protocols like the cellular standards to create a distributed timebase, but 
> quickly becomes non-trivial.
> 
> The B200 does timestamping in the FPGA and can use a 1PPS signal to align to 
> GPS time. It is more expensive than the
> HackRF, but much less than the $3000 mentioned above.
> 
> On Wed, Aug 16, 2017 at 4:20 PM, Sylvain Munaut <address@hidden> wrote:
> > On Wed, Aug 16, 2017 at 5:11 PM,  <address@hidden> wrote:
> > > What type of hardware? I thought from hardware point of view only precise 
> > > clock is required and all the other
> > things in
> > > firmware. I've naively thought i could modify hackrf firmware to get this 
> > > feature.
> > 
> > Mostly a FPGA and a PPS input from a GPS receiver.
> > 
> > Each individual sample has a timestamp of when it has been received.
> > And you can also reset the timestamp on the next  PPS edge.
> > 
> > Technically it would be possible to modify the hackrf firmware and
> > repurpose some GPIOs and have all samples transmitted to the host be
> > in timestamped packets and implements timestamping in the on-board
> > ARM.
> > For additional hardware you'd only need an external GPS receiver (or
> > some other way to have both a single freq reference + single sync
> > pulse).
> > 
> > Cheers,
> > 
> >     Sylvain
> > 
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> 
>