So, my takeaway from this is: * Tagging by the scheduler /before/ work is called doesn't make overly much sense, because of the blocking wait that might happen before the first sample is received * Tagging by the block is kind of prone to inconsistencies in tag format (though gr-uhd probably established a standard for this), and of course would need modifcation of a lot of blocks * If you need the precision, you must go for a device with hardware timestamping, anyway * Tagging by the scheduler /after/ the first work call returned means that one could "count backwards" to the first sample, based on sampling rate, but will of course be pretty rough, too
All in all, I think having host-clock time stamping as part of the hardware source block seems to more sensible way – there's already Ettus Hardware handling (libusrp-era USRPs, IIRC) that does that for older devices that don't have hardware timestamping. Actually, thinking about this: abusing ctrlport's callbacks, couldn't we just let an "external" observer get notified when a work function returns? That observer would ideally be the software cross-correlator that syncs up your different hardware. Cheers, Marcus On 19.07.2016 23:23, Marcus D. Leech wrote: > On 07/19/2016 05:13 PM, Marcus Müller wrote: >> Question arising: would you want the tag to contain the time when >> general_work() was called, or rather the time of the first sample? Note >> that sources should usually be designed to be blocking until there's >> data available! That way, between entering general_work() and actually >> having the first sample, a lot of time could pass. It might make a lot >> more sense to let the block that knows the hardware handle tagging >> itself, here. >> >> Cheers, >> >> Marcus > The idea is to tag when the first samples come off the hardware. > > Indeed, if one had an N-channel "tag these when the samples show up" > block, it would likely be useless, since the scheduler will have > nicely arranged for all of the those channels to have something > useful to do the first time it calls work(). > > >> >> >> On 19.07.2016 23:00, Marcus D. Leech wrote: >>> On 07/19/2016 04:53 PM, Sylvain Munaut wrote: >>>> I don't see why this has to be done by the scheduler, I'd do that in >>>> the source, or even as a separate OOT that just inserts that tag. >>> Doing it in the source means modifying each source to insert, but, >>> maybe that's the right thing to do. >>> >>> >>>> Cheers, >>>> >>>> Sylvain >>> >>> _______________________________________________ >>> Discuss-gnuradio mailing list >>> Discuss-gnuradio@gnu.org >>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> Discuss-gnuradio@gnu.org >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
