Hi That’s one possible application. Jim’s VLBI in the back yard is another possible application. If this is aimed at “distributed VLBI” then the requirements are … errr … pretty tight.
Bob > On Apr 14, 2018, at 4:43 PM, Chris Caudle <[email protected]> wrote: > > On Sat, April 14, 2018 8:37 am, Bob kb8tq wrote: >> big an issue as the TCXO. If it's a single location and the time is >> arbitrary, then maybe not so big a deal. >> If it's all arbitrary why worry about drift? >> >> GPS on the board looks like a good thing to have to me > > The application is time stamping separate free running devices, in this > case different video and audio recorders. So the absolute time is > arbitrary, but all the devices in use have to agree on the rate of time > progression for as long as they are being used together. > The typical requirement is that all the free running devices have timecode > which will be aligned within one video frame, so ca. 33ms, at the end of > the time of use. > So for example, you are making some kind of video, you put all the > timecode devices together and get their time synchronized, at which point > they get separated and connected to various audio and video recording > devices to output a timecode signal that the video and audio devices > record along with their primary recordings, so that later you can line up > the recordings from different machines and match same recording from > different locations, angles, etc. and know they were from the same time. > You want the last work of the day to still be synchronized to within > closer than 33ms, so the maximum time you want to be able to work without > getting your timecode generators back together to synchronize defines your > drift rate which defines your acceptable accuracy. > From common specifications it seems that the commercial products converged > on 24 hours as the use time limit, so 33ms/24 hours -> 0.033s/86400s ~ > 0.4ppm > > Yes, in principle you could use an arbitrary clock rate as well as an > arbitrary starting time, but that could only work if all the devices were > exactly the same rate, so if you have to adjust the devices anyway, and > some may be coming from 3rd parties that you don't have access to prior to > use, then the only practical approach is for everyone to calibrate their > devices to standard rate. > > I'll let the original poster ponder on whether GPS on board is a good > thing or not, but I think you cannot count on GPS being available in use > (could be inside a steel building, or a steel reinforced concrete > building, with no RF reception), so you would still need a local > oscillator which could hold the rate tightly enough to guarantee less than > 33ms of phase drift over the course of a day. Maybe you could relax that > to "working day" and say it's only over 12 hours, not 24 hours. > > What I think makes this potentially interesting to time-nuts is that the > time requirements are pretty loose by time-nuts standards, but potentially > some of the tricks that people come up with for getting ns level accuracy > on hobby budgets could be applied to this to find a way for non-nuts (or > at least not-yet-nuts) to get started on a really low budget. > > -- > Chris Caudle > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
