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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.