On 2013-01-27, [email protected] <[email protected]> wrote: > @unruh: You are exactly right. Whatever network jitter there is I > just need to have a calibration run sufficiently long so that the > derived frequency is close enough. Given infinite time I can make
Well, no. the problem is that as time goes on, the clock itself has frequency shifts ( temperature changes, crystal aging,...) > arbitrarily precise frequency measurements. Of course the limiting > factor is user bother in putting up with a long calibration run. I > hope to mitigate that problem by the following: > > 1. This calibration is only needed once when the user installs my app. > 2. The calibration, once initiated, will run unattended. So the user > will be instructed to start the calibration and then leave the > smartphone alone. During that time our app will have to be running > continuously, so we will recommend that the user leave the phone > plugged in to a charger, preferably overnight. When he wakes in the > morning the calibration will have been completed. See above. After a few days, the sound crystal frequency will have drifted ( whether enough to bother you I do not know). There is a concept of the Allen minimum which gives essentially the ideal time between calibrations. ntp has this problem and tries to alter the calibration time scale to get to that minimum. > > Unfortunately the calibration software cannot be interrupted by a > phone call. So if it is interrupted the user will just be notified > that the calibration was aborted because of the interruption and he > will have to start the calibration run over again. But I don't see > that as a big problem. My estimates of the time span now, based on > expected network jitter, is about one hour. It is not that hard to > have one hour of uninterrupted time, especially if the user starts the > calibration run just before retiring for the night. > > > In case you are wondering, my app is a professional piano tuning app. > The standard in this industry is that tuning devices should be > accurate to 12 parts per million. I know that is probably overkill > for tuning pianos, but that is what the professionals expect from > their equipment. Ah. I would expect 1 cent, which is more like 500PPM. > > Robert Scott > Hopkins, MN _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
