On 08/28/11 06:53, David Woolley wrote: > Harlan Stenn wrote: > >> >> ntp *does* a fit/figure of the expected needed adjustment - your >> sentence implies that it does not (at least that's how it reads to me). > > No it doesn't. It is basically a feedback controller. > > If you take a modern central heating controller, which varies the output > by varying how much of a ten minute cycle the pump is running, you have > something similar to version 3 ntpd, except that it used 4 seconds. > > The central heating controller will use the measured difference from the > target temperature to adjust the on to off proportion, and also include > some of the integral of that, to, eventually, remove any remaining offset. > > A fitting process would be more like the controller measuring the rate > of temperature change when the heat was off, and when it was on, and > then calculating exactly how much on to off time to apply in one go. (In > practice, it isn't as simple as that for the central heating system, as > there is a lag involved for the heat to get from the boiler flame to the > the thermostat. >
I think that there might be something to the process that Bill supports, at least in some situations. As you know, I have had problems with NTP and larger initial frequencies. (I know I owe you testing on the latest NTP. Sorry, I have been busy. I'll get to it soon.) But if you think about it, we already do something like this for offset. Of course, NTP uses a PLL in the general case. (It used to use a FLL when it settled in, but I seem to remember Dr. Mills saying that was removed.) but with iburst, we futz with the algorithm to get the offset right quickly. Couldn't we do something similar for frequency? Particularly in the case where there is no drift file, the initial frequency could be very far off. Save the first 16 (say) poll samples, correct for the offset adjustment, do a best fit analysis and some sanity checking and use the result for an initial frequency. You could keep this up, doing best fit analysis until it got to within a certain error interval and then switch to the normal regime. If you only did the first analysis, you could trivially prove that it does not break the PLL because you are left with an initial condition that potentially could have occurred anyway. But in most cases, you are going to zero in on the right frequency pretty quickly. -- blu Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live. - Martin Golding -----------------------------------------------------------------------| Brian Utterback - Solaris RPE, Oracle Corporation. Ph:603-262-3916, Em:brian.utterb...@oracle.com _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
