[ntp:questions] Change reference clock soon after DCF-signalislost

unruh unruh at wormhole.physics.ubc.ca
Thu Jul 15 16:33:17 UTC 2010

On 2010-07-15, Matuschka, Sebastian <Sebastian.Matuschka at gcd-solutions.de> wrote:
>> I think this isn't a good design.
>> In my experience, DCF-77 reception is simply not stable enough to
>> directly
>> use the pulses from the receiver as clock ticks.
>> When there are thunderstorms, local interference, and sometimes
>> propagation
>> problems, there can be spurious extra pulses that you do not want to
>> count.  Or pulses can be missing.
>> A good way of using DCF-77 is to collect the 59 pulses that make up a
>> minute, average the offset between the pulse start and the local clock
>> tick,
>> decode the time from the pulse lengths, and then at the end of the
>> minute
>> decide if all this information is valid and should control the clock
>> (adjusting the clock offset/frequency), or should be discarded as a
>> whole.
>> This is also what the DCF-77 drivers in ntpd do.
> Sorry, i forgot to mention an important thing:
> The DCF-77 Signal is generated by a precise clock that is connected via
> fiber optics to the device the ntpd runs at.
> The distance isn't very long, so i can expect that the signal is very
> precisely and there is no jitter nor any (relevant) offset.
> So, it has not really has anything to do with a normal DCF-77 receiver.
> I hope my questions make more sense now :)

And you are using the DCF-77 driver why? Seems like one of the more
inapproriate drivers to use. I would have used the shm driver instead.
You are also still hiding stuff and expecting us to guess.

> Best Regards
> Sebastian

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