[time-nuts] can of worms: time-of-day in a community radio station

[Hal Murray]

Replies to several messages collected here to reduce traffic.


artgodwin at gmail.com said:
> Off-the-wall thought :   could you discipline a well-insulated raspberry pi
> to NTP using heaters or workload to modify its temperature ? 

Yes.
  https://blog.ntpsec.org/2017/03/21/More_Heat.html

The other approach is to measure the temperature and correct for it.

NTP temperature compensation
Mark Martinec, 2001-01-08 
  https://www.ijs.si/time/temp-compensation/

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David J Taylor  said:
> I suppose if you have a poor or overloaded internet connection  server
> quality doesn't matter as much - well, almost.  My ISP is 200/20, and  used
> to be 200/12.  Talk about asymmetric!

Assume a NTP packet with all the protocol overhead is 100 bytes.  That's 800 
bits.  At 12 megabits/sec, that's 66 microseconds.  Half of that is the worst 
offset you can get and it would be lost in the noise from other sources.

There are 2 big ones.  The first is queuing delays.  The usual suspect the 
last hop to your house.  You have some control over that.  But there are 
queues out in the great big internet that you probably can't measure.

The second is asymmetric routing.  Your packets probably go out mostly on the 
server's ISP and back mostly on your ISP.

If you get caught in the crossfire of nation-state level cyber games, packets 
between cities in the US may go via Iceland in one direction.

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kb8tq at n1k.org said:
> At some odd hour of the day, one day a week, the tech geek fires up WWVB and
> pipes it into an audio trunk. He / she then wanders around with  a set of
> headphones. Each clock in the station gets set to “the right time”. Any clock
> that is out by more than some defined amount gets flagged  for repair.  

What was the ballpark for "defined amount"?

Any guess on how far a typical clock drifted in a week?


> In some places, the clocks that got the set process had tags on them that
> told you to use them for the correct time. Indeed even so, people  still
> would look at their wrist watch ….. 

40ish years ago, the time servers on the Xerox internal network were mostly 
set from my watch.  I don't remember doing anything fancy to set it.  I 
probably used POP-CORN.  We did have crude drift correction.  I think the 
units were seconds per day, probably an integer.

I remember an interesting improvement in Alto timekeeping.  Ed Taft got annoyed at how poorly they kept time.  He tracked it down to the handoff between designers, builders, and programmers.  The Alto was designed with a 170 ns cycle time.  Crystals are ordered by MHz rather than ns.  The crystals said 5.88 MHz.  That's off by 400 ppm.  A magic constant deep within the OS was tweaked and things got much better.


-- 
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