[chrony-users] 'fallbackdrift' and temporarily connected ref-clocks

[simon@xxxxxxxxxxxxx]

I am experimenting building a DIY SMTPE TimeCode system, based around 
Chrony and libLTC.

An external TimeCode generator would be temporarily connected as 'LTC' 
(previously connected long enough to build good drift files) and local 
time would be 'JamSynced'. Once the generator is removed local time 
would freewheel, maintained by Chrony's drift calculations.

The TimeCode is injected via SHM, 'chrony.conf' reads
--
local stratum 10 orphan
fallbackdrift 6 7
log refclocks

refclock SHM 0:perm=0666 trust refid LTC
refclock SHM 1:perm=0666 noselect refid MON
--

Note: I have set a 2nd source 'MON' so that I can monitor drift after 
'LTC' has been removed.


I am confused about 'fallbackdrift'; if my understanding is correct the 
above will cause the real time drift corrections from 'LTC' will be 
dropped between 16s and 32s.

Q. Is this understanding correct?
Q. Is there a better way to handled a disappearing ref-clock?


From looking at the refclock logs of a trial run, at the point of 
removal I see:
--
2020-07-16 23:13:20.001717 LTC     0 N 0 -1.958000e-03 -1.717746e-03  
3.700e-08
2020-07-16 23:13:21.000135 LTC     1 N 0 -3.720000e-04 -1.357082e-04  
3.700e-08
2020-07-16 23:15:34.191050 MON     6 N 0 -1.910500e-01 -1.910500e-01  
3.700e-08
2020-07-16 23:15:35.003651 MON     7 N 0 -3.651000e-03 -3.651003e-03  
3.700e-08
2020-07-16 23:15:36.002157 MON     8 N 0 -2.157000e-03 -2.157003e-03  
3.700e-08
--

And using 'Chronyc/sources' I can confim that 'LTC' is dropped, '*' 
changes to '?' and my reported Stratum drops to '10'.
--
chronyc> sources
210 Number of sources = 2
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#* LTC                           0   4   360    86    +61us[ +405us] +/- 
1223us
#? MON                           0   4     0     -     +0ns[   +0ns] +/- 
   0ns

chronyc> sources
210 Number of sources = 2
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#? LTC                           0   4     0   147    +61us[ +405us] +/- 
1223us
#? MON                           0   4     0     -     +0ns[   +0ns] +/- 
   0ns
chronyc> tracking
Reference ID    : 7F7F0101 ()
Stratum         : 10
Ref time (UTC)  : Thu Jul 16 23:15:49 2020
--

Q. Is there a (better?) way to confirm drift compensation source?



Some (considerable) time later, I can see that local time has drifted 
off.
--
2020-07-17 15:08:17.129213 MON     1 N 0 -1.292130e-01 -1.292133e-01  
3.700e-08
2020-07-17 15:08:18.127694 MON     2 N 0 -1.276940e-01 -1.276943e-01  
3.700e-08
2020-07-17 15:08:19.126076 MON     3 N 0 -1.260760e-01 -1.260763e-01  
3.700e-08
2020-07-17 15:08:20.124516 MON     4 N 0 -1.245160e-01 -1.245163e-01  
3.700e-08
2020-07-17 15:08:21.123108 MON     5 N 0 -1.231080e-01 -1.231083e-01  
3.700e-08
2020-07-17 15:08:22.121534 MON     6 N 0 -1.215340e-01 -1.215343e-01  
3.700e-08
--

Cheers,
Simon.

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