[time-nuts] Building a mains frequency monitor

[Hal Murray]

> record the duration of each cycle directly
> 5) Double wide cycles are detectable but missed cycles are not.

What do you mean by a double wide cycle?
What do you mean by a missed cycle?
They seem like the same thing - if you miss one, the next one will be twice 
as wide.

> Here are the advantages of the timestamping method:
> 5) Extra or missing cycles are easy to detect and repair with no loss of
> phase information. 

I'd expect the extra or missing cycles would be easy to spot if you were 
looking at the duration.  The duration would either be twice normal or less 
than half of normal.  In the latter case, you have to figure out which is the 
extra pulse.

I agree that working with time stamps seems simpler.  I wonder if that's 
because I got started that way and/or wanted to watch phase drift?  I'll bet 
durations work just as well if the data collection code remembers the round 
off and includes it in the calculations for the next cycle.

To me, this is the important advantage of working with time stamps, but 
that's because I was interested in tracking phase which turns into clock 
error.

Cycle duration:
> 3) With period or frequency measurements, if you lose even a single reading,
> you lose track of phase (timekeeping). 

Timestamps:
> 3) You get perfect long-term phase tracking, even if there is noise or
> glitches or lost or corrupted data. 



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