[time-nuts] TEC party file format?

Tom Van Baak tvb at LeapSecond.com
Tue Jun 28 21:45:58 UTC 2011


> I see two interesting problems with this sort of approach.
> 
> One is glitches on the line, either lightning/whatever causing extra counts, 
> or dropouts causing missed cycles.  Does anybody know how often this sort of 
> stuff happens?  Does anybody have scope pictures?

The beauty of time-stamping each cycle (each byte) is that you
have a lot of data to work with to identify and correct short-term
glitches like this. Normal frequency counters are at the mercy
of glitches, but continuous time-stamping counters can, if they
want, apply heuristics to a CW sequence and easily pinpoint
cycles that are missing or doubly counted.

If course if you are missing minutes of data it may be impossible
to know for sure if you're off by a cycle or two. But to isolate bad
data over the span of a few seconds, it's easy.

> The other would be glitches on the PC.  I can easily keep a system running 
> for a week or a month, but every now and then I need to move the power plug 
> or I get the urge to play with some software and ...

Correct. Then again, all you have to do is split the 60 Hz pulse
train to two serial ports on different PC's. Serial ports are easy
that way.

> The PIC could also send out a RS-232 text message with the count in it, or 
> modulate the pulse width, say double the width every 10000 cycles.  Mumble 
> there are lots of opportunities.

See the start of these TEC thread(s). I'm collecting all my data
with a PIC: 60 Hz in, RS232 time-stamps out. I can send one
to you if you want to try it.

> An alternative would be to feed the 60 Hz into the audio port on the PC.  No 
> need for the 555 or whatever.  The crystal driving the audio ADC is another 
> variable but I think that won't be a major problem.

Yeah, this was mentioned earlier. It turns out the crystal isn't
a problem since all you're doing is counting 16 ms cycles within
the realtime waveform capture buffer. In this case the ADC isn't
used as much for edge timing as it is for edge counting. The
sample rate or rate stability can be very low.

/tvb



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