[time-nuts] Re: Low cost synchronization, kitchen appliances

[Alberto di Bene]

Chuck Harris wrote:

> Let's see, your graph shows for that small time interval that the
> frequency
> is within 0.05% of being correct.  If it continued at that rate of error,
> it would result in an error of about 42 seconds in 24 hours.    Sounds
> pretty
> bad, until you factor in a few other variables.  One, the accuracy of
> the spectrum
> analyzer you used to measure the plot, and the other is the fact that
> the powerline's
> variation is cyclical by design.  Your spectrum analyzer appears to be
> soundcard
> based.  The absolute accuracy of such a system is not so good
> Soundcards don't
> even trim their crystals for frequency.  I would venture that the
> typical accuracy of
> a sound card's clock is certainly no better that +/-0.01%.

Hmmm, the sound card used is the M-Audio Delta 44, a professional sound
card, used also by musicians and composers for studio works. I don't
know the accuracy of its time base, but certainly it is quite good.

>   If you plot the
> powerline frequency over  a longer period, I believe you will find
> that it keeps coming
> back to reality.  The usual pattern for summer time is to lose time
> during the day, and
> to gain it back during the night.  Winter is usually the opposite.

Of course. I have added another plot to that page, taken half an hour
later, and things now look better. And I have beside my bed an alarm
clock driven by the 50 Hz of the mains (I know this for sure, it was
written on the instructions), and I have to reset it very, very seldom.
The seconds are not displayed, just the minutes, and I reset it when the
error is greater than 1 minute, compared to my DCF-77 clock.
So the short term accuracy of the mains seems to be quite bad, but by
averaging on a long period things get definitely better...

73  Alberto  I2PHD