Newsgroups: comp.risks
X-issue: 11.21
Date: Sun, 3 Mar 1991 02:58:20 GMT
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Red clocks run faster than green ones!

	[[ responding to someone who had observed that clocks with
	   red displays often ran faster than clocks with green displays ]]

I think the key words are "more sophisticated".  That is, I would conjecture a
correlation between choice of display and sophistication of the rest of the
circuitry.  A clock that keeps time based on the power frequency works by
counting zero crossings of the AC waveform, and it matters how carefully this
is done.

The simplest method is to just count each time the voltage crosses zero.  There
is an inherent problem here: noise can turn one zero crossing into several, and
if the circuit responds quickly enough, it will count them all.  The potential
for trouble increases greatly if some sort of signalling is also being done
over the power lines, because one very popular way of doing such signalling is
to send a short burst of data around the time of the zero crossing, when
voltage is low and transmission is easy.  This almost guarantees multiplication
of zero crossings!

A more sophisticated circuit will incorporate safeguards against noise that
will also be effective against zero-crossing signalling.  One way is to know
the approximate frequency of the AC waveform and reject overly-frequent zero
crossings as spurious.  Another is to charge a storage element from the *peak*
of the AC waveform, discharge it at a zero crossing, and count discharges, so
that only the first zero crossing after a peak is counted.  (I've built both
types of circuit.)

LED displays are cheap and easy to drive from digital circuits, so it would not
be too surprising to find them in the cheapest clocks.  The green displays are
probably vacuum-fluorescent types, which are perceived (or at least
advertised!) as better for some reason, but need more complicated and costly
drive circuitry.  So plausibly the green clocks are less cost-critical and get
better counting circuitry as well.
                                         Henry Spencer at U of Toronto Zoology
                                          henry@zoo.toronto.edu   utzoo!henry