At 4:53 PM +0000 12/13/09, [email protected] wrote:
Date: Sun, 13 Dec 2009 17:42:12 +0100
From: Magnus Danielson <[email protected]>
Subject: Re: [time-nuts] 60Hz mains clocking in computers
To: Discussion of precise time and frequency measurement
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Joe Gwinn wrote:
At 1:44 AM +0000 12/13/09, [email protected] wrote:
Date: Sat, 12 Dec 2009 17:29:17 -0800
From: Colby Gutierrez-Kraybill <[email protected]>
Subject: [time-nuts] 60Hz mains clocking in computers
To: [email protected]
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I'm trying to get to the bottom of whether or not any computing
equipment made around the advent of UNIX systems (or any time-slicing
system) used the mains cycles of 60Hz as phase lock for the internal
system clock. My guess is that perhaps they did not as the computing
logic is DC based, but, I have memories of using an 68000 based UNIX
system that I thought had its internal clock based off of the 60Hz
mains... Not sure the vendor anymore.
>
In the 1980s and 1990s, before networks capable of carrying NTP time to
the millions became common, the computer local clock was very often
derived from the local AC power mains, and the frequency was steered to
match atomic time once per day. The POSIX standards reflect this common
approach by the tolerance on CLOCK_REALTIME, 20 milliseconds, this being
one cycle of 50 Hz power.
Which does not perfectly match the 60 Hz being used in some countries or
for that matter traditional division for PC clocks (derived from
14,31818 MHz, over 4,77 MHz and what the 8253 divider allows).
The CPU logic clock was not generally phase-locked to the AC power
lines, instead being generated by a cheap crystal having a very large
tempco. The exception to this was that video generators were (and still
are) often locked to the AC line so that hum bars would not drift across
the screen.
I have never seen this in any of the devices I've seen. It is certainly
not what we do in the TV world either. Examples would be good.
It was certainly true in black and white TVs, and I'm pretty sure
that the early Macintosh computers did the same.
NTSC color TV runs almost at 60 Hz, so the hum bars drift slowly
enough to not be visually disturbing.
The Television Handbook will have the relevant standards for TV.
Because NTSC was intended for North America, behavior with 50 Hz
power was not a concern. European standards are different, but they
must have dealt with hum bars somehow.
When all the vacuum tubes (except the picture tube) disappeared from
TVs, hum bars became far less of a problem, and subsequent TV
standards don't worry about the effect.
Joe Gwinn
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