[time-nuts] 60Hz mains clocking in computers

[Joe Gwinn]

At 4:53 PM +0000 12/13/09, time-nuts-request at febo.com wrote:
>Date: Sun, 13 Dec 2009 17:42:12 +0100
>From: Magnus Danielson <magnus at rubidium.dyndns.org>
>Subject: Re: [time-nuts] 60Hz mains clocking in computers
>To: Discussion of precise time and frequency measurement
>	<time-nuts at febo.com>
>Message-ID: <4B251964.5040808 at rubidium.dyndns.org>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>Joe Gwinn wrote:
>>  At 1:44 AM +0000 12/13/09, time-nuts-request at febo.com wrote:
>>>  Date: Sat, 12 Dec 2009 17:29:17 -0800
>>>  From: Colby Gutierrez-Kraybill <colby at astro.berkeley.edu>
>>>  Subject: [time-nuts] 60Hz mains clocking in computers
>>>  To: time-nuts at febo.com
>>>  Message-ID: <3058527A-CC99-4174-BE75-21DD92334155 at astro.berkeley.edu>
>>>  Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes
>>>
>>>
>>>  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