[time-nuts] 60Hz mains clocking in computers

[Lux, Jim (337C)]

On 12/13/09 7:52 AM, "Joe Gwinn" <joegwinn at comcast.net> 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
>> 
>> 
> 
> 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.
> 
> 
Only for black and white TV before color.  The frame rate of pre color TV
was chosen to be 60 Hz so that hum bars wouldn't move.  BUT, come color in
the early 50s, and the move to 59.94 Hz (which was as close as you could get
with integer division ratios from the master reference, which had something
to do with solving a problem with color encoding), there's a slight
difference. So that line frequency interference slowly crawls up the screen
in 20 seconds. Now, most people who are not time-nuts would say that 59.94
and 60 are the same, but to us, that's a 1000 ppm difference.

Woe to the person these days who tries to sync line frequency widgets (like
synchronous motors) to video and film cameras (film is nominally 24fps, but
if they're shooting for conversion to video, they'll sync to .999*24fps, so
that the 3:2 pulldown matches the video, without having to do the dropframe
thing to keep it lined up.)..

I haven't done much of this in the last 15 years, but in the early 90s,
there were directors of photography who used "quartz locked" cameras and
those who didn't. Shooting with video monitors in the scene was always
tricky to make sure that the vertical retrace was synced with the camera
shutter; which you'd usually do by just starting the camera several times
until you got the right phase or changing the camera speed slightly (by
eye.. On a film camera, the viewfinder shows you the scene when the film
isn't exposing. So your eye is the detector of the optical sampler.) The
fancy lock boxes had a "phasing" knob. I got my start in the special effects
business doing graphics software on modified DOS boxes that I could phase
lock to the camera sync. Somewhere out in the garage I have a bunch of CGA
and VGA cards with external oscillator inputs.

The other strategy was to run the monitors at 24fps, and genlock the video
to the camera sync output (but that was expensive, you'd only see that in
feature films or long duration series..).  There were some VGA cards that
you could reprogram the sync rates to 24fps, too, but you had to make sure
the monitor could actually lock it (and usually, you'd run a little program
that started at 30fps and slowly moved it to 24, so the monitor wouldn't
assume it had lost lock and try to resync) As chroma-key became cheaper and
easier, a lot of times, they'd just paint the screen of the monitor blue or
green, and do it in post.  I'm out of that business now, but I'll bet that
doing it in post is by far the most common these days.  It's easy to do the
projective geometry needed to warp the desired image to whatever it is in
the scene (you click on the 4 corners in a key image, and the software
tracks it as it moves, so you don't have to worry about camera moves)