[time-nuts] PC clock generator without 14.318MHz

jimlux jimlux at earthlink.net
Tue Oct 18 18:51:03 EDT 2016

On 10/18/16 2:30 PM, Tom Van Baak wrote:
> Hi Vladimir,
> Some of these numbers survive to the present. I'm typing this post on
> an XP laptop where QueryPerformanceCounter() has a Frequency.QuadPart
> of, you guessed it, 3579545 Hz, which is why my Win32 laptop's
> high-res clock has ~279 ns resolution.
> For more fun with time, frequency, oscillators, and prime numbers,
> see: http://www.poynton.com/PDFs/Magic_Numbers.pdf

and this is why clocks in film movies on TV run slightly slow<grin>.. 
because the film was shot at 24 fps, and it's converted to 29.97 frame 
rate (in the US) by a 3:2 pulldown scheme.

I am sure that all the time nuts here notice that 0.1% rate difference. 
Over a half hour TV program it adds up to almost 2 seconds of offset. 
(that's just because we watch things like movies shot of counters running).

Hmm.. there's probably film footage of things with a running counter in 
the scene counting tenths or hundredths of a second (sporting events, 
nuclear bomb tests, etc.) I wonder if you could see that difference by 
single framing something like a filmed 100 meter race where they have an 
onscreen timer.

You don't have to go back very far and film cameras used mechanical 
governors for speed control.. "quartz lock" is a relatively recent 
addition, and as recently as 20 years ago, you had to pay extra for it 
when renting camera gear.

When I was in that business, one of the things I used to do was modify 
PCs so that they could be locked together - the frequency tolerance on 
PCs is pretty bad, so if you have a set with a bunch of PCs they could 
adjust the camera shutter phase and frame rate so you didn't get the 
sync bars on one screen, but not on all, and furthermore, over a long 
take, they would drift relative to each other, so even if you had them 
all lined up to start...

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