[time-nuts] PC clock generator without 14.318MHz
magnus at rubidium.dyndns.org
Tue Oct 18 19:25:50 EDT 2016
On 10/19/2016 12:51 AM, jimlux wrote:
> 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.
The time-code of TV and film production runs with a frame-counter.
Now, since the 30/1.001 factor is uneven, to get things into shape the
factor is compensated using the drop-frame method. The sad thing is that
when you do the math, the drop-frame method only partially compensate
the 1.001 factor, so over a day you still drift, albeit slower. So, for
a TV-station setup, you throw a wrench into the clock machinery in order
to jam the gears into time again. Now, when you do that, all the
decoding stuff can "jump" in unexpected way, so you try to schedule it
for when you are off air or not transmitting anything important. Oh, and
we have to inherit all this into the new stuff too.
Interesting when you have to explain to them that leap seconds is
introduced at the same time, regardless of local time-scale. So,
leap-seconds would also have to be jammed-in.
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