[time-nuts] PC clock generator without 14.318MHz
jimlux at earthlink.net
Tue Oct 18 19:40:58 EDT 2016
On 10/18/16 4:25 PM, Magnus Danielson wrote:
> 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
>> 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.
SO that compensates in the "big sense" so that "timecode" and "wall
clock" line up..
But when they do the original telecine, they're basically running a
30fps (interpolated from 24 fps) sequence of frames at 29.97. Over the
air, there will usually be a commercial break and they can add/drop any
arbitrary number of frames to get it to line up (should they even care
about whether the on-screen clock ticking the seconds actually lines up)
So I was thinking about something where you get a broadcast (or maybe a
video conversion on DVD/tape/online) that is a continuous piece of film.
Seems that something like 100 meter race, which lasts 10 seconds, and
will have an on screen timer to hundredths isn't quite long enough to
see the 1.001 error (and would it be one continuous shot, or would they
have edited film together from different viewpoints).
What about a filmed rocket launch with a countdown timer or similar?
they might have one continuous piece of film long enough.
Partly, its going to be limited by the magazine size of the camera: a
400 ft magazine is a bit more than 6 minutes (1 ft = 1 second in rough
terms), so that's plenty long to see the difference.
What you really want is continuous footage lasting, say, a minute, of
some event (motivating the coverage) where there's an accurate clock
visible in the scene, where the film was originally shot at 24fps, and
has been converted to video.
An interesting quest....
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