[Gllug] Two Questions

[Chris Bell]

On Thu 10 Feb, John Hearns wrote:
> 
> On Wed, 2005-02-09 at 23:58 +0000, Andrew Farnsworth wrote:
> 
> 
> > that it synchronizes with the flash which is why when you take a picture 
> > of a TV or monitor you get strange banding in the picture.  The 
> > Macintosh used to have a tool that would fix this problem, one of the 
> > many reasons that the Macintosh was used in to many movies, this tool 
> > basically set the monitor refresh rate to 60 hz.
> 
> 
> I thought this was a box called a Genlock.
> Chris Bell can tell us I'm sure!
> 
> I thought that to avoid the banding appearing you connected any CRT
> type monitor to the Genlock, so that the screen refresh cycle starts
> at the same time.
> 
   Correct for analogue, we used to generate early master reference pulses
and then insert adjustable delays to time all sources to be correct at the
mixing point(s). We could also generate audio tones which could nudge the
stable oscillator frequency and/or division ratio of a remote source so that
it was within tolerance despite being long distances away, or mobile with
Doppler effects. Anything that could not be locked in this way could be
passed through a frame store synchroniser, which is now standard practise
with digital. Unfortunately some pulse generators and early synchronisers
were less than reliable, and everything used to drift with temperature
variations, despite keeping crystals in battery maintained ovens. There is
also a problem when using synchronisers because there is a noticeable delay,
and sequential synchronisers at different mixing points can result in
lip-sync problems.
   We used to lock our mobile generators to the TV frame frequency, and had
one or two occasions when that locking failed and the generator just carried
on increasing speed until it disintegrated. A diesel can suck its own
lubricating oil past the piston seals and continue to run on that until the
oil runs out. We had the fire brigade pouring water on one generator while
they watched red-hot metal being ejected from the exhaust pipe. It is not
much better when a generator runs too slowly as mains transformers start to
look like dead shorts. We did not carry the full set of several thousand
spare fuses.

-- 
Chris Bell

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