ah, so the 24 jerks per second is the noise you hear on audio recordings 
where they were using a film projecter?
Sort of a generator like noise?

Regards
Colin
-----Original Message----- 
From: Martin McCormick
Sent: Tuesday, November 3, 2015 1:22 PM
To: [log in to unmask]
Subject: Useless Knowledge (was Laser Light)

A lot of what is left of my brain is useless knowledge by
today's standards. When I worked as a technician with the OSU
Audio Visual Center in the eighties, I repaired 16-millimeter
film projectors among many other things and I have a whole
treasure trove of trivia in my head that doesn't get you very far
today.

Did you know, for instance, that if the film stops moving
for even a fraction of a second that the heat from the projection
lamp is sufficient to melt it? Most projectors had a freeze-frame
function but it only worked because a special piece that greatly
reduced the amount of heat/light was supposed to drop down
between the film gate and the film, itself.

We had a few machines that came in because a mechanical
part that held the filter piece up and out of the way failed and
the piece would slowly swing down and cause the light output to
appear to go very dim.

How does a person who is blind work on a film projector?
Most of the mechanical failures that befall 16-millimeter
projectors cause audible issues also.

Sound film projectors must yank the film through the gate
in a series of 24 very fast jerks per second so that each frame
of the picture is still for 1/24 of a second. When the film
reaches the sound pickup, it must not have any trace of the jerky
motion and moves as smoothly as audio tape. There are failure
modes that cause the film to move more smoothly than it should
through the film gate and other failures that make it move in a
jerky manner through the sound pickup. Those usually make the
machine clatter with an extra loud sound or the sound has a
terrible flutter in the speed which is unacceptable. It is
necessary to examine the film transport to diagnose the problem
and fix the broken parts.

There is a Walt Disney movie which I think might be
"Bambee" in which there is a scene where animals are talking to
each other while under water. Their voices all sound fluttery
like someone with water in their throat. One time, my wife and I
watched this film because she wanted to see it again after many
years and I had never watched it at all. I almost fell out of my
chair when that scene came on. Some technician simply blocked the
rotation of the sound drum and fed that audio in to the movie as
a special effect. I thought it was brilliant and never imagined
that someone would actually want that sound to appear, but it was
perfect for the scene.

I remember running across many weird problems that were
usually the fault of wear and tear on belts, gears and clutches.
That's where I accumulated all this useless knowledge.

So, what makes the film stop and melt? Ripped out sprocket
holes along the edge. That will turn torn-up film in to torn-up
melted film. If this had been back in the really old days, the
film which used to be made of celluloid would have burst in to
flame and started a real fire.

Martin

Colin McDonald <[log in to unmask]> writes:
> ah, I love teachable moments!!!
> They call me a walking encyclopedia of mostly interesting, but useless
> facts...