Ah yes, I got my AV card in junior high so I could thread the 
films in to those projectors.  They wouldn't even have to turn 
the lights on when something happened during a movie if I was 
there.  I bet I still have that AV card in my wallet somewhere, 
73, Jim WA6EKS

 ----- Original Message -----
From: Martin McCormick <[log in to unmask]
To: [log in to unmask]
Date sent: Tue, 3 Nov 2015 21:53:47 -0600
Subject: Re: Useless Knowledge (was Laser Light)

	Basically, yes.  It is sort of a fast sewing machine sound
as in click-click-click-click on top of the whirring sound of the
motor.

	Briefly, a movie projector works as follows:

	Film feeds in to the bottom of the machine from the
supply reel.  The film is 16 millimeters wide which is about half
an inch.  One edge has a row of holes down it's total length 
while
the other edge has a wavy line down it's entire length that is
the optical sound track.

	When a projector is properly loaded, there is a loop of
film at the top of the film gate and another loop of film at the
bottom of the gate as if somebody left some extra slack.  There
are sprocket wheels above and below the loops and gate and those
wheels have teeth that fit precisely through the holes.

	The jerky movement happens when a tiny metal claw darts
out of a slot in the film gate and yanks the film forward one
hole.

	The top loop gets a little bigger and the bottom one gets
a little smaller for an instant but the sprocket wheels keep
tugging the film at a relatively steady speed.

	The sound pickup is near the bottom of the projector and
there is a small lamp there that shines through the wavy line
opposite the holes and this lamp is focused through a lens
assembly that makes a thin beam of light shine through the sound
track.  A photo cell sees this varying amount of light and turns
that back in to an electric current which goes through a
conventional amplifier to the loud speaker.  Just as this 
happens,
the film is partly wrapped around the sound drum which looks
something like the capstan roller on a tape recorder.

	That roller turns a heavy fly wheel which smooths out the
last vestiges of jerkyness and gives you normal sound or wattery
sound if the wheel isn't spinning.

	On occasion, the film slips just enough in one direction
that the sound track drifts a little too far out and the sound
optics begin to pick up a little bit of the pictures and that
sounds pretty bad, like an engine idling in the background.  This
can be bad film and it usually goes away on it's own.

	The film gate where the film stands still for 1/24 of a
second has the little window where the bright projection lamp
shines through.  The claw is right next to that window and the
click you hear is it yanking the film one frame at a time.

	Finally, there is what looks like a 3-bladed fan between
the film gate and the projection lens.

	The blades of this device are not tilted like fan blades
but flat like cutters.  They interrupt the light 3 times for 
every
frame of film which means that viewers don't see quite as much
flicker.

	These beasts were complex and there is a lot of history
behind the mechanism.

Colin McDonald <[log in to unmask]> writes:
 ah, so the 24 jerks per second is the noise you hear on audio 
recordings
 where they were using a film projector?
 Sort of a generator like noise?