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Vintage 16mm optical sound cameras - what made them tick?


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Hi team,
I've been at National Camera Exchange in Golden Valley, Minnesota for the last 10 years, and for 6 of those I've run their video transfer lab.  One medium I've encountered there is optical sound 16mm film, and as you may know, it has an optical sound stripe exposed along its edge which reproduces an audio track.  I have always wondered how the camera exposes that track on there.  I've Google image searched for schematics or mechanisms of action on the 16mm optical sound movie cameras to no avail.  Do any of you know how those cameras worked?
Thanks,
Mark Stang
Excelsior, MN

Optical sound film.jpg

Edited by Mark Stang
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A sound module was fitted in the film cavity of a camera and laced so that it exposed the sound track slightly before the visual exposure in the camera gate (26 frames was a common offset). I believe the optical sound modules used a mirrored galvanometer and a slit light source to vary the width in response to the sound frequency and intensity.

You might find patent descriptions if you search for them, I think RCA were the ones who developed variable width optical sound:

https://en.wikipedia.org/wiki/RCA_Photophone

There are film societies that still play with optical sound recording:

http://mononoawarefilm.com/may-19-optical-sound-16mm

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That reference image presents digital optical tracks which can only be made in the lab, not in camera. The variable area track can be made in camera using a small slit and a rotating mirror altered by magnetic field generated by the audio signal. 

Most in camera optical tracks are variable density which is the simplest one to create

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The Auricon could be obtained with either a Variable Area or a Variable density track recording option. The Maurer could be outfitted with a Tobis Klangfilm-like multiple trace Variable Area track.

It's a black art, making those galvos.  Not many people who walk the face of this Earth can service them or build them.

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