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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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Variable area tracks look like an audio waveform in a digital edit timeline.

Variable density tracks are a constant width with what looks like very fine bars across the full width of the track. 

Before magnetic audio recording media came along, there were optical audio recorders. Some like Auricon were integrated into the camera itself. Others were separate boxes which rolled film and recorded an optical soundtrack and was used just liike a tape recorder. The optical track was later mated to the final film negative to strike release prints. Synchronisation was achieved by using AC motors in cameras and audio recorders driven by a common source.

My understanding was that variable density recording was by using a galvanometer which rocked a mirror instead of a display needle on a gauge, disturbing  the direction of a beam of light. There may have been a fine slot aperture and the mirror and light beam may have been larger than the film track itself and scaled to the area of the track using a lens. 

My understanding of variable area recording is that there was a light emitting device which was similar to the very old visual recording level green displays on old tape recorders. It was some kind of discharge tube. The length of the band of light varied with audio level. Such a display may have been scaled to film track width via lenses and maybe narrowed by a transverse slot aperture to improve higher audio frequency response. 

Calibrating and setting levels must have been a nightmare but there may have been a live-viewing method for calibrating.

Some of the old tech was large and brutally heavy. 

If anyone has more definite information and can correct my comments, please do so.

Edited by Robert Hart
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Lauste was probably the first. https://books.google.ch/books?id=86JXA-G2ovsC&pg=PA71&lpg=PA71&dq=eugène+augustin+lauste+sound+recording&source=bl&ots=KrdtPP-ES5&sig=ACfU3U13oqjho4H0BUqbEdrWYNO1ekgyWA&hl=de&sa=X&ved=2ahUKEwjzwNn3kPDyAhVEhf0HHdfADdQ4ChDoAXoECA0QAw#v=onepage&q=eugène augustin lauste sound recording&f=true

Berglund held the first public presentation of a synch sound film in Stockholm, Sweden, in 1921.

Of the three original processes two subsisted. The longitudinal process that yields similar records as the intensity does was never used. Variable-intensity sound recordings were relatively easily made with the aid of the Kerr cell. That system works linearly up to 100 kHz. The main problem that had to be solved was film stock sensitive to very short exposure times, the Schwarzschild effect corrected. Until noise reduction was introduced VI tracks suffered from noticeable hiss. Variable-area tracks do not use halftones, therefore noise or hiss from the granulation plays a minor role. Also, VA recordings are more forgiving for less than perfect exposure and development. VI tracks need to be mastered photographically, else you have distortions. On the other hand VI tracks are more robust in reproduction.

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