photo- cell.

[cinema1]

I would like some information on how the audio sgnal is developed
on a photo-cell in a 16mm sound projector.
I have tried to fined informtion in old radio books plus books
on sound film projection , but none of these books
exsplain how the audio sgnal is developed.
can any one help.
cinema1.

[Alan Stepney]

The light from either an exciter lamp, or part of the light from the main projection lamp, goes through the "sound strip" and on to the cathode of the photocell.

The sound strip varies in area or density in accordance with the sound level and frequency.

The cathode is coated with a material (e.g., caesium)that emits electons when illuminated. That provides a voltage, a minute one, that can then be amplified.
Given that the light falling on the cathode will vary in accordance with the illumination, which will depend upon the width or density of the track, the voltage generated will also vary accordingly.

Amplify, then listen and enjoy.

[cheerfulcharlie]

A similar system was used to run the old speaking clock I believe, except the squiggly recording was kept on glass discs .It obviously offered over coventional discs no friction or wear as it was only light passing through.

The variable density track (which looked like a bar code) was invented by a company called Western Electric I think.However this fell out of fashion because the sound was affected by contrast and brightness tweaking by those processing the film.
The variable area track invented by RCA (I think) is the squiggly line one,it is
robust against processing eccentricities, but the wider the area, the better the frequency response..I think , so a 35mm film will have a better sound than a 16mm one. (Obviously talking analogue here)

[kalee20]

The variable area sound track is indeed basically better than variable density. That's because, essentially the variable density track modulates the playback light beam by fast-changing shades of gray. And that relies on the film and processing to be linear, else distortion could be introduced. Whereas, for variable area, the film at any point is either black or clear, so the linearity of the film doesn't matter.

One crucial aspect for both systems, the light from the exciter lamp must be focussed to form a very narrow line-of-light across the film soundtrack, for best treble response. This is similar to tape recorders, where the narrower the head gap, the better the treble. Also, the faster the film movement, the better the treble response, which is why 35mm film travelling at 18 inches per second is so much better than 16mm at just over 7 inches per second.

[Lucien Nunes]

To expand a little on the method of operation with a traditional caesium photocell in a soundhead, one should say that it is a photoemissive cell that does not itself generate electrical output. The cell is fed with a DC polarising voltage (e.g. 75V) through a high value load resistor (e.g 2.2M). When the light reaching the cell varies, the emission varies in proportion. The resulting AC component of the cell current causes an AC voltage to be developed across the load resistor, which provides the audio signal.

This is a different process to that in a solar cell, (a.k.a. photovoltaic cell) that actually converts incident light energy into electrical power and can produce an output without being externally polarised.

Lucien