DIY 16mm optical sound recorder, v2:
Optical sound in a film projector.
The optical sound system in a film projectors consists of a soundtrack1, a photocell (hidden behind the sound drum2), a sound lens3, and an exciter lamp4 with a hair-thin filament5.The filament in the exciter lamp is projected as a microscopically thin line of light on the sound track, right above the photocell.
Sound in the projector is generated by the change in the amount of light that falls onto the photocell: one change from light to dark correspods to one phase of a sound wave. If (for example) a light flashes on and off 400 times in a second onto the photocell - a 400Hz tone can be heard out of the projector speakers.
The purpose of the sound track is to modulate that thin line of light that falls onto the fotocell: in a way - to “flash the light on and off”. The soundtrack moves at a constant speed and consists of transparent and opaque areas that change in frequency which corresponds to the frequency of the sound. If (for example) you draw 400 opaque lines with 400 transparent lines between them on a film soundtrack and play it through the projector - a 400Hz tone can be heard out of the speakers.
If you want to learn more about optical film sound track technology, here is a great video to watch.
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