The Optical Sound Decoder Extracts Audio Tracks From Vintage Film Reels

Emily Velasco’s Optical Sound Decoder is a very cool analog device that is able to extract and play back audio from vintage film reels.

Emily Velasco is one of our favorite makers because her projects are always incredibly imaginative and creative. She has a serious thing for vintage, analog technology and always finds a way to utilize that tech in ways that not many other people would ever think of. For her most recent project, Velasco put those creative juices to work deciphering the analog sound track on vintage 16mm film reels. After figuring out how to extract that audio, she built this awesome Optical Sound Decoder machine that can play back the sound from whatever film is fed through the mechanism.

Early films were, of course, silent and didn’t have an accompanying audio track at all. Theater goers might be treated to some music from a record player or a live orchestra if they were really lucky. Eventually, during the party that was the Roaring Twenties, technology was developed to include audio tracks on film strips so audiences could hear musical scores and actors’ voices. It would still be several decades before digital technology came around and so those audio tracks were completely analog. A thin portion of the film was reserved for the audio track, which was essentially a waveform represented by exposing the film. Light shines through the film and onto a special photodetector that translates the intensity of the light (the Y axis of the waveform) into an analog audio signal.

Back in the day, a vacuum tube photodetector would have been used to amplify the light into a usable electrical signal. Fortunately, Velasco had access to modern technology and didn’t have to mess around with vacuum tubes or their high voltages. Instead, she used a breakout board with an OPT101 chip, which is basically a monolithic photodiode paired with an op amp. That is able to detect the variations in light intensity coming through the film and output an audio signal that can then be used like any other. In this case, it is fed into an amplifier and then speaker inside of an attractive box.

The sensor is held in place in a 3D-printed mount that the film feeds through. A stepper motor is used to spin a wheel that pulls the film. That stepper motor is presumably controlled by an Arduino, a similar development board, or directly by a stepper motor driver. A knob on the front of the speaker box provides speed adjustment for the stepper motor in order to get the audio pitch just right. It is very cool to hear the voices coming from old film reels, and the Optical Sound Decoder is pretty darn good looking to boot.

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