Retrotechtacular: Lighting The Way For Talkie Pictures With Optical Sound Recording

This week’s Retrotechtacular is a 1943 Encyclopædia Britannica film focusing on optical sound reproduction for motion pictures. Both the sound and the images are recorded on film, which is only affected by light. Therefore, the sound waves must be converted to changes in light.

This is done the way you might expect: the sound waves hit a microphone and the changes in current are amplified and used to control the intensity of light falling on the film. Three types of soundtracks are described and wonderfully demonstrated at the end of the film.

All three types are made from a series of thin bars of light, and the corresponding current value is represented by changes in either their length or their width. In the Unilateral Variable Area recording, the bars extend from the right side of the sound track. Bilateral Variable Area recorded bars emanate uniformly toward the edges from the center. In Variable Density recording, all of the bars extend from the left to right extremes, but their thickness varies.

Variable Density recording is done with a light valve, which contains a pair of delicate metallic ribbons in a magnetic field that move like shutters when the sound current flows through them. The light coming through to the film is varied by the slot created in the space between the ribbons. The light patterns are changed back to sound through a photoelectric cell, which converts the variations in light back to changing current. These changes are amplified and run through a loudspeaker. Be sure to watch to the end to catch a demonstration of the recording methods, set to what we’re pretty sure is Camille Saint-Saëns’ Danse Macabre.

[Thanks Thomas]

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.

13 thoughts on “Retrotechtacular: Lighting The Way For Talkie Pictures With Optical Sound Recording

  1. Whats really cool about this whole thing, is that they still used this up until they had fully digital media distribution. They just extended it to either have multiple channels, or even encode digital information, some films have both analog and digital on them, on the edges, and based on what equipment you use, they read what they need to reproduce the audio. Some smaller local theaters still use this kind of equipment, where they don’t have the need or capitol for expensive digital cinema projectors.

  2. This is confusing. If the audio is recorded with the video then there wouldn’t be any sync issues, yet I thought sync issues was the whole reason for the use of clapper boards when filming motion pictures. Those two seem at odds to me. Or is the optical sound recording method only used for theater copy distribution?

    1. It makes sense that the optical method is only used for theater distribution, it would make it difficult to add music in the editing stage and those light valves look mighty delicate.

      Either way, clapper boards would still be useful in editing, disjoint sound or not, to aduibly indicate a cut.

    2. Watch the video again. When filming a movie, there are two separate films being recorded. One film is a negative of the visual scene, the other film is a negative of the sound track. The sound of the clapper board left a distinctly visible dark mark on the negative sound track. The audio engineer could then visually align that mark on the sound track with the frame showing the clap board closing. To produce the master positive film, the negative video was first projected onto the master, then the negative sound track was projected onto the master. The master was then developed, and had both video and sound tracks.

        1. I found an old Leica 16mm splitter that used a small razor to cut the audio track at an antique store. They wanted a lot for a bunch of kit, but since they had sold the 35mm video camera it didn’t make sense to my wallet.

    1. Maybe because a light track like that is mono by nature, and it takes more work (more cost) to generate a synced stereo track? Vinyl records could do quadrophonic audio with ease, and may have been more stable for storage, transport, and were almost certainly a lot easier for the average person to play. Reel to reel film systems have a lot more moving parts and theaters hired dedicated projectionists to master the skills. It wasn’t until years later that cartridge based tape formats became the norm.

      1. They were I believe, they were also used in the Everest records as a mastering media.
        After I post this I am going to look at the you tube above that came up after the film. Twelve tracks in 1922! Yikes! Of course stereo goes back to 1881, not recorded just wire transmitted.
        LP’s could NOT handle more than two channels. Matrix was a sham and CD4 with 50 kHz in the groove just wouldn’t fly.
        Now off to those vids. 12 track 1922, first AV 1888!

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