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Yes. It sits on the Movies & TV shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.

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Mondo Knows Facts

Optical sound is printed beside the picture

A wavering line of light and dark runs down the edge of every frame In an optical soundtrack, sound is recorded as a photographic strip alongside the image, read by a lamp and photocell in the projector. Its width or density varies with the waveform, so picture and sound stay locked together. The system replaced sound on disc by the early 1930s. It sits in Mondo's movies & tv collection, one of 200 facts that each trace back to the reference below.

Where this comes from

Encyclopaedia Britannica

Facts that can change over time are reviewed and updated. If something here looks wrong, Mondo would rather know than be confidently incorrect.

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Technology trivia

More facts like this

Seventy millimetre prints carried magnetic soundStrips of iron oxide were glued to the edges to hold six audio tracks Film stock was slit from 70mm rollsEastman Kodak cut Edison's wide roll in half to make the standard gauge An IMAX frame is about ten times largerThe format runs film sideways through the camera at 15 perforations a frame Safety film replaced nitrate in the 1950sAcetate stock burned slowly, though it decayed into vinegar over time Some films are shot with three perforations a frameUsing less film per frame cuts stock costs and allows longer takes Kodak's 16mm film was launched for amateursKodak launched the smaller gauge in 1923 for hobbyists and schools

Tags: sound, film-formats, projection

Optical sound is printed beside the picture: the short answers

Where does this come from, and can I check it?: A wavering line of light and dark runs down the edge of every frame. The figure is taken from Encyclopaedia Britannica, which is linked in full on this page. Every fact on Mondo Knows names its source rather than repeating an unsourced claim.
Is this really a movies & tv fact?: Yes. It sits on the Movies & TV shelf, one of 2,600 sourced facts grouped into space, animals, the human body, history, science and more. Category pages collect every fact in the same field.
What else can I read about it?: The links under the fact point to a quiz, game, puzzle or calculator on the same subject, and the shelf below shows related facts. The wording here is original, and quoted phrases are marked as quotations.