sound recorded on and subsequently played back from an optical or photographic soundtrack, as opposed to a magnetic soundtrack.
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Origin of optical sound
First recorded in 1930–35
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for optical sound
sound recorded in the form of a photographic image on cinematograph films
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012