Linear (LONGITUDAL) Timecode (LTC) encodes SMPTE timecode data as a Manchester-encoded audio signal. Each frame is terminated by a 'sync word' which has a special predefined sync relationship with any video or film content.
A special bit in the linear timecode frame, the 'biphase mark correction' bit, ensures that there are an even number of AC transitions in each timecode frame.
The sound of linear timecode is a jarring and distinctive noise and has been used as a sound-effects shorthand to imply 'telemetry' or 'computers'.
Care has to be taken with analog audio to avoid audible 'breakthrough' from the LTC track to the audio tracks.
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