Linear timecode (LTC)
SMPTE timecode recorded as an audio signal on its own track, so tape machines and video decks can agree on which frame they are looking at.
Each frame gets an 80-bit word carrying hours, minutes, seconds and frame number, capped with a fixed sync word that tells the reader where the word ends and which way the tape is moving. The bits use biphase mark coding, where every bit starts with a transition. Polarity therefore doesn't matter, and the code reads forwards, backwards and at shuttle speed.
The weakness is at the other end of the speed range. A stationary tape produces no signal at all, which is why video also got VITC, tucked into the vertical blanking interval. In the studio, LTC sounds like a modem with a sore throat and has a habit of bleeding into the neighbouring track, so it was banished to the edge of the reel by long tradition.