The primary goal is to achieve perfect timing of the audio track in relation to the visual footage. This can be achieved in two ways: Traditionally by recording a pilot track in an area of the tape that serves as a "clock" upon playback. This can be 60 Hz (in the US) or 50 Hz elsewhere, where there is a power line frequency of 50 Hz. In the studio, the playback of the original tape "pilotone" is compared to the power line frequency, and servo locks the playback machine to that power line frequency, while the Mag Film recorder's motor runs synchronously, transporting the 16 mm or 35 mm magnetic film being recorded.
The newer methods take advantage of the timing of the microprocessor clock accuracy, and rely on playback timing that is nearly absolute, for the length of a typical "take" anyway. Most 16 mm film loads are limited to 11 min, as the magazines typically hold 400 ft of film. This second method requires that the camera's drive system runs at a nearly absolute speed. Typically, this is achieved using a crystal frequency standard, which runs the camera motor. The motor drives sprocketed film, so there is no error from slippage, as found in the recording tape process.
abctvgorehill.com.au mag unit at ABC in the 1950s, to play the sound locked to a film chain
aes.org
Audio Engineering Society AES E-Library
Magnetic Film Transport Based on the SEPMAG Method: Basis and Developmental Program from the First Unit up to the Present Time
Audio Engineering Society Audio Technology in Berlin to 1943: Magnetic Sound Activities – Heinz H. K. Thiele
