On neo pilot

Neo pilot on tape synch for film is a great and very clever invention.

It operates in a similar way to a balanced microphone input where the RF or other disturbing signal is extinguished.

A push signal is recorded on some upper part of tape. A pull or 180 degrees out of phase or x(-1) or times (-1) signal is recorded on some lower part of tape. The full track mono playback head cannot read their existence. But the information can be extracted. For example by reading each part, multiplying one part with (-1) and adding it to the other. For example by connecting the 2 information  extracting  heads in series and out of phase.

She also:

https://en.m.wikipedia.org/wiki/Pilottone


And a previous post on synchronization