There have been many different systems to provide essentially “electronic sprockets” on audio tape to lock the tape, speed-wise, to film or video being shot at the same time. In 1967 EECO developed a system of time code which was later standardized (I want to say the mid-1970’s, but that date is not certain), by the Society of Motion Picture and Television Engineers (SMPTE) and the European Broadcasting Union as SMPTE/EBU TimeCode. This became the best and most widely used system of locking differing transports together. Here are some links that might be a starting point:
Going back in history, we have a variety of different systems that embedded tones on tape in order to provide a speed reference. The best summary of these old systems I have seen to date is in the EBU-3095 document (European Broadcasting Union). It is now available from the EBU here. The EBU developed a film code in 1985, but I do not know how widely it was used.
One of the later, and more-common versions of this is NeoPilot This Nagra-developed system has two out-of-polarity tracks within the audio track. It is somewhat problematic when reproducing on two-track heads as the pilot tone (60Hz) does not cancel out–another reason to have a full-track mono head assembly.
Nagra, Stellavox, and Tandberg all made units capable of accepting the 60 (or 50) Hz “pilot tone” from a movie camera or crystal synch unit so that the camera motor and the tape were locked together and running at the same speed. On playback, the pilot tone allowed adjustment of the tape speed to accurately reflect the camera speed so when the sound was transferred to sprocketed magnetic media, the sound and picture would line up.
From EBU T3095, these are the methods in the order presented in the document. The EBU has kindly made this document available as a PDF here.
Pilote (not described in EBU T3095) dates back to 1940 in Germany and it has an 18-mil gap at a 90-degree angle with no bias. Nagra’s Pilot-tone was an improvement on this system.
Pilot-tone 50/60 Hz from camera or crystal fed to centre track 90 degrees out of azimuth with the audio. Developed by Nagra and described in DIN 15575 (October, 1965). Pilottone starts when the last frame of film in the camera is exposed by a light inside the camera. This should be recoverable with standard timecode heads.
Perfectone uses a 4.0 mm play head in the centre of the tape with out-of-phase synchronizing signals at 2x the mains frequency recorded on the tape edges. 4-channel heads should recover this with tracks 2 and 3 summed for mono audio. Or the NAB 3-track cartridge heads could be used.
Neopilot is an update to the Pilot system and is also specified in DIN 15575. It uses two opposite polarity mains-frequency tracks. Each track is 0.45 mm wide and they are separated by 0.4 mm and this cluster is centred on the centre line of the tape. The audio is standard full-track. This requires a special head for recovery of the two out-of-polarity tracks.
Ranger (aka Rangertone) developed in 1949 uses a sync head at mains frequency 80 degrees out of azimuth with the audio in the centre of the full-track audio. This might be readable on a centre-track timecode head.
Telefunken this is a 0.8 mm centre track like centre-track timecode with a modulated tone. It is designed for NAB-standard (2.0 mm) stereo heads.
Synchrotone uses a 0.5 mm centre track with mains-frequency recorded on it. The centre-track timecode assembly should be able to extract this.
Leevers-Rich uses a modulated carrier either on one track of a 2-track NAB-standard head configuration (2.0 mm tracks) or a 0.6 mm centre-track. This system apparently used both 1 kHz and 400 Hz carriers, modulated with the supply mains or camera-motor locked pulses.
B.B.C. uses a 2.0 mm (NAB) two-track head with the mains signal on the lower channel.
Fairchild uses a full-track-width off-azimuth head recording an amplitude-modulated 14 kHz carrier. The modulation is at the mains/camera motor frequency. This will require significant modification to a standard head assembly to recover. It might be possible to use the record head, tilted, to acheive azimuth.
In addition, after this standard was published (I believe), Nagra came out with FM pilot for stereo machines. Synchrotone is similar, but I’m not sure if it’s identical with Nagra’s FM pilot.
In July, 1968, Loren L. Ryder published in the Journal of the Audio Engineering Society “Synchronous Sound for Motion Pictures.” In this paper, Ryder (of Ryder Sound in Hollywood) states that Neopilot accounts for 75% of all sync sound recordings made at that time. The remaining 25% in his experience were, at that time, dominated by the original Pilote system, the Ranger system, and a 2-track system (although he doesn’t seem to indicate whether it’s using a carrier or not).
I am aware of some usage of these mains-related frequencies being used on 4-track tapes for speed control. Both 1/4-inch and 1/2-inch tapes were used in this mode.