Sound sync in a Mitchell BNC?

[David Tilotta]

Hi Folks,

I apologize up front for the questions, and hope they're not too naive. I'm working on a project and trying to understand how double system sound worked in the 1960's with a typical 35 mm Mitchell BNC and a Nagra III tape recorder. It's my understanding that the motors of the two needed to have some synchronization signal in order for the sound and picture to be synched. Does the Mitchell supply the signal? Or did they just rely on a common power supply? Or? Any clarity on this on how this worked would be much appreciated!

David

[dan kessler]

You can google up lots of details and many folks here are experts, but you've
got the basic idea.

From the 60's until things went digital, the Nagra tape recorder was the de facto
standard for sync sound recording, using the Neopilotone system.

Cameras like the Mitchell had motors equipped with little signal generators that produced
a 60-cycle signal when running at speed.

That signal was recorded on a separate track on the 1/4" tape during each take.

When the sound was transferred to 35mm mag for editing and mixing, playback speed was regulated 
by comparing the sync signal to an internal crystal-controlled reference oscillator.

Prior to the 60's, sound was recorded directly to 35mm sprocketed mag tape on a recorder equipped
with a synchronous motor just like the camera.  Both ran from a common 60-cycle power supply, thus
they ran in sync.
 

[Steve Switaj]

To further expand on what Dan said;

The issue back in the day was that neither the camera nor the tape recorder could be relied on to run at exactly 24fps.

They would be close, since they were controlled by good mechanical or electrical governors, but in the age before good electronic servos, they were not exact and would drift from each other.

To keep them in sync they had to share a timing reference, and this was a system called pilot tone.

The camera had a little tachmoter on the motor that generated an audio tone at 200x the frame rate. This tone generator was thus mechanically coupled to the speed of the camera and for a camera at 24fps was a nominal 4800 Hz.*

This tone was fed to the audio guy who recorded it on the tape as a separate track. This tone became, in effect, the 'perfs' for the audio track, at 200 perfs per frame.

The camera might drift a bit, but so would the pilot tone, and they'd do it in sync. Later, in post where everything was held to 24 fps, the tape was played back with a machine that kept the reference tone at 4800 hz regardless of what frequency it had originally been recorded at. 

If the camera was a bit slow, say only 23fps (or 4.2% slow), the pilot tone went to tape at 4600Hz. On playback in the post house, the tape machine there would speed up the tape 4.2% to get it back to 4800Hz, and the playback speed would magically match the film, now also running at 24fps. (there would be a slight pitch change, but unless you were doing music it was seldom enough to worry about).

Even after cameras got crystal time references and could run at exact speed, pilot tone still stuck around on the audio side. Tape recorders built for film work, like the Nagra family, would still generate the tone internally and lay it down on the reference track. Since tape used in the field doesn't have perfs, in the days before time code and digital formats this was the only way to get a true speed reference for the audio.

 

*I think there were also systems at different frequencies, I don't know which ones were used in what situations.

 

Edited November 23, 2025 by Steve Switaj

[David Tilotta]

Thanks for the replies and details. Much appreciated!

 

David

Edited November 23, 2025 by David Tilotta