The Iron Pencil - Re-recording and Mixing at Walt Disney Productions
The patent was applied for on July 31st, 1940, the same day as the filing of the two Fantasound patents, which implies to me that it was invented originally for mixing Fantasia, which was released in November that year. The patent was granted June 24th, 1941, in the middle of the infamous strike.
In its simplest form the console would look like the patent application image: in the center would be the master score, and the musician - or Jim MacDonald - would follow it using a metal pin connected to a pantograph system which in turn moved a lamp throwing a light circle, not much larger than an inch in diameter, on copies of the score marked for that specific track. This would be followed by an operator who, based on the lighted information, would control a fader for just that track, which may have been music, effects or vocals...
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It is certainly an archaic system, compared to modern computer-operated mixing consoles, but it was no doubt a vast improvement over having to follow a bouncing ball. Director Jack Kinney believed it still to have been in use around 1973!
Here is the entire patent document: