Hedy Lamarr and George Antheil's 1942 torpedo patent hopped frequencies using player-piano rolls synced by clock motors

The problem they were solving is right there in the patent’s own words. Radio control of a torpedo was easy to break because “the enemy could quickly discover the frequency of the control signals and block control of the torpedo by sending false signals of the same frequency.”

Their fix was to never sit on one frequency long enough to be found. The system “employs a pair of synchronous records, one at the transmitting station and one at the receiving station,” which keep retuning both radios together, “so that without knowledge of the records an enemy would be unable to determine at what frequency a controlling impulse would be sent.”

And the records were paper. The patent says it plainly: “records of the type used for many years in player pianos.” The strip in the drawings has only eight rows of holes. Seven of them work switches that bring tuning condensers in and out of the circuit, and the eighth lights a signal lamp. But the inventors knew where it could go: “In a conventional player piano record there may be 88 rows of perforations, and in our system such a record would permit the use of 88 different carrier frequencies.”

The mechanism reads like a player piano that went to war. Each roll runs past a control head with a row of small passages connected to “a suction pump.” When a hole in the paper uncovers a passage, suction lifts a little piston “against the force of the spring,” and the piston closes a tuning switch. That’s exactly how a pianola turns a hole into a note, except the note here is a radio frequency.

Keeping two rolls in step, one on a ship and one inside a moving torpedo, was the hard part. They did it with clockwork. The rolls are “synchronized by driving them with accurately calibrated constant-speed spring motors, such as are employed for driving clocks and chronometers.”

Starting the two rolls took one more trick. Both rolls had to begin at the same instant, so the patent holds each one at a starting hole with a pin and a solenoid until launch. Firing the torpedo breaks the wire running to it, which cuts the current to both solenoids at once, and both rolls start moving together. Launching the weapon is what starts the clock.

Steering was simple once the link was secure. Two keys, L and R, sent a 100-cycle or 500-cycle tone on the hopping carrier to swing the rudder, and the inventors pointed out that “the transmission of a very short impulse may not be discovered by the enemy at all.” The patent even lets an observer in an airplane do the aiming.

The Hall of Fame describes the frequencies “switching frequencies at split-second intervals in a seemingly random manner that would sound like mere noise to anyone listening.” The patent itself never says how fast a 1942 clock motor could actually step a paper roll, or how close two spring motors would stay after a few minutes underwater. It expired in 1959, and Google Patents lists it as cited by well over a hundred later patents, many of them for wireless systems. How many of those inventors had ever seen a player piano?