Wednesday, March 24, 2010

Player Piano Torpedoes

March 24, 2010 is Ada Lovelace day, an informal holiday to celebrate the achievements of women in technology and science. I'd like to share a fascinating technology story about Hedy Lamarr. Ms Lamarr was a contract star at MGM during the Golden Age of Hollywood, in the 1930s and 40s. She was also a creative and mathematically talented inventor. Today, we would proudly call her a geek.

From US Patent 2,292,387, by Hedy Kiesler Markey and George Antheil:

"This invention relates broadly to secret communication systems involving the use of carrier waves of different frequencies, and is especially useful in the remote control of dirigible craft, such as torpedoes.

Our system... employs a pair of synchronous records, one at the transmitting station and one at the receiving station, which change the tuning of the transmitting and receiving apparatus from time to time..."

Two signals are sent, labelled L and R and controlling the left and right rudders of the torpedo. L is indicated by sending a 100 Hz signal over a carrier, R by 500 Hz. Remotely controlled torpedoes had been used before the 1940s, but were often jammed by the target because the control frequency was relatively easy to detect. The innovation in this patent is the use of perforated rolls of paper to modulate the frequency rapidly enough that the enemy would not be able to predict it, making jamming difficult. The perforated rolls of paper were commonly used in player pianos of the time, requiring no special development.

In the patent application seven rows of perforations were used to control the frequency of the carrier. An eighth row of perforations lights a small lamp at the transmitting station. Three of the seven transmission frequencies were dummies which would not actually be received by the torpedo, while the lamp informed the torpedo operator when the weapon was out of contact. The intent of the dummy frequencies appears to be to mislead the enemy and make it more difficult to determine how the control system worked. Some seemingly valid transmission would not be acted upon by the torpedo, while others would.

Player piano tape
Rows A-G tune the radio to one of 7 frequencies.
Row H controls a lamp for the operator when the dummy frequencies A-C are in use.

For the transmitter and receiver to frequency hop in sync, the tape reels must begin rolling at very close to the same time and the speed of the winding must have a reasonably tight tolerance. Machined springs available in the 1930s were sufficiently precise to maintain this for several minutes, long enough to guide a torpedo to its target.

All in all its a fascinating invention which repurposed existing technology for a new purpose, in fighting the Pacific War. Unfortunately the rest of the story is not a happy one, as the invention was not taken seriously by the War Department. By the time the communication industry reinvented spread spectrum communications in the 1950s, this patent had expired.

In 1997 the EFF recognized Ms Lamarr and Mr Antheil's achievement with a Pioneer award.