Sunday, March 06, 2005

Thank Hedy Lamarr For Bluetooth

The next time you are using your wireless cell phone headset or synchronizing your PDA, be aware that one of the key technologies in Bluetooth enabled wireless devices was co-invented by a beautiful and famous 1930's film actress.

I first ran across this amazing story in the Spring, 1997 edition of American Heritage of Invention & Technology Magazine. In fact, the cover boldly proclaimed "Hedy Lamarr, Munitions Inventor." Munitions? Bluetooth? Movie actress? What sort of bizarre tale is this?

The story begins on the eve of World War II. Hedy Lamarr, the former Hedwig Eva Maria Kiesler, was married to Fritz Mandl, a preeminent Austrian arms manufacturer. She divorced Mandl in 1937 and moved to Hollywood where Louis B. Mayer of MGM gave her the name Lamarr. What she learn while married to Mandl apparently stayed on her mind, though. In 1940 she struck up a conversation with her neighbor, a noted music composer. Hedy had an idea for radio controlling torpedoes by constantly moving the control frequency so the radio signal wouldn't be detectable or easily interfered with. What she needed was some practical way to make it work.

This is where George Antheil, her neighbor and composer, thought it might be done like a player piano. Why not? A player piano roll causes a constant set of frequency shifts by changing notes to play a song. If you synchronize two identical piano rolls, one at the transmitter and one at the receiver, you can change transmission channels at will. The transmitter roll switches a different capacitor in the oscillator circuit to change frequency and the receiver roll does likewise to retune the receiver. Anyone else trying to tune in this signal will only catch it for a moment before it changes. The eavesdropper doesn't know what frequency is next on the roll, so he doesn't know where to tune next.

George Antheil and Hedy Lamarr, by then remarried with the name Hedy Kiesler Markey, are the two inventors named on the 1942 patent called "Secret Communication System." It's patent number 2,292,387 if you want to get a copy. The Navy didn't jump on the idea, thinking the piano roll scheme was too hard to get working reliably. In 1957, engineers at Sylvania redesigned the scheme using electronics and it became the basis for secure military communications.

So what's this got to do with Bluetooth? You might recognize the methodology by its current name: Frequency Hopping Spread Spectrum or FHSS. The twin characteristics of FHSS are secure transmissions and interference resistance. Interfering signals might drown out one of the frequencies in the hop sequence, but any data lost during that brief outage can be resent on another frequency. That's a good characteristic to have for low power data communications on the crowded 2.4GHz band shared with WiFi.

The Bluetooth transmission protocol uses a set of frequencies between 2.40 and 2.48 Gigahertz. It selects from among 79 frequencies and chooses a new one 1600 times a second. That would be tough to do with piano rolls, but the implementation in integrated circuits still follows the basic patent awarded to a music composer and a movie actress. Thanks Hedy! Thanks George!

If you can't find the Spring, 1997 issue of Invention & Technology, you can read a similar account of Hedy Lamarr's story at

Learn more about Bluetooth communications at The Official Bluetooth Wireless Info Site

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