In wireless communications spread spectrum radio is a transmission technique where the frequency of the signal is intentionally varied. This gives the signal a much greater bandwidth than if its frequency had remained constant. In the conventional transmission and receiving of signals, the frequency does not change over time, except for small fluctuations due to modulation. The signal is kept on a single frequency so two people communicating can exchange information, or so a listener in the broadcast bands knows exactly where to go to find his favorite station.
That is all fine and dandy for typical uses of radio. But as radio has developed the inventors and researchers who expanded the state of the art found a couple of hitches that made it problematic for certain types of signals to remain parked on one frequency. The first was interference caused by deliberate jamming on the desired frequency. This category also included other non-malicious interference coming from transmissions on nearby frequencies. The second issue with using only one frequency in a communication is when the information being transmitted is of a sensitive nature. Constant-frequency signals are easy to intercept. The military and others can make use of codes and encryption to veil transmissions on single frequencies, but codes can be broken. Radio researchers found that another layer of communication security could be added by the use of frequency-hopping which was the first technique established in spread spectrum radio.
Though attributed to multiple inventors, the first patent for frequency hopping was granted to actress Hedy Lamarr and composer George Antheil in 1942 for their "Secret Communications System" that was designed to protect Allied radio-guided torpedoes from being jammed by the Axis powers. Both Hedy and George are most remembered for their main fields of activity, movies and music, but they each had a touch of the polymath inside of them, and their other passions allowed them to make a significant advance in the radio arts.
Hedy was born in 1914 in Vienna and started training in the theater as a teenager in the 1920's. By the age of eighteen she had married her first of six husbands. Friedrich "Fritz" Mandl was a wealthy ammunitions manufacturer whose weapon systems later gave her inspiration for the patent. During this time she had started a career in film in Czechoslovakia with the 1933 film Ecstasy which became controversial for its frank depictions of nudity and sexuality. Hubby Mandl got a bit ticked off by these movie scenes and attempted to stop Hedy from continuing her career as an actress. In her autobiography Ecstasy and Me she claimed that she was kept virtually a prisoner in their Austrian castle home. She wrote, "I knew very soon that I could never be an actress while I was his wife.... He was the absolute monarch in his marriage.... I was like a doll. I was like a thing, some object of art which had to be guarded—and imprisoned—having no mind, no life of its own". And Hedy had a keen mind with natural talent for science and invention.
Both Mandl and Lammar had Jewish parents, but Mandl also had business ties with the Nazi government, to whom he sold his weapons. Mussolini and Hitler were among those who attended the lavish parties Mandl hosted at their Schloss Schwarzenau castle. Hedy would accompany him to his meetings where she got to associate with scientists and professionals involved in military technology. It was at these conferences where her interests in inventing and applied science were first sparked.
As her marriage grew unbearable she decided to flee to Paris where she met movie mogul Louis B. Mayer who was scouting for talent. With all the trouble brewing in Europe he found it easy to persuade her to move to Hollywood where she arrived in 1938 and began work on the film Algiers. She was in number of other popular feature films, including I Take This Woman (1940), Comrade X (1940), Come Live With Me (1941), H.M. Pulham, Esq. (1941), and her most famous role in Cecil B. Demile's Samson and Delilah (1949). After starring in the comedy My Favorite Spy (1951) with Bob Hope her acting career started to peter out.
It was during the height WWII and her career when she was also grew bored with acting. Hedy had complained that the roles given to her required little challenge in terms of technique or the delivery of lines and monologues. Mostly the films she had starred in cast her for her beauty rather than her talent and ability. Stifled by the lack of more demanding roles she found an outlet for her intellectual capacities through the hobby of tinkering and inventing which was nurtured by her friendship with aviation tycoon Howard Hughes.
Lamarr had some ideas about using radio controlled torpedoes in the war effort. To help her in its implementation she eventually tapped composer George Antheil, who had also found success in Hollywood scoring films. Antheil had been a part of the Lost Generation, and like many of his contemporaries such as Ernest Hemingway, he had moved to Europe after the horrors of the first World War to live a bohemian and artistic life amidst the cafes and salons of Paris in the 1920's. It was during this time period when he composed his best known work Ballet Mecanique. It began its life as an accompaniment to the Dadaist film of the same name made by Fernand Léger and Dudley Murphy, with cinematography by Man Ray. The techniques Antheil developed in this composition were to be key to the success of his shared frequency hopping patent.
Ballet Mecanique was scored to use a number of player pianos. He described their effect as "All percussive. Like machines. All efficiency. No LOVE. Written without sympathy. Written cold as an army operates. Revolutionary as nothing has been revolutionary." There are no human dancers. The mechanical instruments are what make it a ballet. Antheil's original conception was to use 16 specially synchronized player pianos, two grand pianos, electronic bells, xylophones, bass drums, a siren and three airplane propellers. There were a number of difficulties involved in this set-up that broke away from traditional orchestral arrangements. The synchronization of the player pianos proved to be the largest obstacle. Consisting of periods of music and interludes of relative silence created by the droning roar of airplane propellers. Antheil described it as "the rhythm of machinery, presented as beautifully as an artist knows how."
Besides composing Antheil was a writer and fierce patriot. He was a member of the Hollywood Anti-Nazi League and wrote a book of predictions about WWII titled The Shape of the War to Come. He also penned a newspaper column on relationship advice that was nationally syndicated and he fancied himself an expert on the subject of female endocrinology. His interests in this area was what first brought into contact with Hedy. She had sought him out for advice on how she might enhance her upper torso. After he proposed that she could make use of glandular extracts their conversation turned to the kind of torpedoes being used in the war.
Lamarr was herself a staunch supporter of her adopted country, though she didn't become a naturalized citizen until 1953. Using knowledge she gained from her first marriage with the munitions manufacture she had the insight that radio controlled torpedoes would excel in the fight against the Axis powers. However the radio signals could easily be jammed and the torpedo sent off course. Working with Antheil she devised their "Secret Communications System".
The action of composing for the player pianos helped Antheil with one of the aspects of creating their system, which had a striking resemblance to the still top secret SIGSALY system. It is best described in the overview of their patent number 2,292,387: "Briefly, our system as adapted for radio control of a remote craft, 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, so that without knowledge of the records an enemy would be unable to determine at what frequency a controlling impulse would be sent. Furthermore, we contemplate employing records of the type used for many years in player pianos, and which consist, of long rolls of paper having perforations variously positioned in a plurality of longitudinal rows along the records. 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, from one to another of which both the transmitting and receiving station would be changed at intervals. Furthermore, records of the type described can be made of substantial length and may be driven slow or fast. This makes it possible for a pair of records, one at the transmitting station and one at the receiving station, to run for a length of time ample for the remote control of a device such as a torpedo. The two records may be synchronized by driving them with accurately calibrated constant-speed spring motors, such as are employed for driving clocks and chronometers. However, it is also within the scope of our invention to periodically correct the position of the record at the receiving station by transmitting synchronous impulses from the transmitting station. The use of synchronizing impulses for correcting the phase relation of rotary apparatus at a receiving station is well-known and highly developed in the fields of automatic telegraphy and television."
Although the US Navy did not adopt their technology until the 1960s the principles of their work continue to live on and are now used in everyday devices such as Wi-Fi, CDMA, and Bluetooth technology. Spread spectrum systems are also used in the unregulated 2.4 GHz band and on some walkie-talkies that operate in the 900 MHz portion of the spectrum. Other spread spectrum techniques include direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), and chirp spread spectrum (CSS).
In 2008 Elyse Singer wrote the script for an off-Broadway play, Frequency Hopping, that features the lives of Lamarr and Antheil. It won a prize for best new play about science and technology. Hedy and George's pioneering work eventually led to their posthumous induction into the National Inventors Hall of Fame in 2014.
Ecstasy and Me by Heddy Lamarr
The Bad Boy of Music by George Antheil
George Antheil, Ballet Mecanique: Digital Re-creation of the Carnegie Hall Concert of 1927, Conducted by Maurice Peress, Music Masters Inc. 1992.