Skip KH6TY wrote: "In fact actress Hedy Lamarr invented spread spectrum, and you can read that here: http://en.wikipedia.org/wiki/Hedy_Lamarr.";
While frequency-hopping was first introduced in a patent filed by Nikola Tesla in 1900, I've always been fascinated by the role of Austrian actress Hedy Lamarr in the development of spread-spectrum. According to Wikipedia, (following is quoted from their site): Avant garde composer George Antheil, a son of German immigrants and neighbor of Lamarr, had experimented with automated control of musical instruments, including his music for Ballet Mecanique, originally written for Fernand Léger's 1924 abstract film. This score involved multiple player pianos playing simultaneously. Lamarr had learned about the problem at defense meetings she had attended with her former husband Friedrich Mandl, who was an Austrian arms manufacturer. Together, Antheil and Lamarr submitted the idea of a secret communication system in June 1941. On August 11, 1942, U.S. Patent 2,292,387 was granted to Antheil and "Hedy Kiesler Markey", Lamarr's married name at the time. This early version of frequency hopping used a piano roll to change between 88 frequencies and was intended to make radio-guided torpedoes harder for enemies to detect or jam. The patent came to light during patent searches in the 1950s when ITT Corporation and other private firms began to develop Code Division Multiple Access (CDMA), a civilian form of spread spectrum. Jim - K6JM ----- Original Message ----- From: KH6TY To: digitalradio@yahoogroups.com Sent: Monday, February 22, 2010 5:30 PM Subject: Re: [digitalradio] Re: Curious sound card modes question - It will be spread spectrum if the tone frequencies are controlled by a code as explained in the ROS documentation: "A system is defined to be a spread-spectrum system if it fulfills the following requirements: 1. The signal occupies a bandwidth much in excess of the minimum bandwidth necessary to send the information. 2. Spreading is accomplished by means of a spreading signal, often called a code signal, which is independent of the data. 3. At the receiver, despreading (recovering the original data) is accomplished by the correlation of the received spread signal with a synchronized replica of the spreading signal used to spread the information. Standard modulation schemes as frequency modulation and pulse code modulation also spread the spectrum of an information signal, but they do not qualify as spread-spectrum systems since they do not satisfy all the conditions outlined above. Note that all three conditions must be met to be considered spread spectrum. I don;t know if it would be possible to send the data in less bandwidth, but, for example, PSK31 accomplishes the same typing speed in a bandwidth of 31 Hz, instead of in 2000 Hz, so ROS is probably truly spread-spectrum. Remember that spread spectrum was conceived as a way of coding transmissions so they could not be intercepted and decoded. In fact actress Hedy Lamarr invented spread spectrum, and you can read that here: http://en.wikipedia.org/wiki/Hedy_Lamarr. The difference is the use of a code to spread the data and signals to avoid detection and monitoring by those without the same code. Download the documentation from www. rosmodem.wordpress.com and read about spread spectrum and the ROS implementation. That will make it clear I think. Remembering that a single tone creates a single RF carrier makes it easy to see how just about anything can be done with tones, including sending data over several tones at once so if one carrier is lost, others carry the same data, or using a psuedo-random code to determine the carrier frequencies, as I think is done in ROS. That documentation also explains the difference between FHSS and modes like MFSK16. However, a main point is that the data does not have to be scattered over such a wide bandwidth to achieve communication, but ROS does, so it qualifies as spread spectrum. If you have a receive bandwith of 10,000 Hz, and you spread over that bandwidth, you really are using way more bandwidth than necessary to send the same data at a given speed. MT63 uses 64 carriers with the data divided among the carriers for redundancy and about 40% of the signal can be obilterated by QRM and still produce good copy. I think the difference with ROS is that the carrier frequencies are varied according to a code, instead of being at a fixed position, but I am no expert on modes, so someone else can probably explain it better and with more accuracy. Generally it is qualifies as spread spectrum if a code is used for the spreading, and in military communications (and even cell phones, I think) the code prevents anyone else from reconstructing the signal so that the intelligence can be recovered if they do not possess the same code. 73 - Skip KH6TY
