On Mon, Feb 22, 2010 at 08:30:29PM -0500, KH6TY wrote: > 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.
She invented FHSS as a torpedo control technique; most folks don't know that she had an EE degree. DSSS came about later, as a classified technique called "Phantom", to permit transmissions with a low probability of interception ("LPI"). With a typical 3 KHz bandwidth receiver, or even a 50 KHz wide panadaptor, you won't see all the spectrum from a wideband (say, 100 KHz spreading code) DSSS transmission. You may notice only a slightly raised noise floor. But that's only part of the deal with DSSS. The correlation and despreading produces a really nice gain in noise immunity, as well. -- Mike Andrews, W5EGO mi...@mikea.ath.cx Tired old sysadmin