ion (FM) is similar to amplitude modulation except that instead of changing the loudness of Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. First Things First Chapter 1 the signal, we change the number of signals that pass a point in a given second (see Figure 1-16). The left side of the graph contains a lower frequency signal component, while a higher frequency component appears to its right. We can use this technique in the same way we used AM: If we let a high-frequency component represent a zero and a low- frequency component represent a one, then I can transmit our 00001111 series by transmitting four high-frequency signals followed by four low- frequency signals. An interesting historical point about FM: The technique was invented by radio pioneer Edwin Armstrong in 1933. Armstrong cre- ated FM as a way to overcome the problem of noisy radio transmission. Prior to FM's arrival, AM was the only technique available and it relied on modulation of the loudness of the signal and the inherent noise to make it stronger. FM did not rely on amplitude, but rather on frequency modulation; therefore, it was much cleaner and offered significantly higher fidelity than AM radio. Keep in mind that signals pick up noise over distance; when an amplifier amplifies a signal, it also amplifies the noise. Many technical historians of World War II believe that Armstrong's invention of FM transmission played a pivotal role in the outcome of the war. When WWII was in full swing, FM technology was only available to Allied forces. AM radio, the basis for most military communications at the time, could be jammed by simply transmitting a more powerful sig- nal that overloaded the transmissions of military radios. FM, however, was not available to the Axis powers and, therefore, could not be jammed as easily. Phase Modulation Phase modulation (PM) is a little more 