arefully, we notice some interesting things about it. First of all, every time the cir- cle completes a full revolution (360 degrees), it draws the shape shown in Figure 1-14. Thus, halfway through its path, indicated by the zero point on the graph, the circle has passed through 180 degrees of travel. This makes sense, since a circle circumscribes 360 degrees. The reason this is important is that we can manipulate the charac- teristics of the wave created in this fashion to cause it to carry varying amounts of information. Those characteristics-amplitude, frequency, and phase-can be manipulated as follows. Amplitude Modulation Amplitude is a measure of the loudness of a signal. A loud signal, such as that currently thumping through the win- dow of my office from the subwoofer in the back window of the car that belongs to the kid across the street, has high-amplitude components (but very low-frequency components, as evidenced by the fact that I can hear him coming when he's still in southern Vermont), while lower volume signals are lower in amplitude. Examples are shown in Figure 1-15. The Figure 1-13 Creating a sine 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 First Things First Figure 1-14 Sine wave Figure 1-15 Amplitude modulation dashed line represents a high-amplitude signal, while the solid line represents a lower amplitude signal. So, how could this be used in the data communications realm? Simple: Let's let a high-amplitude signal represent a digital zero and a low-amplitude signal represent a digital one. If I then send four high-amplitude waves followed by four low-ampli- tude waves, I have actually transmitted the series 00001111. This tech- nique is called amplitude modulation (AM); modulation simply means "variation." This is how AM radio works-and why it's called "AM." Frequency Modulation Frequency modulat