uit is low. Overall, vibrations in the diaphragm (its "pushing back" and "popping out") cause the same vibrations (frequencies) to appear in the current of the electrical circuit (via those carbon granules). The channel is a copper wire. The vibrating current generated by the transmitter is carried along this wire to the receiver. Channel (copper wire) eardrum electromagnet 4v power supply Windpipe Vocal carbon granules cords diaphragm Monica TRANSMITTER RECEIVER Figure 1.4 Monica and Carl talking on a telephone Chapter One The receiver consists of two parts: the receiving (top) part of the telephone, and Carl's ear. The current, sent along the copper wire, arrives at the top end of the tele- phone. Inside this top end is a device called an electromagnet and right next to that is a diaphragm. The current, containing all of Monica's talking frequencies, enters into the electromagnet. This electromagnet causes the diaphragm to vibrate with all of Monica's talking frequencies. The vibrating diaphragm causes vibrations in the air, and these vibrations travel to Carl's ear. His eardrum vibrates, and these vibrations cause electrical signals to be sent to his brain, which interprets this as Monica's sound. 1.3 Analog and Digital Communication Systems The last part of this chapter is dedicated to explaining what is meant by analog and digital communication systems, and then explaining why digital communication systems are the way of the future. 1.3.1 Some Introductory Definitions An analog signal is a signal that can take on any amplitude and is well-defined at every time. Figure 1.5(a) shows an example of this. A discrete-time signal is a signal that can take on any amplitude but is defined only at a set of discrete times. Figure 1.5(b) shows an example. Finally, a digital signal is a signal whose amplitude can take on only a finite set of values, normally two, and is defined only at a discrete set of times. To help clarify, an example is shown in Figure 1.5(c). Figure 1.5 (a) An analog signal; (b) a discrete time sign