rily follow the exact same frequencies but they do have similar frequency bands. Communications Engineering Desk Reference; ISBN: 9780123746481 Copyright © 2001, Ed da Silva. Published by Elsevier Ltd 2001. All rights reserved CHAPTER 2.1 Basic features of radio communication systems writes postal envelope carrier reads message post box envelope system message sends radio envelope selective receives envelope message carrier modulator receiver demodulate message signal system signal Fig. 2.1-1 Analogy between the postal system and a radio system. We start by writing a letter-message, putting it in an amplitudes to operate the message reading devices such envelope, and sending it through a post-carrier (postal as a loudspeaker and/or a video screen. carrier system) to our destination. At the other end, our In addition to the main functions mentioned above, recipient receives the letter from the post office, opens we need a post box to send our letter. The electrical the envelope and reads our message. Why do we carry equivalent of this is the transmitting antenna. We require out these actions? a letter box at home to receive letters. The electrical We write a letter because it contains the information equivalent of this is the receiving antenna. we want to send to our recipient. In radio communica- tions, we do the same thing; we use a message signal, which is an electrical signal derived from analog sound or 2.1.2.2 Summary of radio digitally encoded sound and/or video/data signals, as the communications systems information we want to convey. The process of putting this information into an 'envelope' for transmission A pictorial summary of the above actions is shown in through the carrier is called modulation and circuits Figure 2.1-1. There are three main functions in a radio designed for this purpose are known as modulation communications system. These are: modulation, trans- circuits or modulators. mission and demodulation. There are also supplementary We use the post office as the carrier for our letters funct