ions in a radio communications system. These in- because the post office has the ability to transmit clude transmitting antennas, 3 receiving antennas, selec- messages over long distances. In radio communications, tive circuits, and amplifiers. We will now describe these we use a r.f. carrier because a radio carrier has the methods in the same order but with more detail. ability to carry messages over long distances. An r.f. carrier with an enveloped message impressed on it is often called an enveloped carrier wave or a modulated 2.1.3 Modulation and demodulation carrier wave. When the post office delivers a letter to a destination, Before discussing modulation and demodulation, it is the envelope must be opened to enable the message to be necessary to clarify two points: the modulation in- read. In radio communications when the enveloped car- formation and the modulation method. rier wave arrives at its destination, the enveloped carrier In the case of a letter in the postal system, we are free must be 'opened' or demodulated to recover the original to write our messages (modulation information) in any message from the carrier. Circuits which perform this language, such as English, German, French, pictures, function are known as demodulation circuits or data, etc. However, our recipient must be able to read demodulators. the language we use. For example it is useless to write our The post office uses a system of postal codes and ad- message in Japanese if our recipient can only read dresses to ensure that a letter is selected and delivered to German. Hence the modulation information system we the correct address. In radio communications, selective use at the transmitter must be compatible with the or tuned circuits are used to select the correct messages demodulation information system at the receiver. for a particular receiver. Amplifiers are also used to Secondly, the method of putting information (modu- ensure that the signals sent and received have sufficient lation method) on the letter is important. For ex