ABSTRACT
It is often the case in instrumentation and communication systems that an information-bearing signal may not be in an optimal form for direct use. In such cases, the information-bearing signal may be used to alter some characteristic of a second signal more suited to the application. This process of altering one signal by means of another is known as modulation; the original information is called the baseband signal, and the signal modulated by the baseband signal is termed the carrier (because it “carries” the information contained in the baseband signal). Recovery of the original information requires a suitable demodulation process to reverse the modulation process. A prominent use of modulation techniques is found in communication systems. The extremely long wavelengths of electromagnetic waves at frequencies found in a typical audio signal make direct transmission impractical, because of constraints on realistic antenna size and bandwidth.
