ABSTRACT
Information theory can be used to choose codes and correct modulation techniques, to predict the effect of noise and error rates, and also to relate channel capacity and the noise penalty incurred in using multiple-level codes. System designers must specify levels of signal strength such that the system, in an environment with noise, can identify each of the particular code levels. Information can be superimposed on a carrier signal by a technique called modulation and then removed at the receiving end in a reverse process called demodulation. Amplitude modulation (AM) is derived from early carrier telegraph systems as an outgrowth of the dc telegraph system. Frequency modulation (FM) is a technique in which the frequency of the signal is varied at a rate equivalent to the value of the modulating signal. Frequency division multiplexing (FDM) consists of dividing a high-capacity circuit into compartments on the basis of frequency bands.
