. B ANDWIDTH OF S IGNALS In a communication system, the message signal can be voice, music, picture or computer data. Each of these signals has different ranges of frequencies. The type of communication system needed for a given signal depends on the band of frequencies which is considered essential for the communication process.
For speech signals, frequency range Hz to Hz is considered adequate. Therefore speech signal requires a bandwidth of Hz ( Hz – Hz) for commercial telephonic communication. To transmit music, an approximate bandwidth of kHz is required because of the high frequencies produced by the musical instruments. The audible range of frequencies extends from Hz to kHz.
Video signals for transmission of pictures require about . MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated MHz of bandwidth for transmission. In the preceeding paragraph, we have considered only analog signals.
Digital signals are in the form of rectangular waves as shown in Fig. . . One can show that this rectangular wave can be decomposed into a superposition of sinusoidal waves of frequencies + , + , + , + ...
n + where n is an integer extending to infinity and + = / T . The fundamental ( + ), fundamental ( + ) + second harmonic ( + ), and fundamental ( + ) + second harmonic ( + ) + third harmonic ( + ), are shown in the same figure to illustrate this fact. It is clear that to reproduce the rectangular wave shape exactly we need to superimpose all the harmonics + , + , + , + ..., which implies an infinite bandwidth. However, for practical purposes, the contribution from higher harmonics can be neglected, thus limiting the bandwidth.
As a result, received waves are a distorted version of the transmitted one. If the bandwidth is large enough to accommodate a few harmonics, the information is not lost and the rectangular signal is more or less recovered. This is so because the higher the harmonic, less is its contribution to the wave form.