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.
(a) Bandwidth of speech : For speech signals, frequency range 300 Hz to 3100 Hz is considered adequate. Therefore speech signal requires a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for commercial telephonic communication.
(b) Bandwidth of Music : To transmit music, an approximate bandwidth of 20 kHz is required because of the high frequencies produced by the musical instruments.
(c) Bandwidth of Audio signal : The audible range of frequencies extends from 20 Hz to 20 kHz.
(d) Bandwidth of video signal : Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission.
(e) Bandwidth of digital signal : Digital signals can be ideally reconstructed if we have superimpose fundamental frequency and all the harmonics and hence implies an infinite bandwidth. However, in practice, 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.
