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, a frequency range of 300 Hz to 3100 Hz is considered adequate. Hence pair 1 is correct. Therefore speech signal requires a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for commercial telephonic communication. To transmit music , an approximate bandwidth of 20 kHz is required because of the high frequencies produced by the musical instruments. Hence pair 2 is correct. The audible range of frequencies extends from 20 Hz to 20 kHz. Video signals for transmission of pictures require about 4.2 MHz of bandwidth. Hence pair 3 is correct. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission. Hence pair 4 is correct. The contribution from higher harmonics can be neglected, thus limiting the bandwidth . As a result, received waves are a distorted version of the transmitted ones . 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 , the less is its contribution to the waveform.
