The following factors responsible for the need of modulation are:
(a) PRACTICAL ANTENNA LENGTH: We know that lingth (l) of transmitting and receiving antenna is given by
l =(1/4) wave – length = [ 1/4 * velocity/frequency(in hertz) ] meters
Now, for audio frequency range (20hz to 20khz) antenna length can be calculated as
l = 1/4 * (3*108)/(20*103) = 3750 meters
Hence to transmit an audio frequency of 20khz an antenna of 3750 meters is required which is practically impossible. Therefore, if a carrier wave of say 1Mhz is used to carry the signal, we need an antenna of about 75 meters that can be easily constructed.
(b) OPERATING RANGE: The energy of any wave depends on its frequency. so if the frequency is more, the energy possessed by it is also more. the audio signal with high frequency can be transmitted over large distances. therefore, a high frequency wave as the carrier wave is needed.
(c) To remove interference and reduction of noise: when a signal is sent using high frequency carrier, there is less distortion of the signal and less noise in the received signal. also using several modulation techniques, the effect of noise can be minimized.
(d) NO MIXING OF AUDIO SIGNALS: In modulation technique each audio signal is carried by a separate high frequency carrier wave and therefore no mixing of audio signals occurs.
