Modulation is necessary for: 

i) transmitting a low frequency signal to a distant place. 

ii) avoiding mixing up of signals from different transmitters. 

iii) so as to keep the height of antenna small.

iv) Modulation helps us to filter out the unwanted noises present in the signal and process out a more clearer form of information.

The three basic units of communication system are:

(i) Transmitter: It convert the message signal produced by the source of information into a form suitable for transmission.

(ii) Communication channel: The signal is passed through the channel for fulther processing of the information. 

(iii) Receiver: It operates on the received signal in such a way that it is recognisable by the user of information. 

The range of transmission of signals by a TV tower can be increased by the following ways:

(i) By increasing the height of the transmission tower. 

(ii) By increasing the height of the receiving antenna, so that it may directly intercept the signal from the transmitting antenna.