1. Modulation is the process by which some characteristics, like amplitude, frequency or phase angle of a high frequency carrier wave is varied in accordance with the instantaneous value of the audio signal.
2. Modulation is necessary due to the following reasons.
The height of the transmitting antenna should be at least 1/4th of the wave length. So to transmit audio frequency, we need very long antenna.
The power radiated by an antenna is inversely proportional to the square of the wavelength. In audio frequency, wavelength is higher.
3. It is the ratio of the change in amplitude of the modulated carrier wave to the amplitude of the original carrier wave.
