Modulation is a technique of superimposing a low frequency information signal over a high frequency carrier signal for its transmission. This is done by modulating one of the parameters of the high frequency carrier wave in accordance with the instantaneous amplitude of the information signal, also known as the modulating signal. The carrier wave can be modulated in terms of amplitude, frequency, phase etc. For example: - In case of amplitude modulation, the amplitude of the carrier wave is varied in accordance with the amplitude of the modulating wave. Its maximum at the positive peak of the modulating wave, equal to the original carrier amplitude when the modulating wave amplitude is zero and is minimum at the negative peak of the modulating wave. The frequency of the carrier in this modulation scheme remains constant throughout.