(A) Amplitude Modulation: A continuous wave goes on continuously without any intervals and it is the base band message signal, which contains the information. This wave has to be modulated.
According to the standard principle, “The amplitude of the carrier signal varies in accordance with instantaneous amplitude of the modulating signal.” Which means, the amplitude of the carrier signal containing no information varies as per the amplitude of the signal containing information, at each instant. This can be well explained by the following figures.
The first figure shows the modulating wave, which is the message signal, the next one (b) is the carrier wave which is a high frequency signal and contains no information while the last one (c) is the resultant modulated wave.
It can be observed that the positive and negative peaks of the carrier wave, are interconnected with an imaginary line. This line helps recreating the exact shape of the modulating signal. This imaginary line on the carrier wave is called as envelope. It is the same as that of the message signal.
(B) Frequency Modulation: In telecommunications and signal processing, frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. This contrasts with amplitude modulation, in which the amplitude of the carrier wave varies, while the frequency remains constant.
In analog frequency modulation, such as FM radio broadcasting of an audio signal representing voice or music, the instantaneous frequency deviation, the difference between the frequency of the carrier and its center frequency, is proportional to the modulating signal.
Advantages of FM:
The big advantage of frequency modulation is its noise reduction ability, because most of noise is appeared as additional amplitude and in FM the amplitude of signal is hold-fixed. In FM, recovered voice depends on frequency not on amplitude. Hence the effects of noise are minimized in FM.
FM bandwidth covers all the frequency range which human can hear. Hence FM radio has better quality of sound in comparison with AM radio.
Disadvantage of FM:
At higher frequency, FM modulated signal pass through the ionosphere and do not get reflected. Hence FM has lesser coverage compare to AM signal.
(C) Phase Modulation: Phase modulation (PM) is a modulation pattern that encodes information as variations in the instantaneous phase of a carrier wave. The phase of a carrier signal is modulated to follow the changing voltage level (amplitude) of modulation signal. The peak amplitude and frequency of the carrier signal remain constant, but as the amplitude of the information signal changes, the phase of the carrier changes correspondingly. The analysis and the final result (modulated signal) are similar to those of frequency modulation.
Phase modulation is widely used for transmitting radiowaves and is an integral part of many digital transmission coding schemes that under lie a wide range of technologies like Wi-Fi, GSM and satellite television.
