Here ec is instantaneous
value, Ec is amplitude (maximum value), wc
is angular frequency and q is the phase angle of
carrier voltage.
Corresponding to the three variables Ec, wc and q, we can have three different types of modulation.
Amplitude of Modulation
In this type if modulation, wc and q are kept constant. The amplitude Ec of the carrier wave is varied in accordance with the modulating wave.
Frequency Modulation
In this type of modulation, Ec and q are kept constant. The frequency wc of the carrier wave is varied in accordance with the modulating wave.
Phase Modulation
In this type of modulation, Ec, wc are kept constant. The phase angle q of the carrier wave is varied according to the modulating wave. The common name for "phase modulation" and "frequency modulation" is "angle modulation".
The digital transmission of analog signals involves pulse modulation. In analog pulse modulation, time is represented in discrete form. In digital pulse modulation, both time and signal amplitude are represented in discrete form.
