The transmission of the voice signal by a carrier is represented as a voltage signal that varies continuously with time. In amplitude modulation and frequency modulation. The carrier signal is varied continuously in analog manner; this continuous transmission of information in an analog manner is used in the Frequency Division Multiplex system, while in the Time Division Multiplex system it is not necessary. The Time Division Multiplex system use pulse modulation. In the Pulse modulation, the continuous signal is converted into a series of pulses, each proportional to the amplitude of the signal and corresponding in time to it. Thus in pulse modulation, a series of pulses carries the information instead of continuous modulated signal.
The modulating signal can be used to control one characteristic of the pulses. Thus pulse modulation has the following common methods.
- Pulse Amplitude Modulation (PAM)
- Pulse Width Modulation (PWM)
- Pulse Frequency Modulation (PFM)
- Pulse Code Modulation (PCM)
These four methods of pulse modulation are shown in Below Figures.