The pulse modulation methods, the carrier is no longer a continuous signal but consists of pulse train, some parameter of which is varied according to the instantaneous value of the modulation signal.

There are two types of pulse modulation system as under

1] Pulse Amplitude modulation (PAM)

2] Pulse time modulation (PTM)

In pulse amplitude modulation (PAM) the amplitude of the pulses of the Carrier pulse train is varied in accordance with the modulating signal whereas in pulse time modulation (PTM), the timing of the pulses of the carrier pulse train is varied.

They are two types of PTM as under

1] Pulse width modulation (PWM)

2] Pulse position modulation (PPM)

In pulse width modulation, the width of the pulses of the carrier pulse train is varied in accordance with the modulating signal whereas in pulse position modulation (PPM) , the position of pulses of the carrier pulse train is varied.

Pulse modulation methods (PAM, PWM and PPM) are called analog pulse modulation methods because the modulating signal is analog in nature in PAM, PWM and PPM.

PAM: Pulse amplitude modulation

The amplitudes of regularly spaced rectangular pulses vary according to instantaneous value of the modulating or message signal.

In fact, the pulses in a PAM signal may be of flat top type or natural type or ideal type.

Drawbacks:

Since the amplitude of the PAM pulses varies in accordance with the modulating signal therefore the interference of noise is maximum in a PAM signal, Thus noise cannot be removed easily.

PWM : Pulse width modulation in PWM, the width of the modulated pulse varies in proportion with the amplitude of modulating signal.

The amplitude and the frequency of the PWM wave remain constant only the width changes.

Advantages of PWM:

Less effect of noise i.e. very good noise immunity.

Synchronization between the transmitter and receiver is not essential.

Disadvantages of PWM:

In order to avoid any wave form distortion, the bandwidth required for the PWM communication is large as compared to bandwidth of PAM.

PPM : Pulse position modulation.

In PPM, the amplitude and width of the pulses is kept constant but the position of each pulse is varied in accordance with the amplitude of the sampled values of the modulating signal.

The position of the pulses is changed with respect to the position of reference pulses.

Advantages:

Due to constant amplitude of PPM pulses, the information is not contained in the amplitude, hence the noise affef to PPM signals does not distort the information. thus, it has good noise immunity.

Disadvantages:

Large bandwidth is required to ensure transmission of undistered pulses.