Firstly let's talk about Pulse Modulation:-

                                                                       Pulse modulation is a type of modulation in which the signal is transmitted in the form of pulses. It can be used to transmit analogue information. In pulse modulation, continuous signals are sampled at regular intervals. Pulse modulation can be classified into two major types:

i) Analogue: Indication of sample amplitude is infinitely variable

ii) Digital: Indicates sample amplitude at the nearest predetermined level.

A block diagram showing the basic classification of modulation techniques is given below:





 Introduction of PAM:-

                          It is the simplest form of Pulse Modulation. In this type of modulation, each sample is made proportional to the amplitude of the signal at the instant of sampling. 

The PAM signal follows the amplitude of the original signal, as the signal traces out the path of the whole wave. Here a signal which is sampled at Nyquist rate can be reconstructed by passing it through an efficient Low Pass Filter (LPF) with exact cutoff frequency.

It is very easy to generate and demodulate PAM. This technique transmits the data by encoding in the amplitude of a series of signal pulses. PAM is illustrated in the figure below:

 


Though the PAM signal is passed through an LPF, it cannot recover the signal without distortion. Hence to avoid this noise, flat-top sampling is done as shown in the following figure:

Flat-top sampling is the process in which sampled signal can be represented in pulses for which the amplitude of the signal cannot be changed with respect to the analog signal, to be sampled. The tops of amplitude remain flat. This process simplifies the circuit design.