Pulse Amplitude Modulation (PAM) is an analog modulating scheme in which the amplitude of the pulse carrier varies proportional to the instantaneous amplitude of the message signal.
The pulse amplitude modulated signal, will follow the amplitude of the original signal, as the signal traces out the path of the whole wave. In natural PAM, a signal sampled at the Nyquist rate is reconstructed, by passing it through an efficient Low Pass Frequency with exact cutoff frequency.
The following figures explain the Pulse Amplitude Modulation.   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. Pulse amplitude modulation is defined as the data transmission by altering the amplitudes (power levels or voltage) of every pulse in a regular time sequence of electromagnetic pulses. The possible number of amplitudes can be infinite, but mostly it is some power of two so that the final output signal can be digital. For example, in level-4 PAM there are 22 discrete pulse amplitudes; in level-8 PAM there are 23 discrete pulse amplitudes.
Pulse Amplitude Modulation Circuit:- There are two major types of PAMs:
1. Single polarity PAM: Here, an appropriate fixed DC bias is integrated with the signal to assure that all pulses are positive.
2. Double polarity PAM: Here, the pulses are both negative and positive.
In some PAMs:
A. The amplitude of every pulse is directly proportional to instantaneous modulating amplitude when the pulse occurs.
B. While in other PAM, the amplitude of every pulse is inversely proportional to instantaneous modulating amplitude when at the occurrence of a pulse.
C. In some other systems, the intensity of every pulse is based on particular characteristics of modulating signal other than strength like instantaneous phase or frequency.
Applications:
1. PAM is mostly applied in non-based modulating transmission of digital data and applications replaced by pulse-code modulation and 2. pulse-position modulation.
3. Particularly all phone modems faster than 300 bit/s use quadrature amplitude modulation.
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