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Saturday, 17 December 2011

Pulse Modulation & its types


Pulse Modulation
-      consists essentially of sampling analog information signals and then converting those samples into discrete pulses and transporting the pulses from a source to a destination over a physical transmission medium

The four predominant methods of pulse modulation:
-      pulse width modulation
-      pulse position modulation
-      pulse amplitude modulation
-      pulse code modulation

Forms of Pulse modulation


Pulse Width Modulation (PWM)
-      is sometimes called pulse duration modulation (PDM) or pulse length modulation (PLM)
-      the width of a constant amplitude pulse is varied proportional to the amplitude of the analog signal at the time the signal is sampled
-      used in special-purpose communications system mainly for the military but are seldom used for commercial digital transmission
                             

Pulse Position Modulation (PPM)
-      the position of a constant-width pulse within a prescribed time slot is varied according to the amplitude of the sample of the analog signal
-      the higher the amplitude of the sample, the farther to the right the pulse is positioned within the prescribed time slot
-      the highest amplitude sample produces a pulse to the far right, and the lowest amplitude sample produces a pulse to the far left
-      also used in special-purpose communications system mainly for the military but are seldom used for commercial digital transmission



Pulse Amplitude Modulation (PAM)
-      the amplitude of a constant width, constant-position pulse is varied according to the amplitude of the sample of the analog signal
-      PAM waveforms resemble the original analog signal more than the waveforms for PWM or PPM
-      this is used as an intermediate form of modulation with PSK, QAM, and PCM, although it is seldom used by itself



Pulse Code Modulation (PCM)
-      the analog signal is sampled and then converted to a serial n-bit binary code for transmission
-      each code has the same number of bits and requires the same length of time for transmission


                                 
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