Pulse Modulation

Topics: Modulation, Digital signal processing, Pulse-width modulation Pages: 10 (2400 words) Published: March 3, 2011

Transmittal of digital signals between two or more points in a communications system.

Advantages of Digital Transmission
Noise immunity
Ease of processing and combining
Ease of measuring and evaluating

Disadvantages of Digital Transmission
Requires more bandwidth
Necessitates encoding and decoding circuitry
Requires precise time synchronization
Incompatible with older analog transmission systems

Pulse Modulation
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. Pulse Modulation is a method of modulation that represents information as a train of pulses, wherein one or more characteristics of each pulse change with the input signal's variation.

Methods of Pulse Modulation
Pulse Width Modulation (PWM)
Pulse Amplitude Modulation (PAM)
Pulse Position Modulation (PPM)
Pulse Code Modulation (PCM)

Pulse Width Modulation (PWM)
Sometimes called as Pulse Duration Modulation (PDM) or Pulse Length Modulation (PLM) In this method, the width of the constant amplitude pulse varies proportionally with the amplitude of the analog signal at time the signal is sampled.

Pulse Position Modulation (PPM)
In this method, 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.

Pulse Ampitude Modulation (PAM)
In this method, the amplitude of a constant width, constant-position pulse is varied according to the amplitude of the sample of the analog signal.

Pulse Code Modulation (PCM)
In this method, 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.

Graph of each Pulse Modulation Method

Uses of Pulse Modulation
PAM is used to develop other pulse-modulated waverforms and as an intermediate form of modulation in PSK, QAM, and PCM but seldom used to itself. •PWM and PPM are used in special-purpose communication systems mainly for military but seldom used for commercial digital communication systems. •PCM is by far the most prevalent form of pulse modulation.

Pulse Code Modulation (PCM)

Alex H. Reeves

Alex Reeves was born in Redhill, Surrey in 1902 and was educated at the Reigate Grammar School, followed by a scholarship to the City and Guilds Engineering College in 1918, and then postgraduate studies at Imperial College London in 1921.

Brief Time-line
1937 – Alex H. Reeves was credited forinventing PCM while working at AT&Tat its Paris Laboratories. 1960’s – PCM became prevalent with the advent of solid-state electronics Today – PCM is the preferred method of communication within the public switched telephone network.

Pulse Code Modulation
PCM is the only digitally encoded modulation technique that is commonly used for digital transmission. The term pulse code modulation is somewhat a misnomer, as it not really a type of modulation but rather a form of digitally coding analog signal. With PCM, the pulse are fixed in amplitude and length. It is a binary system where a pulse or lack of a pulse within a prescribed time slot represents either a logic 1 or logic 0 condition. PWM, PPM and PAM are digital but seldom binary, as a pulse does not represents a single binary digit.

PCM Receiver
Band-pass filter - limits the frequency of the analog input signal to the standard voice-band frequency range of 300 Hz to 3000 Hz. •Sample and Hold circuit – periodically samples the analog input signal and converts those samples to a multilevel PAM signal. •Analog-to-Digital converter – converts the PAM samples to parallel PCM codes. •Parallel-to-Serial converter – parallel PCM codes to serial binary data. •Transmission line repeaters –...
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