PULSE MODULATION SCHEMES

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Engineers mainly electronics engineers are familiar with the pulse waveform and modulation.Modulation greatly reduces the antenna size and enables the transmission to longer distances.Information can be transmitted, recorded, or otherwise processed in the form of pulses. The technique used to do this may be by pulse amplitude modulation, PULSE DURATION MODULATION, PULSE POSITION MODULATION, Or PULSECODE MODULATION. Although it is the most complicated of all four methods, pulse code modulation can be the most accurate and most efficient technique. Several separate pulse-modulated signals can be transmitted or recorded on one channel by time division multiplexing. In this process, pulseSignals are inserted in the spaces between other pulse signals.

TYPES OF PULSE MODULATION:

The instantaneous amplitude of a signal may be measured (or sampled) at regular intervals, and the measured amplitudes converted to pulses. The pulses may then be transmitted, recorded, or otherwise processed.

There are four types of pulse modulation schemes.

Pulse amplitude modulation

(PAM) is the simplest type of pulse modulation. As the name implies, the amplitude of each pulse is made proportional to the instantaneous amplitude of the modulating signal.The largest pulse represents the greatest positive signal amplitude sampled, while the smallest pulse represents the largest negative sample. The time duration of each pulse may be quite short, and the time interval between pulses may be relatively long. If a radio frequency pulse-amplitude-modulated instead of simply being amplitude-modulated because the much less power is required for the transmission of because the transmitter actually is switched off between pulses. This is one advantage of pulse modulation.

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  Pulse duration modulation

(PDM), also termed pulse width modulation, the pulses have a constant amplitude and variable time duration.The time duration (or width) of each pulse is proportional to the instantaneous amplitude of the modulating signal.In this case, the narrowest pulse represents the most negative sample of the original signal, and the widest pulse represents the largest positive sample. WhenPDM is applied to radio transmission, the carrier frequency has constant amplitude, and the transmitter on-line is carefully controlled. In some circumstances, PDM can be more accurate than PAM. One example of this is in magnetic tape recording, where pulse widths can be recorded and reproduced with less error than pulse amplitudes.

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Pulse position modulation

(PPM) is more efficient than PAM or PDM for radio transmission. In PPM, all pulses have the same constant amplitude and narrow pulse width. The position in time of the pulses is made to vary in proportion to the amplitude of the modulating signal the pulses near the right-hand side of the sampling time period represent the largest positive signal sample and those toward the left-hand side correspond to the most negative samples of the original signal. PPM uses less power than PDM and, essentially, has all the advantages of PDM. One disadvantage of PPM is that the demodulation process to recover the original signal is more difficult than with PDM.

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Pulse code modulation

(PCM), is a the most complicated type of pulse modulation. However, PCM can be the most accurate and the most efficient of the four methods. In certain circumstances, it may be the only type of pulse modulation that can be employed. In PCM, each amplitude sample of the original modulating signal is converted to a binary number. The binary number is then represented by a group of pulses, the presence of a pulse indicating / and the absence of a pulse indicating 0. The four-bit code can represent only sixteen discrete levels of signal amplitude. Thus it is far from accurate. Accuracy can be improved by increasing the number of bits {i.e… pulses) employed. A seven-bit code, for example, can represent 128 discrete levels of signal amplitude, or to an accuracy of better than 1%. The process of converting the signal to standard amplitudes which are to be represented by the binary code is termed quantizing, and the error introduced by this process is referred to as the quantizing error.

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For all four pulse modulation methods, the sampling frequency is determined by the highest signal frequency that must be processed. It can be shown that if samples are taken at a rate greater than twice the signal frequency, then the original signal can be recovered. However, in practice, it is normal to sample at a minimum rate of about ten times the highest signal frequency. For audio, voice transmission, for example, with a “high” frequency of 3 kH, the sampling frequency might be 30 kHz. Another major advantage of pulse modulation is when radio signals are very weak; they may be almost completely lost in a circuit or atmospheric noise. If the modulation method is PDM, PPM, or PCM, the signals can be recovered simply by clipping off the noise. For this, PCM gives the best results, since it is only necessary to determine whether each pulse is present or absent.In the following sections, modulation and demodulation methods are explained for PAM, PDM, and PPM. Before PCM techniques can be understood, time division multiplexing methods must be studied.