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| Pulse Modulation |
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| In amplitude modulation and frequency modulation, the modulation was done by sinusoidal signals. In pulse modulation, short pulses may do the modulation of a radio-frequency carrier. The simplest of these is the conventional telegraph, whereby information is transmitted by a code of dots and dashes, or by turning the carrier on and off. |
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| Sampling refers to the process of taking instantaneous values of the message signal at regular intervals (in the case of uniform sampling.) This makes the time-frame (of the analog signal) discrete. |
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| Most pulse systems are based on sampling of the information signal amplitude at periodic intervals, usually about twice the maximum frequency present, or about 8000 times per second for voice transmission. They transmit a very short pulse of radio-frequency carrier for each sample. The pulse characteristics are varied in some manner proportional to the signal amplitude at the sampling instant. |
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| Multiplex operation may also be employed. In this operation, pulses due to other messages are inserted and transmitted between the pulses of the first message. This requires a synchronized message distributor at the receiver to sort the individual pulses into the proper channels. Synchronism is usually obtained by transmitting a synchronizing pulse at regular intervals. |
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The modulating signal is sampled at the
basic rate, usually  where fm is the maximum value. The amplitude of the modulation determines the amplitude of the transmitted pulse. The band-width will be great for exact pulse waveform, but may be reduced by a low-pass filter, thereby somewhat rounding the pulses. As long as the amplitude at the pulse intervals is preserved, no distortion will be introduced. |
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| The timing or position of the pulse is varied around a fixed mean value and conforms to the signal amplitude at the time of sampling. A positive signal may set the pulse ahead, a negative signal will set the pulse behind the reference time. In reception, the pulse shape received is not important. The time of arrival is the only information necessary for interpretation of the signal. |
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| It is also referred to as pulse-width modulation. It varies the pulse length or duration around a fixed value. This may be done by shifting either the leading or trailing edge, or both, as a function of the sampled amplitude. Greater channel band width is required than for PAM. Design must be co-ordinated to prevent overlap of the variable length pulses in multiplex use. The reception of such a wave is simple. The information can be recovered from the demodulated pulses by passing them through a low-pass filter. |
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| Forms of pulse modulation |
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| It translates the sampled amplitude data into a code. The code is transmitted as a succession of pulses and spaces. The system enjoys greater freedom from noise and interfering signals than is possible with most other pulse systems. This freedom arises from the fact that it is merely necessary to determine that a pulse was or was not transmitted, and it is not necessary to measure any characteristic of the received waveform. If the signal has sufficient amplitude, it is possible to operate circuits at the receiver, which will regenerate the original pulse, free from noise. This permits ready use of repeater stations for long distance transmission, without cumulative effects on the waveform or information. |
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