Introduction to Modulation
Naturally occuring or artificial signals occur at frequencies which are
controlled mostly by the physical properties of the source of the signal.
The physical properties of the media used to transmit signals from one place
to another is such that it only transmits signals in frequency bands that are
usually different from the frequency band of the signal to be transmitted.
In order to transmit a signal across such a medium it is necessary
to shift the spectrum of the signal to a frequency band which is not
absorbed by the medium or to an "allocated" portion of that frequency band.
This process is called modulation.
Many types of modulation are possible and the choice of which one to use
depends on efficiency, power requirements, bandwidth and S/N ratio.
Generally S/N ratio can be
improved by increasing bandwidth. Other practical problems like
coupling of the signal to the medium may also play a part.
In the case of digital communications the modulating+coding system
is chosen to transmit the maximum amount of information within the given
bandwidth subject to meeting the specified error rates.
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COPYRIGHT © 1996 Cuthbert Nyack.