Digital Signal Processing Reference
In-Depth Information
Electric current in a solid state implies movement of electrons in the metallic crystal grid.
The movement of an individual electron from an atom to the neighbouring atom takes
place quite randomly.
Even though the movement of electrons mainly points in the direction of the physical
current this process has a stochastic - purely random, unpredictable - component. It
makes itself heard through noise. There is therefore no pure direct current DC; it is always
accompanied by noise. Every electronic component produces noise, that is any resistance
or wire. Noise increases with temperature.
Noise and information
Stochastic noise means something like absolute chaos. It contains no "pre-arranged,
meaningful pattern" - i.e. no information.
Stochastic noise seems to have no "conserving tendency", i.e. nothing in a given time seg-
ment B reminds one of the previous time segment A. In the case of a signal, the next value
is predictable at least with a certain degree of probability. If for example you think of a
text like this, where the next letter will be an "e" with a certain degree of probability.
Stochastic noise is therefore not a "signal" in the strict sense,
because it seems to contain no information bearing pattern - i.e.
no information.
Everything about stochastic noise within a given time segment is random and unpredic-
table, i.e. its development in time and its spectrum. Stochastic noise is the "most non-
periodic" of all signals!
All signals are for the reasons described always (sometimes more or less or too much)
accompanied by noise. But signals which are accompanied by a lot of noise differ from
pure stochastic noise in that they display a certain conserving tendency. This is charac-
terised by the pattern which contains the information.
Noise is the biggest enemy of communications technology
because it literally "buries" the information of a signal.
One of the most important problems of communications technology is therefore to free
signals as far as possible from the accompanying noise or to protect or modulate and code
the signals from the outset in such a way that the information can be retrieved without
errors in spite of noise in the receiver.
