 - 1) In electronics, a signal is an electric
current or electromagnetic field
used to convey data from one place to another. The
simplest form of signal is a direct current (DC) that is switched on and off; this is the
principle by which the early telegraph worked. More complex signals consist of an
alternating-current (AC) or electromagnetic carrier that contains one or more data
streams.
Data is superimposed on a carrier current or
wave by means of a process
called modulation.
Signal modulation
can be done in either of two main ways: analog
and digital.
In recent years, digital
modulation has been getting more common, while analog modulation
methods have been used
less and less. There are still plenty of analog signals around,
however, and they will
probably never become totally extinct.
Except for DC signals such as telegraph and
baseband, all signal carriers
have a definable frequency or
frequencies.
Signals also have a property called wavelength,
which is inversely proportional to the frequency.
2) In some information technology contexts, a signal is simply "that which is sent or received," thus including both the carrier (see 1) and the data together.
3) In telephony, a signal is special data that is used to set up or control communication. See signaling.
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31 Jul 2001
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