Digital Signal Processing Reference
In-Depth Information
Chapter 8
Classical modulation procedures
All the processes which prepare the source signal for the transmission path are subsumed
under the term
modulation
.
Signals are modulated in order to
•
exploit in an optimum way the physical properties of the medium (e.g.
choice of frequency range),
•
guarantee largely undistorted transmission,
•
optimise the reliability of transmission,
•
respect data protection in telecommunications
•
exploit transmission channels several times over (frequency and time
multiplex) and
•
free signals from redundant information.
Note: Alongside the term
modulation
the term
coding
is often used particularly in
modern digital transmission processes. The two terms cannot be precisely distin-
guished. In this connection see Chapters 11-13. There is also a distinction between
source coding and channel coding, which for reasons of information theory should
always be carried out separately. Source coding serves the concentration of infor-
mation or data compression, i.e. the signal is freed from unnecessary redundant
information. The task of the channel coder is to guarantee a reasonably reliable
transfer of signals in spite of the signal-distorting disturbances occurring along the
transmission path. This is achieved by means of coding processes which recognise
and correct errors. Check elements are added to the signal as a result of which
compression by source coding is again diminished to some extent.
Transmission media
A distinction is made between
•
“wireless” transmission, (e.g. by satellite) and
•
wire communications (e.g. via twin wires and coaxial cables).
An important new medium is the optical wave guide, glass fibre
Modulation with sinusoidal carriers
The classical modulation processes of analog technology use the continuous modification
of a sinusoidal carrier. These processes are still standard in radio and television techno-
logy. Modern digital modulation processes are gaining ground here and will increasingly
replace classical modulation processes in future.
