Digital Signal Processing Reference
In-Depth Information
Conversion process: 3-channel-modulation
0,3-3,4 kHz
Double sideband AM
with surpressed carrier
Bandpass filtering surpresses
lower sidebands (SSB)
0,3-3,4 kHz
Double sideband AM
with surpressed carrier
0,3-3,4 kHz
24 kHz
0 kHz
4kHz
8 kHz
20 kHz
12 kHz
16 kHz
Addition
of the three upper sidebands: 3 telephone channels from 12 bis 24 kHz)
Invers conversion process: demodulation
Carrier 12 kHz
Channel filter
Carrier 16 kHz
Channel filter
Carrier 20 kHz
Channel filter
All sum frequency signals
are surpressed bei LP-filter
Carrier 12 kHz
24 - 28 kHz
0,3-3,4 kHz
Difference frequency signals
Carrier 16 kHz
0,3-3,4 kHz
32 - 36 kHz
Carrier 20 kHz
0,3-3,4 kHz
40 - 44 kHz
Illustration 164:
Frequency multiplex illustrated by pre-group formation in telephone channels
Over 10,000 telephone channels can be transmitted simultaneously by means of a coaxial cable by
„traditional“ analog communication technology. Small groups and bigger groups consisting of several
smaller groups etc are formed by frequency staggering.
The smallest group is the so-called pre-group. Three telephone channels are grouped to form a pre-group
which always occupies the range 12 to 24 kHz. This example illustrates the frequency conversion,
processing and staggering of the channels.
The mirror-image negative frequency domain which together with the positive frequency domain forms a
symmetrical spectrum is not shown here.
Both in the case of AM and single sideband modulation multiplication is the central
signal processing. By means of this frequency bands can be moved around in the spectrum
at will.
It should be noted that in the case of multiplication we have a sum and difference frequen-
cy, i.e. generally (at least) two frequency bands always arise which are accompanied by a
“doubling of information”.
The interpretation of the frequency bands which arise is only
correct if a
symmetrical spectrum with a positive and negative
frequency domain
is assumed so that in the difference
(formation) the frequency bands can change these domains.
