Digital Signal Processing Reference
In-Depth Information
The real problem with the single sideband modulation SSB are the high precision filters
(bandpasses) which are required to be able to filter out one of the immediately adjacent
side bands. In analog technology this was (and is) hardly possible using normal filter
techniques (R-L-C-filters). Among others filters with mechanical resonance circuits or
quartz-crystal filters were and are used.
In the high frequency range, surface wave filters offer excellent solutions. All these filter
types exploit acoustic-mechanical physical effects and the fact that mechanical oscilla-
tions via the piezo electric effect can be easily transformed into electrical oscillations and
vice versa.
Frequency multiplex
Many radio and TV transmitters reach our receiver at the same time via the aerial or cable.
Up to now radio and TV have worked in frequency multiplex, i.e. all the stations are stag-
gered in frequency or lie close together in the frequency band. For example, in the
medium-wave radio range all the transmitters are double sideband modulated (AM with
a carrier), frequency-modulated (FM) in the VHF range.
In frequency multiplex systems all the channels are transmitted
simultaneously and staggered in frequency.
The task of the tuner is to filter out the desired station and then to demodulate the filtered
signal. As the aerial signal is very weak it has to be additionally amplified.
In the case of the telephony of Deutsche Telekom the telephone channels are staggered in
frequency in a particularly economical way by means of the frequency multiplex process.
Here single sideband modulation is used the bandwidth of which is exactly as large as that
of the source signal. 10,800 telephone calls can be transmitted simultaneously (V-10800
system) via a single coaxial cable - as used as a connection between antenna and tuner.
This carrier frequency technique works on the following principle:
• 12 telephone channels with 300 - 3400 Hz bandwidth are assembled to form a base
group, and five primary base groups to form a secondary base group with 60
channels. This is illustrated clearly in Illustration 165.
• According to this principle several secondary groups are combined to form a tertiary
group and several tertiary groups to form a quaternary group, until altogether 10,800
channels are combined to form a bundle. This is also shown in Illustration 165.
• It used to be the case that for technical reasons relating to filters first pre-groups with
three channels were formed, then four of these were combined to form a primary basic
group.
