Carrier System

   Most of the world's long-distance telephony systems utilize 12-channel groups : twelve voice
   channels are all changed in frequency in this way so that a complete group of 12 channel
   occupies a 48 kHz bandwidth, basically from 60-108 kHz.

   Figure 11   is a block schematic of the transmitting equipment required for channels 1 & 2 of a
   standard 12-channel group. The audio input signal to a channel is applied to a balanced
   modulator together with the carrier frequency appropriate to that channel. The input signal
   attenuator ensures that the carrier voltage is 14 dB higher than the modulating signal voltage,
   as required for correct operation of the modulator. The output of the modulator consists of
   the upper and lower sideband  products of the amplitude modulation process together with a
   number of unwanted components.
 

 
   Figure 11
Schematic diagram of
transmitting equipment
for a carrier system
 

   Following the modulator is another attenuator whose purpose is two-fold : firstly, it ensures
   that the following band-pass filter is fed from a constant-impedance source - a necessary
   condition for optimum filter performance - and secondly, it enables the channel output level
   to be adjusted to the same value as that of each of the other channels. The filter selects the
   lower sideband component of the modulator output, suppressing  all other components . To
   obtain the required selectivity, channel filters utilizing piezoelectric crystals are employed. The
   outputs of all the twelve channels are combined and fed to the output terminals of the group.
   The transmitted bandwidth is 60.6-107.7 kHz, or approximately 60-108 kHz.

   The equipment appropriate to channels 1 and 2 at the receiving end of the 12-channel group
   is shown in Figure 12  . The composite signal received from the line, occupying the band
   60-108 kHz, is applied to the twelve paralleled, channel filters. Each filter selects the band of
   frequencies appropriate to its channel, 104.6-107.7 kHz for channel 1, and passes it to the
   channel demodulator. The attenuator between the filter and the demodulator ensures that
   the filter works into a load of constant impedance. The demodulator is supplied with the same
   carrier frequency as that suppressed in the transmitting equipment. The lower sideband output
   of the demodulator is the required audio-frequency band of 300-3400 kHz and is selected
   by the low-pass filter.  The audio signal is then amplitude and its level adjusted by means
   of the output attenuator.
 

 
Fig 12
Receiving equipment
for a carrier system

   The assembly of the basic 12-channel carrier group can be illustrated by means of frequency
   spectrum diagram. The spectrum diagram of a single channel is given in Figure 13  ; the actual
   speech bandwidth provided is 300-3400 Hz but a 0-4000 Hz bandwidth must be
   allocated per channel to allow a 900 Hz spacing between each channel for filter selectivity
   to build up.

Fig. 13  Bandwidth for a
commercial speech circuit
   Figure 14a  shows the frequency spectrum diagram for the 12 channels forming
   a group; the carrier frequency of each channel is given and so are the maximum and the
   minimum frequencies transmitted. It can be seen that all the channels are inverted; that is,
   the lowest frequency in each channel corresponds to the highest frequency in its associated
   audio channel, and vice versa. Since all the channels are inverted, the group may be
   represented by a single triangle  as shown by Figure 14b   .
 
 
Fig.14
Frequency spectrum
diagrams of a
12-channel group
 

   The 12-channel system can be used as a building block for the next larger assembly stage
   or as a system which can be transmitted to line in its own right.

   Five 12-channel groups can be combined to form a 60-channel supergroup, and five
   supergroups make up a 300-channel mastergroup. Three mastergroups then make up a
   3872 kHz bandwidth supermaster group.  Alternatively, 15 supergroups may be assembled
   direct to form a hypergroup, sometimes called a 15 supergroup assembly.
 
 
 
 
 
 

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