GB553220A - Carrier current communication systems - Google Patents

Abstract

553,220. Carrier multiplex systems. STANDARD TELEPHONES & CABLES, Ltd. (International Standard Electric Corporation). March 27, 1942, No. 4066. [Class 40 (iv)] In a system involving a long multiplex line, some of the channels are branched from the line at normal repeater points or at repeaters specially provided for the purpose and taken to subscribers' lines or telephone exchanges. The multiplex system may be of the type in which the low frequency channels have individual first carrier frequencies and groups of intermediate frequency channels modulate a common second carrier but, in the arrangement shown, the first carrier frequency is the same for all channels and the second carriers are individual. In all cases, the first carrier is transmitted with one side band to the second modulator and is used at the receiving end for generating the second demodulating frequency and is interrupted to convey the usual signals necessary for the establishment of a connection. Any of the stations may be supplied in known manner with operating current over the main or branch cables which may be coaxial. Circuit arrangements. Speech from branch line 34 passes toafirstmodulator38. One side-band is selected by a filter 40 to a point of which a connection is taken from the first-stage crystal-controlled oscillator 39. The output of the filter is amplified and fed to a crystalcontrolled second modulator 43 the output being taken to the appropriate path of the repeater in the main circuit. Signals such as dialling signals act on apparatus 60 to cause the output of filter 40 to be short-circuited. Currents for the channel shown arriving at the main circuit repeater are selected roughly at 48, amplified at 49. and filtered at 50 and undergo their first demodulation in a crystal-controlled mixer 51. The output is separated by filter 55 and crystal 52 into side-band and carrier and the latter controls the oscillator 53 of the second modulator 54. The rectified speech-band is taken from this to the low frequency line and the unmodulated carrier is rectified at 59 and taken to the control circuit 60. The crystals for the tubes 43, 51 are kept at a constant temperature in a chamber 71. Resistances 45, 46, 76 and 47, 48, 81 are chosen to make the through circuit and the individual tapped off circuits independent of variation in the number of channels tapped off.