GB575213A - Improvements in or relating to multi-channel signalling systems - Google Patents

Abstract

575,213. Multiplex transmission. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., and MURRAY, L. J. March 7, 1944, No. 4171. [Class 40 (iv)] In a carrier multiplex system, the supervisory signals associated with the various speech channels are transmitted at frequencies lying within the gaps between the speech channels, and the speech and signalling channels are defined by the use of band-pass filters of the crystal resonator type. In a 12-channel system using "go" and " return " pairs, the speech channels are first assembled as the lower side-bands of twelve carrier frequencies 64, 68 ... 108 kc. and these then modulate a group carrier of 120 kc., the lower side bands which lie within the range 12-60 kc. being selected for transmission. For a particular channel, the speech frequencies received over wires 10, 11 pass through the modulator 14, crystal band-pass filter 15, group modulator 20 and group low-pass filter 21, which need not be of crystal type to the " go '' pair 22, 23. Frequencies of 3.75 and 3.9 kc. are selected for signalling, and such frequencies received over wires 12, 13 modulate the same carrier as the corresponding speech wave in a separate modulator 16, and a narrow band crystal filter 17 selects the lower side bands which enter the group modulator 20 with the speech side band.