GB371885A - Improvements in or relating to carrier current signalling systems - Google Patents

Abstract

371,885. Wired wireless systems. STANDARD TELEPHONES & CABLES, Ltd., Columbia House, Aldwych, London.-(Assignees of Green, E. I.; 150, Munn Avenue, East Orange, New Jersey, U.S.A.) Jan. 29, 1931, No. 2891. Convention date, Jan. 30, 1930. [Class 40 (v).] In a carrier-current signalling system wherein a large number of signalling channels are made available preferably by using a transmission line comprising two concentric cables, the carrier frequencies are arranged in groups of channels, and are separated by such small intervals that, in reception, a high-frequency filter having a cut-off characteristic such as A, Fig. 3, is used to select only roughly a given group 1 .. 5. Certain partly suppressed channels, such as 3<1>, 4<1>, 5<1> and 1<11>, 2<11>, 3<11> from adjacent groups will also be passed owing to the sloping sides of the filter characteristic. To remove these, the selected channels are " stepped down on the frequency spectrum by beating with a local heterodyne having a frequency corresponding to the points fa, fd, or the half-way point fb. If the beating frequency is located at fb, the resulting frequency of the channel 1 will be within (fd-fb) of zero frequency, whereas if the heterodyne frequency is located at fa, the resulting frequency of the channel 1 will be (fd-fa) above zero. In each case, low-frequency filters of relatively sharp " cut-off " can then be used to separate the individual speech channels and to eliminate the marginal or unwanted components originally passed by the high-frequency filter A. When the heterodyne frequency is at fb, the arrangement permits of such close spacing between each group that some 245 separate channels can be made in a transmission line having a total frequency range of 1000 kilocycles. The terminal arrangements are shown in Fig. 4. Voice currents from the line TL pass into a channel modulator CM supplied with a carrier frequency from a source CO. One or both side-bands are passed through a band filter BF, and combined with similar channel bands BF1, BF2, &c. of the group. The combined output is then applied to a group modulator GM, supplied with a carrier frequency from a source GO, which steps up the group of channels in the frequency spectrum. The new frequencies are then passed through a group filter GF, and together with the groups from the filters GF1 &c. are applied to the transmission line CXL. The terminal arrangements at the receiving end of the line are shown at the righthand side of the Figure. One of the incoming groups is selected by group filter GF1 and applied to group demodulator GD1 supplied with a beating frequency from source GO1. After demodulation a particular channel is selected by band filter BF1 and the final signal band by a channel demodulator CD1, which may be supplied with a homodyne frequency from a source CO1, before passing through a low-pass filter LPF1.