GB646382A - Carrier current message transmission systems - Google Patents

Abstract

646,382. Multiplex systems. STANDARD TELEPHONES & CABLES, Ltd. June 20, 1947, No. 16345. Convention date, June 25, 1946. [Class 40 (iv)] In a carrier transmission system in which two separate lines each include a number of telephone channels, frequency bands of identical width in the two lines and containing some of the channels are interchanged without segregating any of the individual channels, each band being first selected by coarse filters and then injected into the cleared space in the other line after frequency changing to the optimum frequency range of steep cut-off filters and subsequent demodulation. The coarse filters have attenuation-frequency characteristics as shown in Figs. 4a and 4b, each rejection or transmission band f3-f4 being flanked by intermediate zones of frequencies f2-f3, f4- f5 in which the attenuation varies with frequency. The transmission lines L1 and L2 each transmit a number of telephone channels simultaneously. Line L1 is connected through amplifiers 7 and 8 to a band-stop filter 9 and a band-pass filter 10 which select the frequency band to be transposed. The intermediate zones of filter 10 are separated by band-pass filters 11, 13 and applied to steep cut-off filters 17, 19 after frequency changing to a common frequency band by modulators 14, 16. The frequencies in each intermediate zone are subsequently re-converted to their initial values by demodulators 20, 22 and applied to the outgoing line through amplifiers 26, 28. A phase-retarding network 29 and attenuation correcting networks 23, 25 ensure that the intermediate zones of frequencies are superimposed on similar zones from filter 9 in the correct phase and amplitude relationship. A similar modulation and demodulation network interconnected by lines X, Y and an identical equipment (not shown) associated with the line L2 selects and interchanges the transmission frequency bands. In a modification, Fig. 3, all the signal channels are applied through amplifiers 31, 33 and a phase retarding network 32 in opposition to signal channels from an amplifier 34 which have the selected frequency band excluded by a band stop filter 30. The resulting selected frequency band and its intermediate frequency zones are then separated by filters 11, 12, 13 as previously described. The through channels to the retarding network 29 are applied through an amplifier 38. The incoming line may be connected through a band-bass filter 37 to a terminating impedance RO or, alternatively, it may be correctly terminated by omitting the amplifier 33 and filter 37 and inserting an amplifier in front of filter 30.