GB664483A - Toll switching system for telephone and the like connections - Google Patents

Abstract

664,483. Exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Oct. 7, 1949 [Oct. 8, 1948], No. 25799/49. Class 40 (iv). In a system for connecting an incoming 2- wire line to one of a group of outgoing lines which. may be 4-wire, repeatered 2-wire or non-repeatered 2-wire, the incoming line 1, Fig. 1, is connected to an asymmetric hybrid 2, connections being made to the first two types of lines on a 4-wire basis, but to the last type on a 2-wire basis over the low loss arm 5 of the hybrid. By this means the loss in the hybrid which would be necessary for connecting up a non-repeatered line on a 4-wire basis is avoided. The high loss arm of the hybrid contains the usual low-pass filter 3 and an amplifier 4 for compensating for the line and hybrid loss. As shown, the first choice 4-wire lines 10 are connected to the earlier outlets of the switch 6, the 2-wire repeatered circuits (which may likewise contain asymmetrical hybrids 12) are connected to later outlets, and the overflow or last choice non-repeatered outlets 17 are connected to the last outlets of the lower bank only. The connections may be established manually instead of over an automatic switch as shown. The asymmetric hybrid, Fig. 2, comprises two transformers 14, 15 connected as shown, the turns ratio of the windings in the line and balance circuits respectively being 1 : m and m : 1 by which means the incoming power is divided between the upper and lower arms in the ratio 1 : m<SP>2</SP>. Likewise, energy received over coil 5 is split between the line 1 and balance 9 in the ratio m<SP>2</SP> : 1. A phantom tap 16 is supplied in the line circuit. The signalling receiver 13 may advantageously be located in the low loss arm 5, thereby permitting the use of signalling levels, especially in the case of low frequency signalling such as 17 or 50 c.p.s., which will not overload the incoming amplifiers. The amount of signalling current received over the upper arm may be further reduced by giving transformer 14 a low inductance thereby making it inefficient at low frequencies. Care is then taken to make equal the leakage inductance of the windings of the two transformers connected to the balance network, in order to maintain an accurate balance of the transformer.