GB489204A - Improvements in or relating to signalling arrangements in telephone and the like systems - Google Patents

Abstract

489,204. Transmission systems. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.) July 27, 1937, No. 20757. [Class 40 (iv)] [Also in Group XL] In a telephone transmission system, ringing currents are separated from the speech currents bv means of a shunt circuit which includes a resistance element of a substance having a large negative temperature coefficient, such as silver sulphide. The resistance of the element drops to a low value at a critical temperature to which it is heated by ringing current, which is of high voltage compared with speech. When the ringing frequency lies within the speech range, the shunt circuit is tuned to that frequency. Fig. 1 shows one end of a line L which provides one low frequency and one carrier frequency channel separated by filters 10, 34, the terminal apparatus for the carrier channel being of the type disclosed in Specification 427,605. The two channels terminate in jacks 11, 31 with which are associated drop indicators 51, 50, and the carrier channel jack 31 is shunted by a silver sulphide resistance 54 in series with the primary of a transformer 55. When the jack 11 is plugged into, the ringing current (e.g. 20 cycles) flows over the low frequency channel and operates an indicator similar to 51 at the far end. When the jack 31 is plugged into, however, the ringing current, which is of considerable voltage, passes through the shunt resistance 54 and transformer 55, and flows in simplex fashion over the low frequency channel to the far end, where it operates an indicator similar to 50 through a transformer similar to 55. Figs. 3, 4 show arrangements applicable to the system disclosed in Specification 318,569, [Class 40 (iv)], for receiving ringing currents of a frequency within the speech range by means of a shunt circuit tuned to that frequency and including a transformer 10. In Fig. 3, the silver sulphide resistance 12 and tuning condenser 11 are in series with the primary, while in Fig. 4 they are in series with the secondary. The resistance element is preferably such that its temperature due to ringing current does not reach the critical value until an appreciable interval has elapsed.