Patent GB-542643-A (GB)
Title: Telephone set circuits
IPC: H04M1/58
Filed: 1940-05-17
Abstract: 542,643. Substation circuits. STANDARD TELEPHONES & CABLES, Ltd. May 17, 1940, No. 8841. Convention date, June 10, 1939. [Class 40 (iv)] [Also in Groups XXXVI and XL] The effects of room noise or side-tone or both are eliminated by associating with the transmitter or the receiver or both of a telephone substation or operator's set an element or elements having a non-linear variation of resistance with temperature. These elements may have negative temperature coefficients of resistance (e.g. silver sulphide or a bead of uranium oxide in a glass binder) or positive temperature coefficients of resistance (e.g. chromium oxide in a sodium silicate binder or carbon-coated quartz spheres in cellulose acetate). Heating may be direct, i.e. by means of current through the elements, or indirect. Circuits that eliminate room noise only. In these the resistance element E or V has a normally high impedance and is in series with the transmitter T as in Figs. 8 and 37 and in Figs. 7, 22, 25, 36, 38, 39 and 40 (not shown), or has a normally low impedance and is in shunt to the transmitter as in Figs. 19 and 28 (not shown). Circuits that eliminate side-tone only. In these the resistance element may be in series with the receiver and have a normally low impedance as in Figs. 1, 4 and 29 (not shown) and in Fig. 20, or may be in shunt to the receiver and have a normally high impedance as in Figs. 2, 3, 5, 6 and 23 (not shown) and in Fig. 26. In another arrangement, Fig. 41, in which the resistance V is in series with the receiver, it is controlled by an auxiliary transmitter TA1. Circuits that eliminate both room noise and side tone. These may involve two resistance elements with separate heaters or a common heater controlled by the normal transmitter, for example a normally low-resistance element in shunt to or a normally high resistance element in series with the transmitter and a normally low - resistance element in series with or a normally high-resistance element in shunt to the receiver. Examples of such arrangements are given in Figs. 9 ... 16, 18, 21, 24, 27 and 30 (not shown) and Fig. 17. In other arrangements, Figs. 31 ... 35 (not shown) a separate transmitter, which may be controlled by the same diaphragm as the normal transmitter and is designed'to have a large change in resistance when spoken into without regard to the quality of speech transmission, is used for controlling the resistances. The Specification as open to inspection under Sect. 91 comprises also a number of arrangements which now form the subject matter of a divided Specification 544,364 and further arrangements, Figs. 61, 62 and 64 (Cancelled) (not shown) and Fig. 63 (Cancelled), in which a rectifier network such as 35, 39, 40 is associated with the transmitter so as to act normally as a high series impedance and another rectifier network such as 36, 41, 42 is associated with the receiver to act as a high shunt impedance. Rectified transmitted speech brings both networks to the low-impedance condition. Adjustable resistances 56, 60 vary the amount of sidetone and room noise permitted. The transmitter may be coupled to the circuit by an impedance-matching auto-transformer. This subject-matter does not appear in the Specification as accepted.