Patent GB-1164281-A (GB)
Title: Improvements in or relating to Telephone User's Local Apparatus.
Inventor: GILMOUR WILLIAM DUDLEY
IPC: H04M1/725
Filed: 1965-09-18
Abstract: 1,164,281. Inductive signalling; pulse modulation. ELECTRIC & MUSICAL INDUSTRIES Ltd. 16 Sept., 1966 [18 Sept., 1965], No. 39898/65. Headings H4L and H4U. In a telephone instrument of the kind having a handset and a cradle unit the usual cord is replaced by means not involving a physical connection between the handset and the cradle unit. As described, a fixed inductive loop is connected to the cradle unit and surrounds the area in which the handset is to be used, the handset being provided with a coil or coils with a ferrite rod core. Duplexing may be effected by modulating carriers of different frequencies for the two directions of transmission, or by modulating two sets of pulses which are interleaved in time. Power for the handset equipment is obtained from a suitable secondary battery, Fig. 4 (not shown), which is contained therein and which is disconnected, when the handset is replaced upon the cradle unit, by a transistor, although a permanent magnet operated reed relay may be provided. Charging is effected from the telephone line via two contacts equally disposed about a central contact on the cradle unit, a bridge rectifier in the handset allowing it to be put either way on the cradle without reversing the battery on the line. To ensure that the exchange line relay will drop out a CR circuit is arranged to discharge as the cradle switch opens, and so decrease the current from the exchange momentarily to zero. In one embodiment the telephone line is connected to a hybrid 1, Fig. 2, and incoming signals are applied to a monostable multivibrator 18, which is triggered by recurrent uniformly spaced pulses A, Fig. 3, from an oscillator 17, so as to modulate its ON time and produce pulses B, which are fed to a loop 4, the receive side of the cradle unit equipment being inhibited by a gate 19. Differentiated pulses C are received in the handset in a ferrite cored coil 5 and are applied to a trigger 20 which turns ON for a positive pulse and off for a negative pulse, so producing an output D, corresponding to the input B to the coil 4, from which audio signals are extracted by a low-pass filter 21. The leading edge of the waveform D is delayed in a delay circuit 22 so producing recurrent uniformly spaced pulses E interleaved with the pulses A. These trigger a monostable multivibrator 23 whose ON time is modulated by a handset microphone 10 to give an output F which is applied to the coil 5, the receive side of the handset equipment being inhibited by a gate 26. Differentiated pulses G are received by the loop 4 and are applied to a trigger 24 which produces a duration modulated pulse output corresponding to F. Audio signals are extracted by a low-pass filter 25 and are sent to line via the hybrid 1. In another embodiment, duration modulated pulses produced in the cradle unit are fed to a loop 4, Fig. 5, and their trailing edges switch on a bi-stable element 42 in the circuit for reception on the said loop. The pulses are received in differentiated form C1 (shown in Fig. 6) on a winding 34 on a common ferrite rod 5 in the handset and are applied to switch a bi-stable element 36 on and off, so producing an output D1 similar to the input B1 to the loop 4. The trailing edges of the pulses D1 trigger a monostable multivibrator 23, the ON time of which is modulated by a handset microphone 10, so producing an output E1. The trailing edges of the output E1 pulses are differentiated by a condenser 38 and are applied as signals F1 to a winding 40 on the rod 5. At the cradle unit the received signals F1 are used to reset the bistable element 42 the output G1 from which constitutes pulses which are duration modulated according to the input to the handset microphone 10. Short guard periods during which the receiving circuits are desensitized are provided by means of pulse stretching networks 43, 44. The Specification describes detailed transistorized circuit arrangements for the handset and the cradle unit. A tunnel diode may be used as a bi-stable element (Fig. 7, not shown) and a unijunction transistor may be used in a timing oscillator (Fig. 9, not shown). Integrated circuits may also be used (Figs. 8, 9, not shown).