Patent GB-378064-A (GB)
Title: Improvements in or relating to combined telegraph and telephone exchange systems
IPC: H04M11/06
Filed: 1931-04-28
Abstract: 378,064. Code telegraphy STANDARD TELEPHONES & CABLES, Ltd., SEARLE, A. M., HUNT, G. L., and ANDREWS, R. J. M., Connaught House, Aldwych, London. April 28, 1931, No. 12491 and 19461. [Class 40 (iii).] Telephone systems in which certain subscribers are provided with teleprinter facilities have the following features : (1) The telegraphic signals for controlling the teleprinters are sent between substations in the form of a.c. impulses of voice frequency, this frequency being generated at the substation by a valve oscillator or by an extra winding on the teleprinter motor. (2) The oscillator and the amplifier-detector for receiving the voice-frequency signals are fed from local supply mains over a battery-eliminator circuit. (3) At an attended station, the subscriber's line is switched over from the telephone set to the teleprinter and the latter is started up by the operation of a press-button key. (4) At an unattended station, a slowacting relay controlled by prolonged ringing current, e.g. through a contact on the bell armature, connects the mains supply to the teleprinter equipment and sends back a buzzer signal to the calling party who may then complete the switch-over by transmitting an impulse of voice-frequency current. (5) A call to an unattended teleprinter station is metered when the teleprinter is connected up ready to receive signals. (6) A subscriber may have two lines to the exchange, one for telephony and one for teleprinting which may thus take place simultaneously, the lines being selectable over two sets of terminals at the exchange in the manner of lines to a P.B.X., and the sets of terminals being so associated with the lines that the first set normally gives access to the telephone line and the second set is marked busy, but when teleprinting is taking place the first set is switched over to the teleprinter line and the second set is available for conversation. The first Provisional Specification states that the source of voice-frequency current may be at the exchange. First system, Fig. 1. On the receipt of prolonged ringing current, thermal relay AA controlled by relay A operates, connecting the power supply P1, P2, to battery-eliminator BE and to buzzer Bu, and applying battery to the a-wire to cut off the ringing current. Teleprinter transmitting magnet T also operates closing contacts S1 for motor Mo. After an interval, thermal relay C operates in the anode circuit of valve DV in the amplifier detector, connecting up thermal relays E, F, and connecting buzzer Bu to the line a, b. The calling subscriber then throws his key K1 .. 5, energizing his relays E, F, D, to switch in his teleprinter, and transmitting a pulse of a.c. from his oscillator O over the line. At the unattended station, relay C releases in response to the pulse, connecting up relay D which by short-circuiting condenser N provides a directcurrent loop to effect metering. Telegraph signals are sent by pulses of a.c. from the calling party's oscillator O under the control of transmitting contact TM, and are received at the unattended station over amplifier-detector AMP, DV, and receiving magnet T. Release of the equipment is automatically initiated by the printer after a definite idle period by the opening of contact S1 and the temporary opening of contact S2 at the same time. Release in the case of a call from a subscriber not employing the teleprinter is effected by the same means or by a further thermal relay BB. Two subscribers both present at their substations can use the teleprinter service by operating their keys K1 .. 5 and may then revert to telephony by means of keys Y1, 2. If the unattendedstation feature is not required, the relays of figure 1 may be replaced by a key having makebefore-break contacts for associating the line alternately with the telephone equipment and the teleprinter, and simple make contacts for the power supply. Second system, Fig. 2. An ordinary call comes in over the upper set of contacts and the upper line L1 to the subset SS. If it is desired to switch over to the teleprinter, key K1 .. 3 is operated energizing relay AC which connects the supply mains to eliminator BE. Relays BC, DC operate over the low-tension supply, and voice-frequency current from oscillator O passes over the lower line L2 to operate relay X at the exchange. Relay Y also operates and when X releases on the restoration of key K1 .. 3, relays YY, Z, operate, connecting the upper set of contacts to the teleprinter line L2 and the lower set to the subset SS. At the same time the normal busy mark on the second set of contacts is removed. Relay M operates over the line L2 and teleprinting takes place by pulses of voice-frequency current from oscillator 0. At the end of the message the opening of contacts S1 after a pause disconnects the mains and during the interval between the release of relays BC, DC, the line loop is opened to release the exchange apparatus. If a telephone call is in progress at this time, relay P maintains relay Z operated and a new loop is completed for relay M which lights a guard lamp GL.