GB1347929A - Telephone subscribers instruments - Google Patents

Abstract

1347929 Impulse transmitters STANDARD TELEPHONES & CABLES Ltd 17 Feb 1972 7315/72 Heading H4K In an impulse transmitter a semi-conductor device is connected in series in a loop with a telephone line and is switched between a high and low impedance state under the control of a pulse waveform whereby the loop is effectively opened and closed to perform impulse transmission over the telephone line. More particularly the semi-conductor device is a transistor which replaces the line relay contacts in a subscriber station. The pulse waveform may be derived from a pulse waveform generator and the dialling information for controlling this generator may be derived using push-buttons, cord or repertory selective signalling. Several digits may be stored in a data store, the contents of the store controlling the waveform generator. The pulse waveform may be associated with data transmission. Transistors also replace the off-normal contacts of the subscriber station for muting the speech circuits during impulsing. The station may have a local battery which is charged over the telephone line when the subscriber is off-hook except during impulsing. The circuit comprises an impulsing transistor TR1, off-normal transistors TR2, TR3, speech transmission circuit 11 and a battery charging circuit 12. A diode bridge 15 is connected to the line 10 to render the circuitry immune to polarity reversals on the line and a voltage limiting device 16, e.g. a Zener diode or a non-linear resistor, is used to protect the transistors against over voltages on the line. When the impulse transmitter is operative the charging circuit 12 is arranged together with the speech transmission circuit 11 to drop the telephone line voltage to the level required to hold the transistor TR1 on i.e. the line looped. When the transistor TR1 is turned off by the application, by means not shown, of a pulse to its base, logic circuitry causes transistors TR2, TR3 to be turned on thereby disabling the speech transmission circuits and muting the receiving transducer 14. The transistor TR1 may be suitably replaced by a thyristor, a triac, or a semi-conductor gate controlled switch. The receiver muting transistor TR3 may also be arranged in series with the receiving transducer 14 in which case it would be turned off during impulse transmission; this transistor could be replaced by either an N- channel insulated-gate enhancement, or an N- channel junction-gate, FET. In an arrangement where the local battery is charged with the subscriber on-hook or where it is charged from an external source, alternative means, not disclosed, must be used in place of the battery charging circuit to drop the line potential to that required to hold transistor TR1 on.