| 摘要 |
1348141 Party line systems STANDARD TELEPHONES & CABLES Ltd 22 April 1971 10691/71 Heading H4K A subscriber carrier telephone system includes a switch at the subscriber station operable to cause an "out-of-band" signal to be transmitted and when detected in the exchange. carrier equipment causes a required condition to be set up. The switch may be used to provide a recall or call transfer facility. In the system described the exchange equipment transmits speech signals on a second carrier frequency of 64 KHz with ringing signals provided as bursts of carrier interrupted at 17 Hz by the operation of signal detector ESD. These signals are detected in the subscriber unit and the bursts of carrier operate a 25 Hz ringer. When the subscriber station telephone instrument SCT is off hook a control circuit SCC starts oscillator SCO to transmit a first carrier frequency of 28 KHz to the exchange. This carrier is detected and causes relay A1 to operate to loop the line DEL. Dial pulses are sent by interrupting this first carrier frequency to cause relay A1 to act as impulsing relay. Speech may be transmitted by amplitude (circuit of Fig. 1) or frequency (circuit of Fig. 4) modulation of the respective carriers. Operation of switch contacts RC causes 7 KHz oscillator SRO to start and either to modulate the existing first carrier or to replace it to provide the out of band signal. In the exchange the signal is detected in detector ED2 and operates relay B 1 to earth the line RL through contacts b1. In a modification a single oscillator may be used in the subscriber station and closure of switch contacts RC causes a transistor to conduct to short circuit a resistor in the oscillator circuit thus causing a different frequency to be transmitted, Fig. 2 (not shown). In a further modification the detector in the exchange unit may be replaced by a logic circuit, including a monostable circuit, where the output level of the logic circuit changes if the received frequency changes from greater than to less than half the period of the monostable circuit, Fig. 3 (not shown). Both the exchange and subscriber stations are provided with AGC loops which change the transmission level to compensate for the length of line. |
