| 摘要 |
1,272,830. Party line systems. SIEMENS A.G. 8 Aug., 1969 [9 Aug., 1968; 21 Feb., 1969 (4)], No. 39776/69. Heading H4K. In a telephone exchange system in which two (or more) subscribers share a common subscribers line, one subscriber station employs the line as a direct voice frequency link and the other employs different carrier frequencies for each direction with double side-band transmission on each carrier. At each end of the line a common unit demodulates the signals to provide normal working as for separate lines. The arrangement allows conventional substation equipment to be employed and allows for simultaneous use of the substations connected to the line. In one embodiment in which two subscribers employ a common line pair the first subscriber employs the speech frequency band K1 and the other subscriber employs double side-band transmission in a band K2 for transmission from the exchange and similar transmission in a band K2<SP>1</SP> to the exchange. A 25 c/s tone is transmitted from the exchange to ring either subscriber and the polarity of the D.C. supply from the exchange is used to indicate the subscriber being called, the polarity being alternated to ring both subscribers at the same time. The first subscriber has an oscillator which transmits a 4 kc/s tone on the line when that subscriber is off-hook and this tone is keyed on and off in response to dial pulses. At the exchange end of the line a tuned circuit responds making and breaking a contact in the line to the exchange. For the second subscriber the carrier frequency is used in similar manner. A 16 kc/s tone is transmitted from the exchange end to provide metering signals. In another embodiment the exchange to subscriber carrier frequency is used to call the second subscriber and metering signals for that subscriber are transmitted as an upper side-band modulation of that carrier. In this form further subscribers employing additional carrier frequencies may be included. The common unit at the subscribers end employs Zener diode voltage regulators, a transistor current regulator and various transistor switches which are described in detail. |
