| 摘要 |
<p>1,259,302. Automatic exchange equipment. INTERNATIONAL STANDARD ELECTRIC CORP. 23 Jan., 1969 [26 Jan., 1968], No. 3814/69. Heading H4K. A single line pair is used to connect two independent subscribers to an exchange by providing a single carrier channel for the additional subscriber with additional equipment provided at the exchange, Fig. 2 (not shown), and the subscriber, Fig. 3 (not shown). The carrier frequency is used for signalling purposes and provision is made to prevent interference between signalling signals. The additional equipment includes modulators, demodulators, amplifiers and filters for two-way transmission with hybrids to combine the send and receive voice channels together with the transistor switching and oscillator circuits associated with signalling and ringing. In operation, a ringing signal is produced by the exchange as bursts of carrier signal as transistor (71) is intermittently energized. This signal operates a transistor (131) at the substation to control a ringing generator which supplies normal ringing current. When the subscriber goes off-hook, the line is looped, the transistor switch in the ringing circuit is disabled and a carrier signal transmitted to the exchange. Dialling interrupts this signal and a relay (57) in the exchange follows to produce dialling impulses. A low-pass filter is provided at the terminations of the line for the existing subscriber. A local battery is used to power the substation equipment and is charged from the exchange battery during idle periods. The system may be used to provide an additional exchange line for a PBX.</p> |
