Alarm system using telephone network - which automatically senses messages to central station using small numbers of parallel lines and of switches

摘要

This automatic alarm system uses an ordinary telephone line network for passing the alarm message to a central station. It can also be used to report non-serviceability of subscribers telephone instruments. As the system uses ultrasonic coded signals, the ordinary speech channel, i.e. 300 to 3400 Hz is not disturbed and normal calls can continue during an alarm report. The alarm signal is in the form of a 9 bit digital signal with one start bit and one stop bit. The frequency is about 20 kHz, and the signal is subjected to frequency shift key modulation before transmission. An alarm station can be independent of a subscriber instrument, e.g. a push-button and appropriate circuit or part of the telephone instrument.