| 摘要 |
1512171 Substation arrangements INTERNATIONAL MOBILE MACHINES CORP 23 Jan 1976 2668/76 Heading H4K A substation arrangement for allowing ringing signals from an exchange to reach the bell of a telephone only if the caller sends an extra predetermined signal comprises a relay which when energized opens a contact in the ringing path, actuating means coupled to the exchange and arranged to energize the relay during detection of a ringing signal, a switch means arranged to deenergize the actuating means upon detection of the predetermined signal and power supply means arranged to store energy from the ringing signal and to supply power to the substation equipment. In one form ringing signals on a line 12, 14 rectified in a bridge 16, 18, 20, 22 enable a relay 44 the normally closed contacts 70 of which open to prevent the ringing signals reaching the telephone bell. A multi-frequency code transmitted by the caller is filtered at 64 and converted in a compression amplifier to a constant level signal which is mixed with each of the two frequencies of a first multi-frequency digit such that a correct first digit will produce a beat frequency capable of passing through a low pass filter. The output steps a flipflop chain to cause a matrix to select two further frequencies for the second digit and if this is correct to disable via a transistor 42 the relay 44 allowing the ringing signal to reach the substation bell. The power for the equipment is stored on a capacitor 50 charged by the ringing signals. The flip-flop chain is reset by a slowly charging capacitor 54. |
