Push button telephone number selection circuit - compensates for signal attenuation due to long lines by using receive line potential as reference in evaluation circuit

摘要

The application is to telephone systems in which the subscriber is connected to the exchange by four lines, two for transmission and two for reception. The length of line joining the subscriber to the selector in the exchange may vary greatly in length and therefore the signal strength available at the selector may be considerably attenuated by the line impedance. When the push button for selecting a number is closed the appropriate controls put positive potential on one transmit line. This potential is applied through routing diodes to the other transmit line and to one of the receive lines. The potential of the latter line is connected through an operational amplifier to charge a capacitor whose potential is then applied as a reference to the inverting input of a second amplifier. The potential of the transmit line is applied to the other input. The validity of the incoming signal is thus checked before being accepted.