Monolithically integratable telephone circuit for generating control

Abstract

A telephone circuit which may be monolithically integrated, for generating control signals for displaying the telephone charges to a telephone user, is coupled to a AC voltage signal generator having a predetermined amplitude and frequency which are constant over time. The circuit includes a voltage generator for generating voltage signals which are spaced over time and have a trapezoidal pulse waveshape. A multiplier circuit calculates the product of the signals supplied by the two generators and supplies a signal which is sent to the speech circuit of the subscriber's line and is added to the speech (i.e.--conversation) signals. The circuit also includes a circuit for receiving an image signal of the overall signal adapted to the telephone line; a high-pass filter for eliminating the speech signal components from the image signal; a rectifier circuit for rectifying the filtered signal and a comparator for comparing the rectified signal with a reference signal. If the amplitude of the rectified signal is greater than the amplitude of the reference signal, the comparator generates a signal designed to stop the increase in the level of the trapezoidal pulse signals generated by the voltage generator thereby regulating the amplitude of the trapezoidal pulse signals.