Circuit arrangement for feeding a call progress tone into the speech path of a telephone switching system switching digitally in accordance with the time-division multiplex principle

摘要

This circuit arrangement has the effect that a clock generated with the aid of counters and logical combinations is only produced at uniform intervals in the connections into which a call progress tone is to be inserted. This clock is supplied via a shift register to a send gate which applies a zero-volt potential to the PCM bus for as long as it is activated. Since this process occurs at periodically repetitive time intervals, a call progress tone having the corresponding fundamental frequency becomes noticeable. In addition, a cadence can also be inserted into the logical combinations.