RECEIVER FOR PUSH-BUTTON PCM-ENCODED MULTIFREQUENCY SIGNALS

摘要

To decode multifrequency dialing signals transmitted by a pushbutton-equipped subscriber station of a telecommunication system and quantized upon arrival at a central office, a receiver at the office includes two digital filters separating the incoming binary-coded samples from sine waves in a higher and a lower frequency range and supplying the respective sets of samples to two interpolators converting them into respective square waves fed to a decoder. Each interpolator comprises a sign-bit extractor and an arithmetic unit which emits a numerical value-based on the absolute values of two consecutive samples-representing a delay expressed as a number of clock pulses recurring at a multiple of the sampling rate. A logic network responds to a polarity change of the corresponding sine wave, as determined by the sign-bit extractor, to start a clock-pulse counter measuring a corrective time interval by which the resulting square wave must be delayed so that its pulse flanks substantially coincide with the zero crossings of the sine wave shifted by one sampling period. The delay is accomplished by an Exclusive-OR gate with inputs connected to the logic network and to the sign-bit extractor.