Abstract: A system for detecting the presence, in an audio channel which may contain voice signals, of a tone at a frequency selectable from among a plurality of selectable frequencies. A bandpass filter receives the audio input; its center frequency is selectable in response to a control signal. A first detector circuit provides from the output of the bandpass filter an output signal when a tone containing substantial energy at about said center frequency is input to the filter. A band-stop filter having the same center frequency as the bandpass filter also receives the input audio; a second detector circuit demodulates and thresholds the band-stop filter output.

Abstract: A circuit for detecting dial pulses at a receiving telephone transmitted from a sending telephone after the telephones are interconnected. Detecting circuits, including bandpass filters and summing and threshold circuits transmit a probable pulse signal. A probable pulse signal indicates that the signals on the telephone line have the characteristics present whenever the dialing mechanism at the sending telephone is actuated. A digital correlation circuit receives all the probable pulse signals and advances a counter which stores the number representing the dialed digit if a correlation exists.