Programmable multi-tone voice message starting system

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. A micro-controller provides the control signal to set the center frequencies of said filters in response to user selection and analyzes the detector circuit output signals and generates an output signal when the output signal from the first detector circuit indicates the presence of a tone for a predetermined time and the output signal from the second detector circuit indicates the lack of presence of substantial energy at frequencies other than the center frequency for the same predetermined time. Optionally, the invention is applied to substitute for the operator a more pleasing signal than a tone at said filter center frequency.