Abstract: Detection of valid two-out of-N multifrequency signals is realized by employing a microprocessor in conjunction with a controllably adjustable or settable reference threshold level and a plurality of threshold detectors. Under control of the processor, a reference threshold level supplied to the detectors is set to a first prescribed level. Upon detection of a tone which exceeds the first threshold level the processor controllably sets the reference threshold level to a second prescribed level. Detection of two, and only two, tones which are present for at least a minimum prescribed interval are then considered valid.