Abstract: A demodulator produces mark and space signals in response to mark and tone signals received from a radio receiver. The signals after detection are in the form of characteristic tones and are separated by corresponding band pass filters. After separation, AM detection of the mark and space signals is accomplished in envelope detectors whose output voltages are D.C. voltages. In the absence of noise, the detector voltages are essentially a rounded off version of the binary signal (mark) and a rounded off version of the complement (space) of the binary signal.