Method for superimposing data on amplitude-modulated signals

The principles of a new modulation scheme are described which permit an additional data channel to be superimposed on conventional DSBAM signals without increasing band-width or envelope distortion. This scheme, named zero synchronous frequency modulation (ZSFM), requires additional signal processing and provision for combined AM/FM at the transmitter. At the receiver, the data signal is detected by a simple frequency demodulator, while envelope detection of the AM signal is performed in the usual way.