Modulation techniques for data superposition on amplitude-modulated signals
Double-sideband amplitude-modulation is widely used because of its simplicity in transmission and reception, but in a number of applications the inclusion of a data channel has become an additional requirement. An additional data signal, transmitted with a normal AM broadcast signal, can be used by a specially-equipped receiver to provide for display and control functions. The data signals should not impair the main broadcast signals, and should be imperceptible to listeners with ordinary radio receivers. The principles of a new modulation scheme, which permits an additional data channel to be superimposed on conventional DSB-AM signals without any bandwidth increase or envelope distortion, are described in this thesis. The proposed system uses combined amplitude/frequency modulation to provide for the additional data channel. It is shown that bandlimitation may be preserved by frequency modulating the carrier with frequency-modulating functions related to the complex zeros of the amplitude-modulating signal. The performance of the new modulation technique is assessed by computer simulation. The cross-talk between the two channels due to multipath propagation is considered to be a limiting factor. A novel complex filtering technique to detect complex zeros of realsignals is introduced and discussed. The distribution of the complex zeros of positive bandlimited signals is assessed experimentally, and the possible data rates of the proposed system are discussed.