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.