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A method of contemporary use of digital and analog modulation on the same bandwidth is described. The phase of the carrier of an AM signal is modulated by a digital one. The rate of the digital signal depends on the RF bandwidth of the analog signal. The proposed technique can increase the bandwidth efficiency of existent communication systems. Measurements and results of a full hardware implementation will show the applicability of the method. Index Terms--Amplitude modulation, digital modulation, phase modulation.

In this paper we deal with some of the main blocks which consist a stereophonic RDS modulation system. As such we consider the 19 KHz pilot tone of the FM stereo, the 38 KHz subcarrier modulated by the difference of the two stereo channels and the 57 KHz subcarrier of the RDS digital signal. Also, a digital substitute of the biphase signal generator required for coding and shaping of the RDS data preceding the balanced modulator, is considered. The substitution of the balanced modulator by a multiplying DAC is another consideration. The synthesis of the three carriers takes place by a multiplexed (triple) Direct Digital Synthesizer (DDS). The multiplexed synthesizer can be considered independently for the generation of multiple frequencies.