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Abstract

A radio broadcasting system is provided for transmitting and receiving through free space a composite signal consisting of a frequency modulated (fm) analog signal and a multicarrier modulated digital signal which is especially adapted to be resistive to multipath degradation. The fm signal and the digital multicarrier modulated signal are fully coherent. Further according to the invention, the digital signal comprises a plurality of carriers having a maximum amplitude at least 20 dB below the unmodulated fm signal and preferably, 30 dB below the analog signal. The multicarrier modulated signal is phase locked according to the invention to the recovered analog fm pilot tone at 19 kHz in the composite baseband spectrum of the fm signal which is at least 20 dB above the multicarrier modulated signal, which enables rapid and reliable acquisition of signal for coherent detection. In a specific embodiment, the multicarrier modulated signal is a synthesized vector-modulated signal which is a quadrature phase shift keyed (QPSK) modulated set of synthesized carriers each occupying 9.5 kHz of spectrum replicated twenty-one times within a 199.5 kHz bandwidth with no more than two bits per vector. The broadcast system is designed to be used in support of compressed digital audio programming material. In a demodulation process according to the invention, a demodulator is operative to phase lock to the recovered high-amplitude analog pilot tone, to coherently demodulate the digital signal, and to format the recovered data stream for source decoding.

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First Claim