| 摘要 |
A radio broadcasting system for a composite signal consisting of a frequency modulated (fm) analog signal and a multicarrier modulated digital signal is provided which is resistive to multipath degradation. The fm signal and the digital multicarrier modulated signal are fully coherent. The digital signal uses a plurality of carriers having a maximum amplitude at least 20 dB below the unmodulated fm signal. The multicarrier modulated signal is phase locked to a recovered analog fm pilot tone 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. An efficient channel coding consisting of high efficiency block coding, such as BCH 255, 239 code, is employed in connection with time interleaving of bits to mitigate the effects of frequency selective multipath and broadband multipath. The broadcast system is designed to be used in support of compressed digital audio programming material. In demodulation, a demodulator is phased-locked to the recovered high-amplitude analog pilot tone to coherently demodulate the digital signal, to deinterleave, to decode the block encoded signal and to format the recovered data stream for source decoding.
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