| 摘要 |
The present invention provides a technique and system for modifying the modulation of the 66.5 kHz subcarrier in such a way that its spectrum will not overlap that required by the 57 kHz subcarrier, while at the same time maintaining compatibility with existing subcarrier receivers. A special signal is created whose spectral shape is asymmetrical with respect to the 66.5 kHz subcarrier. In the frequency domain, the lowest frequency part of the symmetrical spectrum is cut off, flipped about the carrier frequency and then added to the highest frequency part of the spectrum. This technique and system takes advantage of the fact that the receiver (22) uses a sampling process (24) which produces frequency aliasing. The spectral shape produced at the transmitter is such that the aliasing caused by sampling in the receiver (22) results in the lower-than-carrier and higher-than-carrier portions of the spectrum being added together, producing the receiver (22) a symmetrical spectrum.
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