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The modulation is 16 or 64 QAM. It is claimed that this can be done on existing short wave transmitters. I suspect that the carrier phase is set at the oscillator and the amplitude is set by the usual AM modulation. This way a vector is made to the constalation point. I ffurther suspect that the output signal is sampled and demodulated as part of a feedback loop to compensate for any nonlinearity in the AM modulation process.