All India Radio tested the AM single channel Digital Radio Mondiale (DRM) simulcast system on a 100 kW transmitter last year. Unlike the DRM simulcast system that was tested around 2002 which used two adjacent channels, which clearly would not work in the USA, this system used operates on a single channel using an ingenious method of squeezing the digital carriers completely within a +/- 5 kHz channel!
This means that the DRM simulcast signal will occupy LESS bandwidth than a normal AM 10 kHz analog signal. Good-bye first adjacent channel hash! The DRM signal described in US Patent 7170950.
The technique starts by bandlimiting the analog signal to 5 kHz and forming it into a modulation that has the structure of Leonard Kahn’s “compatible single sideband” from the early sixties. The big difference is that today we can use a Digital Signal Processor (DSP) to perform the job that Leonard’s analog gear could never quite get right and stay in alignment. The DSP can also learn the transmitter distortions and compensate for them. The DRM signal is a high level OFDM spectrum of the same magnitude as the average analog modulation (not some little trashy signal on your neighbor’s front porch) that is placed in the unoccupied sideband.
The ODFM signal will interfere with the analog sideband, you say! Here comes the elegant part – A mirror image of the digital signal is placed under the analog sideband. This signal, when received by an envelope detector, will cancel out the ODFM signal in the other sideband!