C-QUAM

From HFUnderground

Compatible Quadrature Amplitude Modulation (C-QUAM) is a means of transmitting stereophonic audio in a means that is compatible with monophonic AM receivers. Developed by Motorola in 1977, this technique was one of four systems proposed to allow commercial station in the United States and elsewhere to transmit stereo audio. Although never authorized for service on high frequency broadcast stations, some commercial equipment can be modified for such service. Although rare, some shortwave receivers did include C-QUAM decoding, notably the NRD-545. Today in the SDR age, there are software programs that can natively receive or decode stereo broadcasts. These include SoDiRa, and SdrDX.