Triggered by the FCC's approval of suppressed-carrier-am as the multiplex standard for fm stereo broadcasts, manufacturers are preparing to produce equipment for transmitting and receiving. In the new system, the main carrier is frequency-modulated by the sum of the left and right microphone signals and by the sidebands of a suppressed subcarrier. The subcarrier is amplitude-modulated and suppressed by a left-minus-right difference signal. The suppressed subcarrier is restored at the receiver by a pilot signal transmitted between subcarrier and main carrier.
To receive the broadcasts requires that an adapter be added to a standard fm tuner and dual-channel amplifier. General Electric Co. believes that a one-tube adapter—a double triode—would give satisfactory results, although it says addition of a preamplifier would make possible good reception by a wider variety of amplifiers. The cost of the adapter would be minor compared to that of the complete stereo system.
The system is compatible with existing fm broadcasting standards. Monophonic receivers tuned to stereo transmissions would receive the left-plus-right signal and produce sound only slightly reduced in quality. (Electronic Design, May 10, 1961, p. 6)
FM stereo broadcasting was authorized to begin on June 1, 1961, and regular operations began at WEFM Chicago and WGFM Schenectady. There's a good chronology of FM broadcasting at http://members.aol.com/jeff560/chronofm.html.