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During the past decade commercial sound reproduction has progressed from monophonic to two-channel stereophonic reproduction. Now, four-channel sound sources are rapidly becoming dominant for music reproduction. Since the pilot tone system of broadcasting in use today is limited to two channels, a new but fully compatible four-channel system was developed. This "Dorren Quadraplex System" was designed from the start to meet the following conditions: Â¿full compatibility with all existing equipment; Â¿unimpaired directionality and channel separation; Â¿reproduction of the frequency range 50 to 15,000 Hz in all channels; Â¿minimum distortion and cross-talk between channels; Â¿minimum signal-to-noise ratio impairment Â¿no additional RF spectrum requirements Experimental use of this system for broadcasting began at radio station KIOI (125 kW ERP) in San Francisco in December 1970, with a Station Temporary Authorization from the Federal Communications Commission. Additional broadcasts were done over another major station, CHFI (210 kW ERP) in Toronto, Canada. This test was supervised by the Department of Communications and the Canadian Radio and Television Commission. It was found that, "Compatibility was evident in all cases". Additional testing and use is expected in Japan in the near future. The system has shown itself to offer performance for better than the minimum FCC standards and, therefore, its performance in production can be guaranteed.