<P> The normal stereo signal can be considered as switching between left and right channels at 38 kHz, appropriately band limited . The quadraphonic signal can be considered as cycling through LF, LR, RF, RR, at 76 kHz . </P> <P> Early efforts to transmit discrete four - channel quadraphonic music required the use of two FM stations; one transmitting the front audio channels, the other the rear channels . A breakthrough came in 1970 when KIOI (K - 101) in San Francisco successfully transmitted true quadraphonic sound from a single FM station using the Quadraplex system under Special Temporary Authority from the FCC . Following this experiment, a long term test period was proposed that would permit one FM station in each of the top 25 U.S. radio markets to transmit in Quadraplex . The test results hopefully would prove to the FCC that the system was compatible with existing two - channel stereo transmission and reception and that it did not interfere with adjacent stations . </P> <P> There were several variations on this system submitted by GE, Zenith, RCA, and Denon for testing and consideration during the National Quadraphonic Radio Committee field trials for the FCC . The original Dorren Quadraplex System outperformed all the others and was chosen as the national standard for Quadraphonic FM broadcasting in the United States . The first commercial FM station to broadcast quadraphonic program content was WIQB (now called WWWW - FM) in Ann Arbor / Saline, Michigan under the guidance of Chief Engineer Brian Jeffrey Brown . </P> <P> Various attempts to add analog noise reduction to FM broadcasting were carried out in the 1970s and 1980s: </P>

For fm radio signals which statement is correct