In 1969 Louis Dorren invented the Quadraplex system of single station, discrete, compatible four-channel FM broadcasting. There are two additional subcarriers in the Quadraplex system, supplementing the single one used in standard stereo FM. The baseband layout is as follows:
- 50 Hz to 15 kHz Main Channel (sum of all 4 channels) (LF+LB+RF+RB) signal, for mono FM listening compatibility.
- 23 to 53 kHz (cosine quadrature subcarrier) (LF+LB) - (RF+RB) Left minus Right difference signal. This signal's modulation in algebraic sum and difference with the Main channel was used for 2 channel stereo listener compatibility.
- 23 to 53 kHz (sine quadrature 38 kHz subcarrier) (LF+RF) - (LB+RB) Front minus Back difference signal. This signal's modulation in algebraic sum and difference with the Main channel and all the other subcarriers is used for the Quadraphonic listener.
- 61 to 91 kHz (cosine quadrature 76 kHz subcarrier) (LF+RB) - (LB+RF) Diagonal difference signal. This signal's modulation in algebraic sum and difference with the main channel and all the other subcarriers is also used for the Quadraphonic listener.
- 95 kHz SCA subcarrier, phase-locked to 19 kHz pilot, for reading services for the blind, background music, etc.
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 Brown.
A commercially unsuccessful noise reduction system used with FM radio in some countries during the late 1970s, Dolby FM used a modified 25 µs pre-emphasis time constant and a frequency selective companding arrangement to reduce noise. See: Dolby noise reduction system.