Short for multiplex. The letters “MPX” when found on a button on a cassette deck mean the button activates a 19kHz notch filter to eliminate the FM stereo pilot so it won’t cause gain mistracing in Dolby noise reduction systems. The Dolby playback system would sense the pilot and mistake it for high-frequency content of the music. The pilot can also cause audible birdies by beating with the AC bias used in tape recorders.