A karaoke machine with a multiplex button is capable of removing the vocal track from a sound recording. It can only do this if the karaoke disc is formatted for multiplex playback. A multiplex song has two tracks; one is a full version and the other is a full version minus vocals. When the multiplex option is not selected, both versions play. The listener can't hear the difference because the tracks run at exactly the same time. When the multiplex option is selected, the karaoke machine only plays the track without vocals.
The most common application of a karaoke multiplexer is for practice and performance. While you may wish to sing along with the full track to learn the melody, lyrics and phrasing, when it comes to performing, you want a version without vocals. Rather than swapping a disc or skipping through the track menu to find the full and backing version, the multiplex function enables you quickly and easily to turn the vocals on and off. A karaoke multiplexer also can be used as a standard audio playback device.
Not all karaoke machine manufacturers label the multiplexer function. Some use marketing terminology such as "Vocal Remove," "Karaoke Partner" or "Vocal Changer" to present the function more literally. The technical abbreviation for a multiplexer circuit is "mux."
Some karaoke disc manufacturers elect not to make their products multiplexer compatible. Instead they issue their discs with two versions of the same song. One version has vocals and one doesn't, so the user can manually select whether to have vocal accompaniment. While this takes up more disc space and results in each disc having double the amount of tracks, there are few other functional differences between this approach and the multiplex approach.