Out Of Phase Stereo
Out Of Phase Stereo (OOPS) is an audio technique which manipulates the phase of a stereo audio track, to isolate or remove certain components of the stereo mix. It works on the principle of phase cancellation, in which two identical but inverted waveforms summed together will "cancel the other out".
In practice, the OOPS technique can be performed by inverting the polarity of one speaker or signal lead. It can also be performed using digital audio software by inverting one of the channels of a stereo audio waveform, and then summing both channels together to create a single mono channel. Those components common to both channels (that is, those sounds that are recorded as a mono track, and duplicated identically onto both channels) will be phase cancelled, leaving only those sounds that differ between channels.
Applications in music
This technique has been previously used to eliminate vocals in a stereo track (as vocals tend to be panned centre) to create crude karaoke tracks, or generate surround channels from a stereo source, such as in Dolby Pro Logic. It has also been used in the recording process to include tracks that were only audible once an OOPS technique was applied. This feature can be observed in several of the Beatles' stereo albums. Australian band Cinema Prague recorded a single track Meldatype that contained two songs played simultaneously, one of which was only audible after an OOPS technique was applied.
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