Out Of Phase Stereo (OOPS) is an audio technique which manipulates the

phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

of a stereo audio track, to isolate or remove certain components of the stereo mix. It works on the principle of

phase cancellation In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructi ...

, in which two identical but inverted waveforms summed together will "cancel the other out".

Process

When a sine wave is mixed with one of identical

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

but opposite

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

(ie: of an inverse

polarity Polarity may refer to: Science *Electrical polarity, direction of electrical current *Polarity (mutual inductance), the relationship between components such as transformer windings * Polarity (projective geometry), in mathematics, a duality of ord ...

), the combined result is silence. A two-channel stereo recording contains a number of waveforms; sounds that are panned to the extreme left or right will contain the greatest difference in amplitude between the two channels, while those towards the centre will contain the smallest. A mix of the left channel with the polar inverse of the right channel will reduce centre-panned sounds towards silence, while preserving those towards the extremities. In practice, the OOPS technique can be performed by inverting the

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.

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

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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

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' 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. It consisted of two mono tracks: a loud and distorted electric guitar playing chords repetitively, as well as a quiet vocal track. The guitar had one of the channels inverted, while the vocal track was identical in both channels. During normal playback, the guitar would be heard throughout the entire track. When the channels were summed to mono, however, the regular and inverted guitar tracks would cancel out, revealing the vocal track to the listener.

References

{{reflist Interference Wave mechanics