Out Of Phase

When the same audio program (coincident material) is present on two channels, but the phase on one channel is reversed so that the peak (positive voltage) on one channel corresponds to a dip (negative voltage) of the same modulation on the other channel, this audio program is said to be 180 degrees out of phase.

The phase relationship of audio has many implications with respect to downmixing multi-channel audio, as well as Dolby® stereo decoding. In a downmix, if two channels are out of phase with each other when they are combined, the coincident material will cancel out and disappear from the combined track. In a Dolby stereo decode, any coincident material that is out of phase will be inappropriately steered to the surround channel.