ABSTRACT
A distortion in an audio system produced when signals, originally simultaneous, are heard with a small delay between them. This delay, often occasioned by ultrasonic filters in modern systems, may be as tiny as a thousandth of a second, but it will result in a mismatch of the signal peaks. If the signal peaks are at exact opposite stages of their cycles, they are 180 degrees out of phase and cancel each other out. Out-of-phase program material is a particular problem in stereo playback, as it may cause loudspeakers to vibrate out of step with each other. However, some out-of-phase programming is fundamental to the stereo effect:when signals are separately reproduced through different loudspeakers it is phase shift (at very low frequencies) that suggests to the listener that the sound is coming from somewhere between the two speakers. It seems that among audio components it is the loudspeakers that have the potential to create sufficient phase shift to be audible.
