Zero crossing is commonly used in mathematics and electronics. Its when the mathematical function changes represented by crossing at an axis.

The cast of Zero Crossing - 2000 includes: Anja Salomonowitz as Interviewee

The working of zero-crossing detector is to detect the transition of a signal waveform from positive and negative, providing a narrow pulse that coincides exactly with the zero condition.

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Two sine waves, one with zero crossing at 30 degrees, the other with zero crossing of 45 degrees, have a relative phase angle of 15 degrees. Simply subtract the two angles from each other.