Zero crossing
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In alternating current, the zero crossing is the instantaneous point at which there is no voltage present. This normally occurs twice during each cycle, and is the time at which there is the least electrical noise.
The zero crossing is important for systems which send digital data over AC circuits, such as X10 home automation control systems, and Digital Command Control type systems for Lionel and other AC model trains. This low noise period, occurring 100 or 120 times per second, allows the signal to be sent with the least chance of being corrupted.
Zero crossing is also a method used in speech processing to estimate formants of speech.