Zero crossing

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Zero-crossing in a waveform representing voltage vs. time

Zero crossing is a commonly used term in electronics, mathematics, and image processing. In mathematical terms, "zero crossing" basically means the changing of sign (e.g. from positive to negative), that is represented with a crossing of the axis (zero-value) in a graph of a particular function.

[edit] In electronics

In alternating current, the zero crossing is the instantaneous point at which there is no voltage present. In a sine wave or other simple waveform, this normally occurs twice during each cycle.

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.

Counting zero crossings is also a method used in speech processing to estimate frequencies and formants of speech.

In a system where an amplifier with digitally-controlled gain is applied to an input signal, signal artifacts will occur when the gain of the amplifier abruptly switches between its discrete gain settings while outputting a non-zero current. At audio frequencies, such as in modern consumer electronics like digital audio players, these effects are clearly audible, resulting in a 'zipping' sound when rapidly ramping the gain, or a soft 'click' when a single gain change is made. Artifacts are disconcerting and clearly not desirable. If changes are made at a zero crossing of the signal, the input is zero, so no matter how the amplifier gain setting changes, the output will also remain at zero.

The zero crossing of the mains voltage is the key to determining the proper switching time (phase angle) in both leading edge and trailing edge dimmers.

[edit] In image processing

In the field of Digital Image Processing, great emphasis is placed on operators which seek out edges within an image. They are called 'Edge Detection' or 'Gradient filters'. A gradient filter is a filter which seeks out areas of rapid change in pixel value. These points usually mark an edge or a boundary. A Laplace filter is a filter which fits in this family, though it sets about the task in a different way. It seeks out points in the signal stream where the digital signal of an image passes through a pre-set '0' value, and marks this out as a potential edge point. Because the signal has crossed through the point of zero, it is called a zero crossing. For a more complete explanation, try visiting [1]