Editing Zero crossing

{{short description|Point where a function crosses an axis and changes sign}}
[[File:Zero crossing 2.svg|thumb|right|A zero-crossing in a [[line graph]] of a [[waveform]] representing voltage over time]]

A '''zero-crossing''' is a point where the sign of a [[mathematical function]] changes (e.g. from positive to negative), represented by an [[zero of a function|intercept of the axis]] (zero value) in the [[Graph of a function|graph]] of the function. It is a commonly used term in electronics, mathematics, [[acoustics]], and [[image processing]].

==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 [[interval (time)|cycle]]. It is a device for detecting the point where the voltage crosses zero in either direction.

The zero-crossing is important for systems that send [[digital data]] over AC [[electrical network|circuits]], such as [[modem]]s, [[X10 (industry standard)|X10]] [[home automation]] [[control system]]s, and [[Digital Command Control]] type systems for [[Lionel Corporation|Lionel]] and other AC [[model train]]s.

Counting zero-crossings is also a method used in [[speech processing]] to estimate the [[fundamental frequency]] of speech.

In a system where an amplifier with digitally controlled gain is applied to an input signal, artifacts in the non-zero output signal occur when the gain of the amplifier is abruptly switched between its discrete gain settings. At audio frequencies, such as in modern consumer electronics like [[digital audio player]]s, 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. [[Artifact (error)|Artifacts]] are disconcerting and clearly not desirable. If changes are made only at zero-crossings of the input signal, then no matter how the amplifier gain setting changes, the output also remains at zero, thereby minimizing the change.  (The instantaneous change in gain will still produce distortion, but it will not produce a click.)

If electrical power is to be switched, no [[electrical interference]] is generated if switched at an instant when there is no current—a zero crossing. Early light [[dimmer]]s and similar devices generated interference; later versions were designed to switch at the zero crossing.

==In signal processing==
In the field of [[Digital data|digital]] [[image processing]], great emphasis is placed on operators that seek out edges within an image.  They are called ''[[edge detection]]'' or ''gradient filters''.  A gradient filter is a filter that seeks out areas of rapid change in pixel value.  These points usually mark an edge or a boundary.  A [[Discrete Laplace operator|Laplace filter]] is a filter that 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'''. An example can be found [http://homepages.inf.ed.ac.uk/rbf/HIPR2/zerocdemo.htm here], including the source in Java.

In the field of [[industrial radiography]], it is used as a simple method for the segmentation of potential defects.<ref>{{Cite book|last=Mery |first=Domingo |date=2015 |title=Computer Vision for X-Ray Testing |url=https://www.springer.com/gp/book/9783319207469 |location=Switzerland |publisher=Springer International Publishing |page=271 |isbn=978-3319207469}}</ref>

In the field of [[Natural Language Processing|NLP]], the [[Zero-crossing rate|rate of zero crossings]] observed in a [[spectrogram]] can be used to distinguish between certain phonemes such as [[fricatives]], voiceless stops, and vowels.<ref>{{cite book |last1=Church |first1=Kenneth |title=Phrase-Structure Parsing |date=1983 |publisher=Indiana University Linguistics Club |page=15}}</ref>

== See also ==
* [[Reconstruction from zero crossings]]
* [[Zero crossing control]]
* [[Zero-crossing rate]]
* [[Zero of a function]] (a root)
* [[Sign function]]

==References==
{{reflist}}

[[Category:Signal processing]]