The zero-crossing rate is the rate of sign-changes along a signal, i.e., the rate at which the signal changes from positive to negative or back. This feature has been used heavily in both speech recognition and music information retrieval, being a key feature to classify percussive sounds.
ZCR is defined formally as
where
In some cases only the "positive-going" or "negative-going" crossings are counted, rather than all the crossings - since, logically, between a pair of adjacent positive zero-crossings there must be one and only one negative zero-crossing.
For monophonic tonal signals, the zero-crossing rate can be used as a primitive pitch detection algorithm.
Applications
Zero crossing rates are used for Voice activity detection (VAD), i.e., finding whether human speech is present in an audio segment or not.