The short time average zero crossing rate of a speech signal can be used in conjunction with the short time average energy (or magnitude) to discriminate between voiced speech, unvoiced speech and silence. The short time average crossing rate of a digitally sample speech signal is defined in Digital Processing of Speech Signals
Equation 1 Short time average zero crossing rate
Where:
Equation 2 sgn (sign) function
and w(n) is the windowing function with a window size of N samples:
Equation 3 Windowing function, window size N samples
