US Patent 7508747 Derivative qualified zero crossing detector apparatus and methods

Apparatus and methods are provided for detecting the presence of data in a recording media in a plurality of data locations. A mechanical detector is used to sense a data location and generate an initial signal. A differentiator is provided to differentiate the initial signal to generate a derivative signal. A comparator is used for comparing the derivative signal to a first reference signal to determine a zero crossing point representative of a change in direction of the detection device. At least one qualified signal is generated from the derivative signal. Timing circuitry is provided for comparing the zero crossing signal to the at least one qualified signal to determine the presence of data at a data location.