The present invention relates to a digital data play back apparatus for converting
a digital data a signal read from a recording medium by sampling in a clock cycle,
equalizing this digital data to a waveform which agrees with a partial response
class on the basis of the clock, and detecting data on the basis of the clock with
respect to the equalized signal waveform. Then, this apparatus detects a timing
phase gradient between the equalized signal waveform and the clock, and controls
a clock frequency on the basis of the detected timing phase gradient.
