Abstract: A method for detecting aberrations in a digital data storage medium, such as a magnetic media disk. In a media certifier, a series of signal pulses is produced by reading data previously written to a medium is examined for differential variations between each signal pulse and at least one other of those signal pulses in the local area of the medium. A signal indicative of such differential variations is isolated by means of a CYCOMP circuit (54) and/or a band reject circuit (140). The CYCOMP circuit (54) works by delaying a previously produced signal for comparison to the cycle of the signal presently being produced and then producing a differential signal representing any instantaneous difference between the two signals. The band reject circuit (140) works by eliminating any frequency content that would be present in a series of normal signals and thus isolating any asynchronous signal that is indicative of an aberration.