A simple flaw detector that enables reliable single-pass defect locati
on in magnetic media is described and analyzed. The envelope of the pl
ayback signal is detected in a phase-independent manner, small variati
ons in envelope indicate the presence of a defect. The detector may be
configured as a biased or unbiased estimator of the envelope of the i
nput signal. Synchronous, asynchronous, analog, digital, biased, and u
nbiased forms are discussed.