Certification testing for GMR disks

摘要

In order to ensure that media can meet the demands of drive operation, drive manufacturers place specific magnetic requirements—such as a maximum number and size of magnetic defects per disk—on the media they receive. To ensure that these criteria are met, media factories use certification heads to scan disks for magnetic defects. A defect is recorded when the output of the certification head drops below a predetermined threshold, typically 65% of track average amplitude (TAA). Simplified, the critical defect size is 35% of the track width of the head (1-threshold). If more than 35% of the magnetic material is missing from an area being read, the output over that area will drop and a defect is recorded. Of course, there are a multitude of other factors—like pits, blisters, scratches, and other abnormalities—that confound this. Overall, however, the simple relationship holds true: When the signal drops 35%, a defect is recorded.