Data position measurement

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Data position measurement (DPM) is a copy protection mechanism that operates by measuring the physical location of data on an optical disc. Stamped CDs are perfect clones and always have the data at the expected location, while a burned copy would exhibit physical differences. DPM detects these differences to identify user-made copies. DPM was first used publicly in 1996 by Link Data Security's CD-Cops. It was used in volume on Lademans Leksikon published by Egmont in November 1996.