During a transaction using a magnetic card (16), test data is written to the card (16) at a constantly increasing frequency rate. The test data is then read from the card (16) and the high frequency cut-off point f.sub.0 of the card (16) (i.e. the maximum write frequency above which it is no longer possible...http://www.google.com.au/patents/US6240515?utm_source=gb-gplus-sharePatent US6240515 - Method of authenticating a magnetic card
