1. Apparatus for identifying genuine articles comprising: a magnetic write head; a magnetoresistive read head located near said magnetic write head; means for moving an article having magnetic regions printed in coded patterns on the face thereof with ink containing magnetic pigment past said read and write heads; a drive circuit for energizing said write head with at least one pure tone high frequency recording signal for recording said at least one pure tone high frequency signal on said magnetic regions; a detection circuit including a phase sensitive amplifier coupled to said magnetic read head for detecting the presence or absence of said pure tone high frequency signal in said magnetic regions and for producing a detection signal which is amplified by said phase sensitive amplifier; and phase circuit means coupled to said drive circuit and said detection circuit for producing a phase reference signal proportional to the current and field phase of said at least one pure tone high frequency recording signal and for controlling said phase sensitive amplifier with said phase reference signal by comparing the instantaneous phase of said reference signal with that of said detection signal to improve the detection of low level signals from said magnetic regions.