The invention relates to a method and devices for protecting a reading device (1) for card-shaped data carriers (2) against unauthorized evaluation or copying of magnetically encoded data detected in the reading device (1) for card-shaped data carriers (2). To this end, an electromagnetic noise field (18) is generated by means of a noise field coil (17). The use or disposition of the at least one noise field coil (17) is such that the authorized magnetic field reading head (10) is also affected by the noise field (18) of the noise field coil (17) when the magnetically encoded data of a card-shaped data carrier (2) is being read. An output or sum signal of the authorized magnetic field reading head (10) generated from the wanted signal of a card-shaped data carrier (2) and from the effects of the noise field (18) is detected. The effect of the noise field (18) of the noise field coil (17) in the output or sum signal of the authorized magnetic field reading head (10) is then compensated or filtered out or the wanted signal is selectively filtered out of the output or sum signal of the authorized magnetic field reading head (10).