Abstract: Typical defects are detected on a product moving relative to detecting means. An alternating magnetic field is generated in order to provoke the circulation of eddy currents on the surface of the product. A signal representative of the variations of the eddy currents is collected by means of a differential sensor and is demodulated by projection along an angle of adjustable phase in order to detect the disturbances introduced by the defects to be detected. This angle of phase is determined so that the projection of the collected signal has a minimum mean value, in order to eliminate disturbances of the collected signal due to repetitive surface irregularities. The component of the collected signal obtained by the projection is converted into digital form, and the digital signal thus obtained is filtered using a digital filter whose characteristics are predetermined as a function of a type of defect to be detected.