1. A method of authenticating a media item, the method comprising: measuring a magnetic signal along the media item, converting the measured signal to a calibrated length signal, aligning the calibrated length signal with a reference signal, including shifting the calibrated length signal, determining that a measure of difference between the shifted calibrated length signal and the reference signal is at a minimum, and selecting the shifted calibrated length signal as an aligned signal, extracting features from the aligned signal, and classifying the media item based on the extracted features.