1. A storage device detachably loaded in a data processing apparatus, comprising: tap generation information storage means which stores tap generation information for generating a tap coefficient for each predetermined class for a data conversion process of converting first data into second data higher in quality level than the first data; tap coefficient generator means which generates the tap coefficient from the tap generation information under the control of the data processing apparatus; tap extractor means which extracts, from the first data supplied from the data processing apparatus, a predictive tap which is used to predict target data which is of interest in the second data; class tap extractor means which extracts, from the first data supplied from the data processing apparatus, a class tap which is used to classify the target data into one of a plurality of classes; class classifier means which classifies the target data according to the class tap; and predictor means which predicts the target data from the tap coefficient and the predictive tap of the class of the target data, and supplies the data processing apparatus with the target data.