In digital data conversion apparatus and method, class data are generated in association with reference interpolated data for each of a plurality of classes on the basis of a reference high definition digital video signal which includes a reference standard definition digital video signal in addition to the reference interpolated data. The class data is stored at respective addresses in a memory. A standard definition digital video signal representing pixel values is received and then clustered so as to produce a class corresponding to the pixel values of the standard definition digital video signal. The class data is retrieved from the memory address which corresponds to the class of the standard definition digital video signal, and interpolated data is generated in accordance with the standard definition digital video signal and the retrieved class data with such interpolated data constituting a high definition digital video signal.