US Patent 7151887 Copy controlling method and system of digital contents

In a system for recording digital contents compressed and delivered by the MPEG (Moving Picture Experts Group) standard, in decoding the contents, even a DCT coefficient of the digital contents is required in a discrete cosine transform (DCT) unit, according to the attribute information belonging to the digital contents, of the DCT coefficients within a block, some AC coefficients remain and the other AC coefficients are turned to 0, and the obtained DCT coefficients are again encoded so to create stream data for recording.