DEVICE AND METHOD FOR EMBEDDING AND EXTRACTING DIGITAL INFORMATION AND MEDIUM WITH PROGRAM FOR EXECUTING THE METHOD RECORDED THEREON

摘要

PROBLEM TO BE SOLVED: To provide a digital information embedding and extracting device and a method whose image deterioration due to embedding is small and which have high adaptability to the existing MPEG(moving picture expert group)/JPEG(joint photographic expert group) compression. SOLUTION: A block dividing part 11 divides an image signal 71 into plural blocks according to a prescribed size. A frequency converting part 12 performs frequency conversion of each block and respectively calculates frequency coefficients. A coefficient calculating part 13 selects a specific frequency coefficient string Ca for each block and calculates respective absolute average values M and energy S. A quantizing part 14 performs linear quantization of M by using a prescribed quantization step size Q to calculate a quantization value (q) in the case S is equal to or more than a prescribed threshold K. A signal substituting part 15 substitutes (q) with the value (q') on the basis of (q) and the value of digital information to be embedded. A coefficient correcting part 16 perform inverse-linear quantization of (q') by using Q, calculates an average value M', calculates the difference DM between M' and M and corrects Ca. A frequency inverse converting part 17 reconstructs an image signal 72 due to frequency inverse conversion.