A difference recording apparatus includes a log comparator, a processing unit, a recording unit, and a log update section. The log comparator detects a log difference by using a classification key in a log of input data. The processing unit restores the log by using difference data from the...http://www.google.ca/patents/US5675727?utm_source=gb-gplus-sharePatent US5675727 - Difference recording apparatus having a processing unit, recording unit, log update section, and log comparator using a classification key in a log of input data 
