A) A technique used when error correction fails (see Error Correction). Erroneous data is replaced by data synthesised from surrounding pixels. b) When the error correction programme discovers in the reproduced signal, an error too extensive to permit reconstruction, the redundancy in most image information makes it possible for error concealment to make the error nearly inobvious. Video images are frequently nearly identical from frame to frame. Adjacent video lines frequently have almost the same detail. It becomes possible, therefore, when a “burst error” involving the modification or loss of many recorded bits occurs, to determine from image segments adjacent in time or in space, a most probable substitution. Such substitutions, when infrequent and supported by the image redundancy, are often accepted by the viewers as “correct.” (This is a degree of freedom in image data recording that obviously is not available to scientific and financial data recording. The additional information needed by the algorithm for decision and substitution is usually provided by a datastorage cache established during reproduction.