The video memory requirement of an MPEG decoder, or of an SQTV processor, or of similar devices, wherein the storing of full pages or pixels or portions thereof is performed in decoding or in filtering noise, may be dynamically reduced by ADPCM recompressing and decompressing of the MPEG decoded digital data stream, before and after storing the data in the video memory, respectively. A particularly efficient and simple ADPCM compression method employs a differential variance estimator which, instead of processing bidimensional blocks of pels, instead processes blocks of pels all belonging to a same horizontal video line.