Abstract: A converter receives a compressed video stream conformant to one format and re-encodes selected syntactic elements using superior entropy compression algorithms to produce a smaller stream conformant to a second standard. The converter passes through other compressed syntactic elements without re-encoding them to expedite processing, while still producing a smaller stream conformant to the second standard. In variations, a dual output encoder receives an uncompressed video stream and generates a first compressed stream conformant to one formant and a second stream conformant to a second standard. A second converter may be used to convert back to the original format with little or no loss. In some variations, novel arithmetic coding techniques are utilized to recompress previously compressed data.