A system and method to convert a CIF compressed video to a QCIF video bitstream provides compatibility between the various digital video sources and application uses. A coding mode and a motion vector for a macroblock in a QCIF video sequence are selected from those of a corresponding CIF video sequence without motion estimation.

Methods and systems for processing pixels within an encoded video stream are disclosed herein. Aspects of the method may comprise acquiring on a chip, a prediction weight type from the encoded video stream. Prediction weight data for at least one prediction pixel within the encoded video stream may be acquired based on the prediction weight type. A look-up table entry in a look-up table may be generated for the prediction pixel utilizing the prediction weight data. A plurality of current pixels within the encoded video stream may be estimated utilizing the look-up table entry. The prediction weight type may correspond to a plurality of reference pictures within the encoded video stream. A prediction weight value, offset value and motion vector data for the prediction pixel may be acquired on the chip, if the prediction weight type comprises an explicit prediction weight type.

A transcoder system for compressed digital video bitstreams comprising three frame types: I-frames, P-frames and B-frames. The system includes a picture reader decoder to ascertain the type of each frame prior to transcoding. The system also includes a T.sub.2 transcoder for processing I-frames and P-frames, possibly excepting the last P-frame of a group-of-frames (GOP), a T.sub.1 transcoder for processing B-frames and (optionally) the last P-frame of a GOP and a picture decision block for separating all frames into I-frames and P-frames, possibly excepting the last P-frame of a GOP for processing by the T.sub.2 transcoder and B-frames and (optionally) the last P-frame of a GOP for processing by said T.sub.1 transcoder. Another component is a remerge block for recombining the output of all frames from said T.sub.1 transcoder and the T.sub.2 transcoder into a reconstituted output bitstream, such that most of the frames are processed by the much faster T.sub.1 transcoder, thereby achieving substantial improvement in the transcoding throughput speed while retaining a low error rate.

Code quantity control based on a virtual buffer is executed by using information extracted from picture compressed information conforming to a predetermined system. Thus, a reference quantization scale can be held at a fixed value throughout a frame. As a result, it is possible to generate picture compressed information having a smaller code quantity.

In a bitstream transcoder, the computational cost and the memory area are suppressed so that stream transcoding is performed with a reduced number of times of computing and a reduced memory area. The transcoder includes a VLD portion 11 for performing VLD on a bitstream encoded in MPEG-2 to output quantized DCT coefficients; an IQ portion 12 for receiving the coefficients, and outputting inverse-quantized DCT coefficients; a coefficient table 132 for storing a transformation matrix obtained by multiplying transformation matrices for operating band-limiting filter, IDCT, down-sampling and DCT, respectively, together in advance; a DCT scaling portion 13 for performing down-sampling on the DCT coefficients by use of the transformation matrix; a Q portion 15 for outputting quantized DCT coefficients down-sampled horizontally and vertically; and a VLC portion 16 for encoding the quantized DCT coefficients in MPEG-4 to output a bitstream.