A method is provided for transcoding between video signals in two standards,
DV
and MPEG-2, each standard including discrete cosine transform (DCT) compressed
signals. The each of the signals have macroblocks containing a plurality of DCT
blocks. The DCT blocks are quantized according to respective quantization methods
defined by the standards. The coefficients in each block are zigzag scanned, run-length
coded and variable-length coded. The process variable-length decodes the coefficients
and translates the quantized coefficients in the DV standard into quantized coefficients
in the MPEG standard without fully dequantizing at least some of the DV coefficients
and without performing an inverse DCT operation on any of the DCT coefficients.
DV blocks that are encoded in a 248 format are translated into an 88 format before
they are converted to MPEG-2 blocks. A method for transcoding from MPEG-2 to DV
is also described. The MPEG-2 signals are intra-frame encoded, have a 4:2:2 chrominance
format and an 88 frame-encoded block format. According to this method, converted
88 DV blocks that represent significant intra-field motion are converted from the
88 format to a 248 format. The method also controls which overflow coefficients
in the DV signal are transcoded into corresponding coefficients in the MPEG-2 signal
to control the data rate of the MPEG-2 signal.