Patent ID: 7110451

Claim:
A bitstream transcoder comprising: a variable length decoder (VLD) which performs variable length decoding on a bitstream encoded in a first compression method, and outputs quantized DCT coefficients; an inverse quantizer (IQ) which receives said quantized DCT coefficients from said VLD portion, performs inverse quantizing thereon, and outputs DCT coefficients; a coefficient table which stores a transformation matrix obtained by multiplying transformation matrices for operating band-limiting filter, inverse DCT (IDCT), down-sampling and DCT, respectively, together in advance; a DCT scaler (DCT SCALE) which receives said DCT coefficients from said IQ portion and performs down-sampling thereon by use of said transformation matrix read from said coefficient table; a quantizer (Q) which receives said DCT coefficients, down-sampled horizontally and vertically, from said DCT SCALE portion, quantizes said DCT coefficients, and outputs quantized DCT coefficients; and a variable length coder (VLC) which encodes said quantized DCT coefficients from said Q portion in a second compression method, to output a bitstream, wherein a transformation matrix obtained by multiplying matrices for operating band-limiting filter, IDCT, converting from field DCT to frame DCT, down-sampling and DCT, respectively, together, is stored in said coefficient table in advance, and said transformation matrix is used for down-sampling when said DCT coefficients supplied are based on field DCT in which DCT is performed by field, so that DCT coefficients outputted are based on frame DCT in which DCT is performed by frame.