Bitstream transcoder

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.

A bitstream transcoding method comprising:

• a variable length decoding (VLD) step of performing VLD on a bitstream encoded in a first compression method, and outputting quantized DCT coefficients;
• an inverse quantizing step of receiving said quantized DCT coefficients from said VLD step, performing inverse quantizing thereon, and outputting DCT coefficients;
• a horizontal DCT scaling step of receiving said DCT coefficients inverse-quantized by said inverse quantizing step, reading a transformation matrix from a coefficient table storing transformation matrices, and performing horizontal down-sampling on said inverse-quantized DCT coefficients by use of said transformation matrix, said transformation matrix being obtained by multiplying transformation matrices for operating band-limiting filter, IDCT, down-sampling and DCT, respectively, together in advance;
• a vertical DCT scaling step of receiving said DCT coefficients horizontally down-sampled by said horizontal DCT scaling step, reading a transformation matrix from said coefficient table, and performing vertical down-sampling on said DCT coefficients by use of said transformation matrix;
• a quantizing step of receiving said DCT coefficients down-sampled horizontally and vertically by said DCT scaling steps, quantizing said DCT coefficients, and outputting said quantized DCT coefficients;
• a motion vector scaling step of receiving motion vector values from said VLD step, and performing horizontal and vertical down-sampling thereon;
• and
• a variable length coding step of receiving said DCT coefficients down-sampled by said quantizing step and said motion vectors down-sampled by said motion vector scaling step, encoding said DCT coefficients and said motion vectors in a second compression method, and outputting a bitstream;
• wherein said horizontal DCT scaling step and said vertical DCT scaling step read down-sampling transformation matrices from said coefficient table in accordance with said DCT coefficients supplied, respectively, and computing with said transformation matrix.

An encoding apparatus comprising:

• an ME portion which receives a video signal for every macroblock which is a coding unit, selects a macroblock (hereinafter referred to as “reference macroblock”) closest to a latest coded frame, and outputting motion vectors;
• an MC portion which calculates a difference between said macroblock and said reference macroblock, and outputting a residual;
• an inverse discrete cosine transform portion which receives one of said macroblock and said residual, performing DCT thereon, and outputting DCT coefficients;
• a quantizer which receives said DCT coefficients from said inverse discrete cosine transform portion, quantizing said DCT coefficients, and outputting said quantized DCT coefficients;
• an inverse discrete cosine transformer (IDCT) which receives said quantized DCT coefficients, inverse-quantizing said quantized DCT coefficients, and performing IDCT thereon to thereby create a reference frame for a next frame;
• a variable length coder which receives said quantized DCT coefficients from said quantizing portion and said motion vectors from said ME portion, encodes said quantized DCT coefficients and said motion vectors in a first compression method, and outputting a bitstream;
• a horizontal DCT scaler which receiving said DCT coefficients from said inverse discrete cosine transform portion, and performs horizontal down-sampling thereon;
• a vertical DCT scaler which receives said horizontally down-sampled DCT coefficients from said horizontal DCT scaling portion, and performs vertical down-sampling thereon;
• a coefficient table which stores 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 constituted by said horizontal DCT scaling portion, said vertical DCT scaling portion and said coefficient table;
• a quantizer which receives said DCT coefficients, down-sampled horizontally and vertically, from said DCT scaling portion, quantizes said DCT coefficients, and outputs said quantized DCT coefficients;
• a motion vector scaler which receives motion vector values from said ME portion, and performs horizontal and vertical down-sampling thereon;
• and
• a variable length coder which receives said quantized DCT coefficients from said quantizing portion and said down-sampled motion vectors from said motion vector scaling portion, encoding said quantized DCT coefficients and said down-sampled motion vectors in a second compression method, and outputting a bitstream;
• wherein said bitstream encoded in said first compression method and said bitstream encoded in said second compression method are outputted.

An image information transcoding/transmitting method for performing down-sampling on image compressed information encoded in a first compression method to thereby transcode said image compressed information into image compressed information compressed in a second compression method, and transmitting said image compressed information compressed in said second compression method, comprising:

• a decoding/inverse-quantizing step of decoding and inverse-quantizing said image compressed information encoded in said first compression method, and outputting first DCT coefficients;
• a step of reading a transformation matrix from a storage portion storing said transformation matrix used for down-sampling, said first DCT coefficients being multiplied by said transformation matrix;
• a transformation step of multiplying said first DCT coefficients by said transformation matrix read from said storage portion, and outputting second DCT coefficients;
• a quantizing/encoding step of quantizing and encoding said second DCT coefficients, and outputting image compressed information encoded in said second compression method;
• and
• a step of transmitting, to a network, said image compressed information encoded in said second compression method.