Encoding method, decoding method, and encoding apparatus for a digital picture sequence

Apparatus for digitally encoding a picture sequence, wherein the pictures of said picture sequence are arranged in macroblocks containing pixel blocks and the pictures are encoded in intra and predictive and/or bi-predictive coding types denoted I, P and B, respectively, and wherein said pictures form groups of pictures and each group includes one anchor picture and several non-anchor pictures, wherein a current picture is called an ‘anchor picture’ if all pictures that were previously encoded before this current picture precede it in display order, and a current picture is called a ‘non-anchor picture’ if there exists at least one previously encoded picture that follows the current picture in display order,

• and wherein adaptively, for the purpose of overall bit rate control, a specific picture target number of bits is assigned to these coding types, and wherein said overall bit rate control includes a picture-layer rate control and a macroblock-layer rate control which macroblock-layer rate control selects macroblock quantization parameters, said apparatus comprising:
• means for assigning a target number of bits to anchor pictures but not assigning a target number of bits to non-anchor pictures;
• means for coding the macroblocks of said anchor pictures using macroblock-layer rate control by adaptive macroblock quantization parameters, and for coding the macroblocks of said non-anchor pictures without macroblock-layer rate control by using fixed macroblock quantization parameters.

Method for digitally decoding an encoded picture sequence, wherein the pictures of said picture sequence are arranged in macroblocks containing pixel blocks and the pictures were encoded in intra and predictive and/or bi-predictive coding types denoted I, P and B, respectively, and wherein said pictures form groups of pictures and each group includes one anchor picture and several non-anchor pictures, wherein a current picture is called an ‘anchor picture’ if all pictures that were previously encoded before this current picture precede it in display order, and a current picture is called a ‘non-anchor picture’ if there exists at least one previously encoded picture that follows the current picture in display order,

• and wherein adaptively, for the purpose of overall bit rate control, a specific picture target number of bits was assigned to these coding types, and wherein said overall bit rate control included a picture-layer rate control and a macroblock-layer rate control which macroblock-layer rate control had selected macroblock quantization parameters, wherein a target number of bits was assigned to anchor pictures but was not assigned to non-anchor pictures,
• and wherein the macroblocks of said anchor pictures were coded using macroblock-layer rate control by adaptive macroblock quantization parameters, and the macroblocks of said non-anchor pictures were coded without macroblock-layer rate control by using fixed macroblock quantization parameters, said method including the step of:
• decoding said anchor pictures using correspondingly adaptive macroblock quantization parameters, and decoding said non-anchor pictures using only fixed macroblock quantization parameters.