Apparatus and method for preventing repetitive random errors in transform coefficients representing a motion picture signal

Apparatus for quantizing coefficient blocks of DCT coefficients representing a motion picture signal to prevent repetitive mismatch errors, the coefficient blocks including a current coefficient block derived from a current picture block having a position in a current picture, and a previous coefficient block derived from a previous picture block located in a previous picture at the position of the current picture block in the current picture, the current picture being a P-picture, the previous picture being one of a P-picture and an I-picture processed immediately preceding the current picture, the apparatus comprising:

• step size control means for generating a corrected step size for quantizing each coefficient block, the step size control means including:
• step size calculating means for calculating a current step size for the current coefficient block,
• equality determining means for determining when the current step size calculated for the current coefficient block by the step size calculating means is equal to a previous step size to generate an equality indication, the previous step size being a step size with which the previous coefficient block is quantized,
• step size setting means for setting the corrected step size to be different from the current step size in response to the equality indication from the equality determining means, and otherwise for setting the corrected step size to be equal to the current step size;
• and
• a quantizer, the quantizer receiving each coefficient block, and receiving the corrected step size for the coefficient block from the step size control means.

Apparatus for compressing a motion picture signal, the motion picture signal including successive pictures comprising picture blocks, the pictures including a previous picture and a current picture, the current picture including a current picture block having a position in the current picture, the previous picture including a previous picture block located therein at the position of the current picture block in the current picture, the apparatus comprising:

• means for performing motion compensation on the previous picture to derive a block of differences between the current picture block and the previous picture;
• means for orthogonally transforming the block of differences to provide a current coefficient block;
• step size control means for determining a corrected step size for quantizing the current coefficient block, the step size control means receiving information indicating a previous step size used to quantize a previous coefficient block derived from the previous picture block, the step size control means including:
• step size calculating means for calculating a current step size for the current coefficient block,
• equality determining means for determining when the current step size calculated for the current coefficient block by the step size calculating means is equal to the previous step size to generate an equality indication, and
• step size setting means for setting the corrected step size to be different from the current step size in response to the equality indication from the equality determining means, and otherwise for setting the corrected step size to be equal to the current step size;
• and
• a quantizer, the quantizer receiving the current coefficient block and the previous coefficient block, and receiving the corrected step size for the current coefficient block from the step size control means.