Video signal decoding apparatus capable of reducing blocking effects

Claim 1

An apparatus for decoding an encoded digital image signal having a data signal and an information signal, wherein the data signal includes a plurality of blocks of quantized transformed coefficients which are outputs from an encoder performing a discrete cosine transformation(DCT) of a digital image signal and a quantization of the results from the DCT with a quantization step size(QS), each of the blocks including a DC coefficient and a predetermined number of AC coefficients, and said information signal includes each QS for each of the blocks, comprising: means for performing an inverse quantization on a subjective block with its corresponding QS to produce an inverse quantized DC coefficient and a plurality of inverse quantized AC coefficients; means, responsive to the inverse quantized DC coefficient and the corresponding QS, for determining a DC coefficient range in which all of the DC coefficients are mapped into the inverse quantized DC coefficient, and altering the value of the inverse quantized DC coefficient within the range to generate a predetermined number of candidate DC coefficients; means for performing an inverse discrete cosine transformation(IDCT) on each of the candidate DC coefficients combined with the inverse quantized AC coefficients; and means for comparing each of the results from the IDCT with previously performed IDCT results, so as to enable the selection of an IDCT result producing a minimum blocking effect which occurs because of a luminance pixel difference between the previously performed IDCT results and the selected IDCT result.