PURPOSE: A method and an apparatus for deciding quantization space are to decide the quantization space sufficiently reflecting a viewing characteristic, thereby increasing compression effect of information and also image quality. CONSTITUTION: An edge characteristic classifying portion(200) receives a luminance signal in an input video signal so that a dispersing value of the luminance signal of each sub block decides an edge classifying value according to the number of the sub blocks which is more than the desired first critical value. An equal characteristic classifying portion(202) calculates a minimum value out of the dispersing values of each block included in a macro block. In the equal characteristic classifying portion, if a value calculated by the minimum value divided by the dispersing value is larger than the second critical value, a value according to equal classification is output. If the value is less than the second critical value, a value according to unequal classification is output. A color signal characteristic classifying portion(204) calculates the number of pixels satisfying a well-known sensitivity deciding equation.
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