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Abstract

An image signal coding/decoding system comprises an encoder for encoding an image signal by using the adaptive quantization, and a decoder for decoding the coded image signal by using the adaptive inverse-quantization. The encoder comprises an activity detection element for detecting, every blocks obtained by dividing an image signal every a specific time length, activities indicating degree of changes in the signal in the respective blocks; a quantization class determination element for determining class values of a plurality of quantization classes set in correspondence with respective stages of the adaptive quantization from the detected activities; and for outputting the determined class values as class information to the decoder, a filter element for filtering not only the class values but also class values at the peripheral blocks; and a quantization step width determination element for determining, every blocks, widths of quantization steps corresponding to a value obtained by multiplying these class values by fixed coefficients on the basis of class values subjected to filtering. The decoder comprises a decode element for decoding coded data transmitted from the encoder by using the variable-length codes, and an inverse-quantization element for inverse-quantizing the variable-length decoded signal on the basis of class information transmitted from the quantization class determination element.

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