Abstract: In a method for quantizing a moving picture signal and a circuit therefor, the method includes the steps of calculating horizontal and vertical gradients of first pixel values in a macroblock, determining each maximum value of the horizontal and vertical gradients, classifying the moving picture signal into complexity class if the maximum value is greater than a first threshold value and into edge class if the maximum value is greater than a second threshold value and is less than a third threshold value, calculating an activity value with respect to the first pixel values in the macroblock by calculating the sum of absolute values of DCT coefficients for four blocks in the macroblock, calculating an activity normalization coefficient, using the activity value of the macroblock and an average activity value of all pixels of the same picture type; and calculating a reference quantization step size produced by a backward rate control through a forward estimation considering coding efficiency and a visual charac

Abstract: A motion image coder is provided with a pre-filter for performing a pre-filtering which adaptively restricts the band for characteristics of the coded blocks. The pre-filtered result is used in determining quantization step size, thereby halving the quantization error causing a picture quality deterioration into a resolution error. Therefore, picture quality can be improved.

Abstract: In an encoder having a data formatter, a DCT circuit, a transmission rate controlling apparatus, a quantizer, a variable-length coder and an encoder buffer, for compressing a moving picture signal, the transmission rate controlling apparatus includes a segment complexity estimator for receiving outputs of the quantizer and variable-length coder and estimating the complexity of a segment of a current frame relative to a previous frame, a rate control adjustment coefficient calculator for receiving the output of the segment complexity estimator and calculating a rate control adjustment coefficient, a reference quantization coefficient calculator for receiving the rate control adjustment coefficient and the output of the encoder buffer and calculating a reference quantization coefficient, and an activity calculator for receiving the output of the data formatter, calculating activity by a macroblock unit, performing an operation on the calculated activity using the output of the reference quantization coefficient