ENCODER AND DECODER FOR IMAGE SIGNAL

摘要

PURPOSE: To provide the encoder and decoder for image signal with which the degradation of decoded picture quality can be suppressed to a minimum and transmission/ storage efficiency can be improved. CONSTITUTION: Concerning the encoder for image signal with which encoding is performed by dividing a digital image signal into blocks composed of plural picture elements and a coefficient generated by orthogonally transforming this block into plural hierarchies for each degree of importance, this device is provided with encoding means 101-723 for performing encoding in the preferential hierarchy while defining the coefficient having an absolute value larger than a certain threshold value as an important coefficient and an encoding means 102-714 for performing encoding in the non-preferential hierarchy while defining the coefficient excepting for the important coefficient as a unimportant coefficient. Therefore, when a value is large, not only the quantizing coefficient corresponding to a low spatial frequency but also a quantizing coefficient corresponding to a high spatial frequency are arranged in the preferential hierarchy. At the same time, since all the quantizing coefficients having absolute values smaller than the threshold value are arranged in the non-preferential hierarchy, the ratio occupied by the information of the preferential hierarchy in the whole is reduced.