Quantization noise which is added to reconstructed picture by quantization degrades picture quality. Because reconstructed picture quality is decided visually, the effect of quantization noise in picture is dependent on subjective point as well as kinds of picture. And the subjective effect of quantization noise is varied by spacial luminance difference and temporal movement of picture. Therefore subjective quantizer is designed by consideration of spacial and temporal characteristic of picture.
First, for intraframe DPCM coder, the MSSE of average subjective quantizer which is considered in point of picture characteristic is lower than that of previous subjective quantizer which has been called subjective quantizer up to now by 2.5 dB.
Secondly, in interframe DPCM coder, because intraframe subjective quantizer is designed by intraframe activity function represented spacial luminance difference of picture, intraframe subjective quantizer can not used in interframe DPCM coder. Hence interframe subjective quantizer is designed by interframe activity function defined as spacial luminance difference according to picture movement. In performance comparision, the MSSE of interframe average subjective quantizer is lower than that of interframe previous subjective quantizer and that of intraframe previous subjective quantizer by 2.6dB, 8.3dB respectively.
