Pre/post-filter for performance improvement of transform coding

摘要

A technique for generating independent transform coefficients from any p-dependent signal has been developed. Since these coefficients are independent, the Lloyd-Max quantization efficiency is improved. In addition, these coefficients are shown to be Gaussian distributed. Therefore, the probability density function estimate is bypassed during the quantizer design. An all-pass (encryption) pre-filter is required by the proposed technique, and the filtered signal is obtained efficiently by an algorithm developed in this paper. An added benefit of this technique is the compatibility and higher security with respect to the conventional transform coding (TC), and the method is called the transform encryption coding (TEC). In addition, a post-filter is necessary for signal reconstruction. Due to the all-pass nature of the pre/post-filtering process, the mean-square quantization error for the complete process is equal to that for the intermediate independent Gaussian transform coefficients. Simulation results show TEC achieves about 1 dB coding gain compared with TC. Coded images without blocking effects are obtained at 0.5 bit/pixel (bpp) using TEC. The image quality is similar to that obtained by TC at 1 bpp. Even at 0.35 bpp, TEC performs in the same way as the recent ‘DCT/DST’ technique of Rose et al. (1990) and shows no blocking effects. Most significantly, TEC encoded image is insensitive and robust to the channel noise.