Low bit rates video coding using hybrid frame resolutions

This paper proposes a video coding scheme for low bit rates environments such as video streaming in internet or mobile networks. Since the annoying artifacts at low bit rates result mostly from the coarse quantization of residual signals, we try to reduce this problem by introducing the hybrid resolution scheme and allocating more bits to the transform coefficients. The intra frames are encoded in the original resolution for their importance, and the inter frames are downsampled and encoded by H.264 with finer quantization steps. The images of original resolution are reconstructed by using the decoded images of reduced resolution and previously reconstructed frames. Each block of original resolution is reconstructed by one of four modes, according to the combination of interpolation of the decoded block of reduced resolution, fetching of the previously reconstructed block of original resolution block, and use of the residual images of original resolution. And, the selection of mode is decided by the R-D optimization. The final bit stream consists of side information, bit stream of the H.264 for the reduced frames, and the residual signals of original resolution. Experimental results show that the proposed method yields better performance than the usual H.264/AVC up to some critical bit rates and the PSNR difference increases up to 3.5 dB at low bit rates. In addition, since the motion estimation is performed with the downsampled frames, the computational load for the overall encoding is much reduced. The proposed video coding system is especially efficient at low bit rates environment, thus it is suitable for video streaming in internet and mobile networks.