Research On Rate Control In Video Coding And Transcoding

Along with digital AV and multimedia technology developing, largenumbers of video datum need to be compressed. At present, the InternationalCoding Standards, such as MPEG-1, MPEG-2, MPEG-4, H.26x have beenestablished and applied to different application areas. In order to adapt toheterogeneous client terminals and transmission characteristic of networks,pre-encoded video should be transcoded into another coding format. So, theresources of networks can be shared.When the video datum is transmitted far from here, network congestion,channel bandwidth and disposal ability of terminal should be considered. Targetof rate control is to ensure the channel normal and obtain better image quality byadjusting the parameters of encoder.As the basis of the following research on transcoding and rate control,several related video coding standards are introduced in this thesis first. And then,the effect factors of the bit rate are analyzed. A few typical methods of ratecontrol are expatiated in the third chapter, which are theories basis of the newarithmetic.The research in this thesis starts with rate control in low bit rate. The newarithmetic includes bit allocation optimizing in frame layer, namely, differentbits are allotted to different frames in order to decrease the number of skippingframes. Using human visual character, visual modified factor and matrix areintroduced in macroblock layer. By analyzing the relationship between imagedistortion and quantizing order, along with bit stream syntax, a novel macroblocklayer rate control strategy is proposed. This strategy in which quantization orderis based on complexity is different from traditional method in which quantizationorder is based on scan order. The results of experiments show that subjectiveand objective qualities of the new method are improved.Before the research of rate control in transcoding, the transcodingframework is introduced first. The rate control scheme in heterogeneoustranscoding is presented by the encoding characters in different standards, bitstream architecture and decoded information analyzed and studied. For example,MPEG-2 transcoding to H.263/MPEG-4 is taken into account in this paper. Byutilized decoded information in MPEG-2 and combined rate control strategy inH.263/MPEG-4, the new rate control scheme implements effectively control andimproved subjective and objective quality.