Abstract
The aim of this research is to enhance the quality of service in video applications when they are operating over error prone environments. With high compression ratio and low complexity block transform video coders, such as ITU-T H.263 standard algorithm for very low bit rate video coding, compressed video streams can be used for multimedia services. However, when transmitting compressed video streams over channels with degraded conditions, several problems arise and undermine the video decoder from correctly reconstructing the video signal. This thesis presents several techniques employed to enhance the quality of service of a video communication application under erroneous conditions. First, block transform methods for video coding are examined, and their strengths and weaknesses are assessed in terms of performance and error robustness. Packetised video signals are considered and two different techniques are implemented for packet video networks to improve the quality of service on one hand and help resolve the state of congestion that might occur on any video communication medium on the other hand. Additionally, zero redundancy error concealment techniques are considered and applied on the block based video decoder to improve the quality of the reconstructed video signal without any redundancy added on the video bitstream. Then, the aspects of error resilience issues in block transfom video coders are discussed. Based on the different categories of errors encountered in coded video streams, several novel techniques are implemented to render the video coder more immune to channel deterioration. Some of these techniques are combined to form an error resilience algorithm that is implemented on H.263 to enhance its performance over error prone environments such as mobile radio links. In parallel with the development process of the MPEG-4 video coder, we apply the two-way decoding with reversible codewords on the H.263 standard. Results are shown throughout the thesis to evidence the effectiveness of the proposed techniques and illustrate the improvement on the quality of service on both the objective and subjective scales. We conclude with thoughts for future expansion of error control strategies in block based video coding for mobile multimedia services over the foreseen universal mobile telecommunication systems (UMTS) network.