Video coding for reliable communications

This thesis presents the results of an investigation into the transmission of coded or compressed digital video information over communications networks. Practical networks and transmission media have limitations in the “Quality of Service” that they can provide to transmitted data. The aim of this thesis is to examine the effect of Quality of Service limitations on the transmission of coded digital video and to investigate new techniques by which video quality can be improved in the presence of transmission impairments. The video coding algorithms are chosen from widely used International Standards for video coding and transmission. The effects of Quality of Service limitations during transmission of coded video are investigated experimentally. Based on the results of these experiments, several different techniques to improve the quality of coded video are proposed and tested. These techniques include video rate control algorithms, scalable or layered coding as a means of prioritising selected components of coded video and modifications to the structure of coded video frames in order to minimise the propagation of errors. Errors are introduced into coded video sequences and it is shown that their effects depend on the position of the error in the sequence. The error models include pseudorandom bit errors and cell errors as well as errors due to the User Network Interface in an Asynchronous Transfer Mode network. Prioritisation of selected components in coded video is investigated, by splitting coded video into several layers or by modifying the frequency of resynchronisation markers depending on the picture class of each video frame. The visual impact of transmission errors can be reduced in this way. The effect of encoding parameters (including quantizer scale, slice resynchronisation interval and intraframe interval) on the bit rate of encoded video is quantified. A modified rate control algorithm that jointly controls quantization and intraframe interval is implemented and is shown to have the potential to improve the visual quality of constant bit rate coded video. The results of these experimental investigations are discussed and evaluated in the context of a complete digital video communication system.