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Video Compression with Intra/Inter Mode Switching and a Dual Frame Buffer

Standard video coders often use the immediate past frame as a reference frame with motion compensation for video encoding. In our research, we have used dual reference frame motion compensation in the context of high bandwidth to low bandwidth switching such as from an Ethernet connection to a GPRS system. The implementation is based on MPEG-4. Simulation results show that there is a significant gain in the PSNR for relatively static video sequences.

To evaluate the effectiveness of the dual frame buffer technique, we simulated it by modifying the standard MPEG-4 coder. We allocated additional memory for the long-term frame. An extra bit is transmitted per inter coded MB to inform the decoder which frame it referenced. The intra refresh period was set to 100. Lowering the intra refresh period enhanced the performance of the dual frame encoder, but frequent intra refresh results in higher bit rates, which would exceed the bit rates available for a GPRS system. As inputs, we used the News, Container and the Foreman sequences. To investigate the effects of switching to different low bandwidth networks, we simulated switching from 1 Mbps to low bandwidth networks ranging from 10 kbps (GPRS) to 150 kbps (1xRTT CDMA). Each sequence was encoded employing our dual frame buffer coder as well as by a conventional MPEG-4 coder for comparison.

We found that retaining the high quality frame to be used as the long term past frame for the dual frame encoder results in better video quality as quantified by the PSNR of the decoded sequence at a small cost in memory to retain the dual reference frame.

In this sub-project, we explored end-to-end loss differentiation algorithms (LDAs) for use with congestion-sensitive video transport protocols for networks with either backbone or last-hop wireless links. As our basic video transport protocol, we used UDP in conjunction with a congestion control mechanism extended with an LDA. For congestion control, we used the TCPFriendly Rate Control (TFRC) algorithm. We extended TFRC to use an LDA when a connection uses at least one wireless link in the path between the sender and receiver. We then evaluated various LDAs under different wireless network topologies, competing traffic, and fairness scenarios to determine their effectiveness. In addition to evaluating LDAs derived from previous
work, we also proposed and evaluated a new LDA, ZigZag, and a hybrid LDA, ZBS that selects among base LDAs depending upon observed network conditions.

We evaluated these LDAs via simulation, and found that no single base algorithm performs well across all topologies and competition. However, the hybrid algorithm performed well across topologies and competition, and in some cases exceeded the performance of the best base LDA for a given scenario. All of the LDAs were reasonably fair when competing with TCP, and their fairness among flows using the same LDA depended on the network topology. In general, ZigZag
and the hybrid algorithm were the fairest among all LDAs.

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