Comparative Study of Partial Closed-loop Versus Open-loop Motion Estimation for Coding of HDTV

Motion estimation is a vital function of video coders that utilize motion compensated prediction to exploit the temporal redundancy of video signals. The reference images used in the motion estimation process can be either original images (open-loop architecture) or reconstructed images (closedloop architecture). While the closed-loop architecture is intuitively superior to the open-loop architecture, closed-loop motion estimation is more difficult to implement in a realtime video encoder. A compromise is to perform open-loop integer-pel accurate motion estimation followed by closedloop half-pel accurate estimation. In this paper we present a comparative study of the performance of half-pel closed-loop versus open-loop motion estimation for coding of High Definition Television with the MPEG algorithm. Simulation results indicate that, on average, closed-loop half-pel accurate motion estimation provides slightly better performance. This improvement depends on the bit rate of the coded video stream. For relatively high bit rates, the closedloop implementation often provides no improvement and sometimes an actual degradation of performance. Analysis of why a closed-loop implementation does not guarantee a better performance is presented.