EECE 541 Multimedia Systems Project Proposal: Logo ... - Courses

Figure 2. Open-loop architecture.
Open-loop systems are relatively simple. They do not include motion estimation, motion
compensation, DCT, and inverse DCT (IDCT). Therefore, open-loop structures are
computationally very efficient, but they suffer from low video quality due to the drift
problem. The drift problem is caused by the mismatch between the actual reference frame
used for motion estimation in the encoder and the degraded reference frame used for
motion compensation in the decoder. The drift problem accumulates and causes severe
degradation to the video quality.
To avoid the high complexity problem in the cascaded architecture and the poor video
quality problem in the open-loop structure, closed-loop systems provide a good tradeoff
between quality and computational complexity. In closed-loop structure, significant
complexity saving can be achieved while still maintaining acceptable video quality.
Closed-loop systems provide drift compensation for re-quantized data. They aim at
eliminating the mismatch between predictive and residual components by approximating
the cascaded transcoding architecture. In the simplified closed-loop structure as shown in
Figure 3, only one reconstruction loop is required with one DCT and one IDCT. Some
architecture inaccuracy is introduced due to the non-linear nature in which the reconstruction
loops are combined. However, it has been found that the approximation has
little effect on the video quality.
Figure 3. Closed-loop architecture.
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