The first approach in this category is to intentionally add auxiliary information to the bitstream. The most illustrating example of this technique is the concealment motion vectors mode of MPEG2, an optional feature designed to provide robustness in error-prone channels. In this mode, motion estimation is performed for I frames, as well as for P and B frames. The respective motion vectors are sent and used by the decoder to motion compensate I-frames if errors are detected during their normal decoding.
A second approach is to restrict the prediction domain. This is applied in the
independent segment decoding mode of H.263, where spatial and temporal
prediction is confined within each GOB. With this way, the effect of error
propagation, is reduced significantly compared to the one illustrated in
Figure 4.1, as shown in Figure 4.3. The drawback is
that the compression efficiency is reduced as well, as restricted prediction
does not perform well compared to unrestricted.
