TABLE OF CONTENTS The source encoder of H.263 follows the general structure of the generic DCT-based interframe coding technique used in the H.261 and MPEG-1 codecs (see Figure 3.18). The core H.263 employs a hybrid interpicture prediction to utilise temporal redundancy and transform coding of the residual signal to reduce spatial redundancy. The decoder has motion compensation capability, allowing optional incorporation of this technique at the encoder. Half pixel precision is used for the motion compensation, as opposed to the optional full pixel precision and loop filter used in Recommendation H.261. In the new versions of H.263, use of quarter and even one eighth of pixel precision are recommended [6].
Perhaps the most significant differences between the core H.263 and H.261/MPEG-1 are in the coding of the transform coefficients and motion vectors. In the following sections these and some other notable differences such as the additional optional modes are explained.
In H.261 and MPEG-1, we saw that the transform coefficients are converted via a zigzag scanning process into two-dimensional, run and index events (see section 6.4). In H.263 these coefficients are represented as a three-dimensional event of (last, run, level). Similar to the two-dimensional event, the run indicates the number of zero-valued coefficients preceding a nonzero coefficient in the zigzag scan, and level is the normalised magnitude of the nonzero coefficient which is sometimes called index. last is a new variable to replace the end of block (EOB)...
