Page 73 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

50 Chapter 3. Video Coding: Standards

Flag for INTRA/INTER
Coding control
Flag for transmitted or not
Quantizer indication
Video
in Quantizing index for
- DCT Quantizer transform coefficients
Inverse
quantizer

Inverse
DCT
+
Picture memory with
motion-compensated
variable delay Motion vector

Figure 3.1: H.263 video encoder

each MB, the Y blocks are /rst coded in the order shown (left to right and

top to bottom), followed by the C block and then the C block.
R
B
3.4.3.2 Coding Modes
The coding mode in which interpicture prediction is applied is called the
INTER mode. Prediction can optionally be augmented by motion compen-
sation. If no prediction is applied, then the coding mode is called INTRA.
The coding mode (INTRA=INTER) can be signaled at the picture level (re-
sulting in I-pictures=P-pictures) or at the macroblock level in P-pictures. In
PB-frames (discussed later) the B-pictures are always coded in INTER mode.
The mode selection method is not de/ned by the standard. However, to control
3
the accumulation of IDCT mismatch error, the standard requires an MB to
be coded in INTRA mode at least once every 132 times when coe,cients are
transmitted for this MB in P-pictures. In the INTER mode, a Jag is used to
indicate whether an MB is transmitted or not (conditional replenishment). This
is sometimes referred to as the SKIP mode. Again, the method of reaching a

3 The inverse discrete cosine transform (IDCT) is a common block between the encoder and the
decoder. Di1erences in implementation between the encoder’s IDCT and the decoder’s IDCT cause
mismatches between the reconstructed pictures at both ends. This is called the IDCT mismatch
error. This mismatch accumulates due to interpicture prediction and can be stopped by forced
INTRA updating.