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Section 3.4. The H.263 Standard 49

3.4 The H.263 Standard

3.4.1 Introduction
The H.263 recommendation speci/es a coded representation that can be used
for compressing the moving picture component of audiovisual services at low
bit rates. The recommendation fully speci/es the decoder and the bitstream
syntax but does not explicitly specify the encoder. As already mentioned, this
gives manufacturers a large degree of freedom to propose their own optimized
encoders, as long as the output bitstream conforms to the standard decodable
syntax. However, during the standardization process, a software-based codec
(encoder-decoder) called the test model is developed to study the core ele-
ments of the standard. For example, version 5 of the test model near-term
(TMN) is described in Ref. 76.

3.4.2 Source Format

The standard supports all /ve members of the CIF family described in
Section 2.4.4 and Table 2.1. As a minimum requirement, all decoders shall be
able to operate with SQCIF and QCIF. Encoders, on the other hand, shall be
able to operate with either SQCIF or QCIF and are not obliged to be able to
operate with both.

3.4.3 Video Source Coding Algorithm

The generalized form of the source coder is illustrated in Figure 3.1.
It is a hybrid of interpicture prediction to utilize temporal redundancy and
transform coding of the error signal to reduce spatial redundancy.

3.4.3.1 Picture Coding Structure

The input video consists of a sequence of pictures (or frames). Each picture
is divided into groups of blocks (GOBs). A GOB consists of k × 16 lines,
depending on the picture format (k = 1 for SQCIF, QCIF, and CIF, k = 2 for
4CIF, and k = 4 for 16CIF). For example, there are 9 GOBs in a QCIF picture.
Each GOB is divided into macroblocks (MBs). A macroblock consists of

16 × 16 samples of Y and the spatially corresponding 8 × 8 samples of C B and

C R . If we de/ne a block as 8 × 8 samples of Y ;C ,or C , then a macroblock

R
B
consists of 6 blocks: 4 luma blocks and the 2 spatially corresponding chroma
blocks. Figure 3.2 illustrates the H.263 picture structure for a QCIF frame.
As shown, GOBs are coded from top to bottom in increasing number. Within
each GOB, the MBs are coded from left to right (and from top to bottom if
the GOB contains more than one row of MBs) in increasing number. Within