Page 76 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 3.4. The H.263 Standard 53

the method of implementing the forward DCT. Threshold coding, discussed in
Section 2.6.2, is used to allocate bits to the transform coe,cients, as will be
discussed next.

3.4.3.6 Quantization

The six DC coe,cients of an INTRA MB are quantized using a uniform
scalar quantizer with a step size of
= 8 and no dead zone (this corresponds
to Figure 2.5(a) and Equation (2.8)). All other coe,cients are quantized using
a uniform scalar quantizer with a step size of
=2 × QP and a central dead
zone around zero (this corresponds to Figure 2.5(b) and Equation (2.10)).
There are 31 possible quantization parameters, QP = 1 ::: 31. However, the
quantization parameter is kept /xed for all coe,cients within an MB. A high
QP leads to higher compression but worse quality, whereas a low QP leads
to better quality but less compression. The method to select a QP is not part
of the standard. A change of QP to any of the 31 permissible values can
be signaled in the picture or GOB headers. In the MB header, however, this
change is limited to a maximum of ±2. Again, the method to decide this
change is not de/ned in the standard.

3.4.3.7 Quantized Coe5cients Coding

A quantized INTRA DC coe,cient is encoded using a standard 8-bit FLC
table. Other quantized coe,cients are /rst zigzag scanned, as described in
Section 2.6.2 and Figure 2.10(c). The reordered coe,cients are then encoded
using 3-D RLE. Thus, the reordered coe,cients are converted to an interme-
diate set of symbols or EVENTS of the form (LAST, RUN, LEVEL), where
LAST is an indication of whether this is the last nonzero coe,cient in the
block or not, RUN is the number of successive zeros preceding the coded co-
e,cient, and LEVEL is the nonzero value of the coded coe,cient. The most
commonly occurring EVENTs are coded using a standard VLC table, whereas
the remaining EVENTs are coded using a concatenation of four standard FLC
codewords for ESCAPE, LAST, RUN and LEVEL.

3.4.3.8 Coding Control
The coding control block is responsible for varying several parameters to con-
trol the rate or the quality of the coded video. Examples are the INTER=
INTRA mode decision at the picture or MB level, the update pattern of the
forced INTRA refresh, the TRANSMIT=SKIP decision at the MB level, and
the QP and its change at the picture, GOB, or MB level. Such functions are
not de/ned in the standard.