Page 70 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

Section 3.3. Chronological Development 47

comparisons indicate that H.263 provides the same subjective quality as H.261
but with less than half the bit rate [65]. This performance improvement is due
to optimized coding techniques as well as advanced optional coding modes.
Some of the new features of H.263 compared to H.261 are the support for
more picture formats, half-pel MC, a 3-D (LAST-RUN-LEVEL) RLE instead
of 2-D (RUN-LEVEL), more optimized VLC tables, optional extra head-
ers to increase error resilience, advanced 2-D median predictor for motion
vector coding, more optimized macroblock addressing and quantization adap-
tation, optional extended-range unrestricted motion vectors that can point out-
side frames, optional arithmetic coding, optional advanced prediction with
overlapped motion compensation and four motion vectors per macroblock,
and optional bidirectional prediction. H.263 is described in more detail in
Section 3.4.

3.3.8 H.263+

Technically, H.263+ is version 2 of the H.263 standard [66]. This version was
developed by ITU-T=SG16=Q15 Advanced Video Experts Group (previously
under ITU-T=SG15), with technical content completed in 1997 and approved
in 1998. The H.263+ standard added 12 new optional features to H.263. These
new features support custom picture size and clock frequency, improve com-
pression e,ciency, allow scalability, enhance error resilience over wireless
and packet-based networks, provide supplemental display and external usage
capabilities, and ensure backward compatibility. H.263+ is described in more
detail in Section 3.4.

3.3.9 MPEG-4

In 1993, the ISO=IEC JTC1=SC29=WG11 MPEG group initiated a new stan-
dardization activity called MPEG-4. The target was the very-low-bit range and
the aim was to achieve higher compression e,ciency than could be achieved
by existing conventional techniques. In 1994, it was realized that too few
improvements could be achieved over the H.263 and H.263+ compression
results to justify a new standard. Thus, the group decided to broaden the ob-
jectives of the MPEG-4 e1ort and started an in-depth analysis of the trends
within the audiovisual world. Particular attention was given to the convergence
of the three traditionally separate industries of communications, computing,
and TV=/lm=entertainment. This study concluded that MPEG-4 should sup-
port functionalities that would be useful in future applications but were not
supported or not well supported by the available standards. Eight main new
or improved functionalities were identi/ed and then clustered in three classes: