A
History
of
Existing
Visual Coding Standards
H.
120:
The first international digital video coding stan-
dard [3]. It may have even been the first international digital
compression standard for natural continuous-tone visual
content of any kind (whether video or still picture). H.120
was developed by the ITU-T organization (the International
Telecommunications Union-Telecommunications Stan-
dardization Sector, then called the CCIIIT), and received
fi-
nal approval in 1984.
It
originally was a conditional
replenishment (CR) coder with differential pulse-code mod-
ulation (DPCM), scalar quantization, and variable-length
coding, and it had an ability
to
switch
to
quincunx
sub-sampling for bit-rate control. In 1988, a second version
of H. 120 added motion compensation and background pre-
diction. (None of the later completed standards have yet in-
cluded background prediction again, although a form of
it
is
in the draft
of
the future MPEG-4 standard.) Its operational
bit rates were 1544 and 2048 Kbit/s. H.120 is essentially
no
longer in use today, although a few H.120 systems are ru-
mored
to
still be in operational condition.
H.261:
The first widespread practical success-a video
codec capable of operation at affordable telecom bit rates
(with 80-320 Kbit/s devoted
to
video) [4,5].
It
was the first
standard
to
use
the basic typical structure we find still pre-
dominant today
(16
x
16 macroblock motion compensation,
8
x
8
block DCT, scalar quantization, and two-dimensional
run-level variable-length entropy coding). H.261 was ap-
proved by the ITU-T in early 1991 (with technical content
completed in late 1990).
It
was later revised in 1993
to
in-
clude a backward-compatible high-resolution graphics
transfer mode. Its target bit-rate range was 64-2048 Kbit/s.
JPEG:
A highly successful continuous-tone, still-picture
coding standard named after the Joint Photographic Experts
Group that developed it
[
1,2]. Anyone who has browsed the
world-wide web has experienced JPEG. JPEG
(IS
10918-l/ITU-T
T.81)
was originally approved in 1992 and
was developed as an official joint project of both the
ISO/IEC JTCl and ITU-T organizations. In its typical use,
it is essentially H.261
INTRA
coding with prediction of aver-
age values and an ability
to
customize the quantizer recon-
struction scaling and the entropy coding
to
the specific
picture content. However, there is much more in the JPEG
standard than what is typically described or used. In particu-
lar, this includes progressive coding, lossless coding, and
arithmetic coding.
MPEG-1:
A widely successful video codec capable of ap-
proximately VHS videotape quality or better at about 1.5
Mbit/s and covering a bit rate range of about 1-2 Mbit/s [6,
71. MPEG-1 gets its acronym from the Moving Pictures Ex-
perts Group that developed it [6,
71.
MPEG-1 video
(IS
11
172-2) was a project of the ISO/IEC JTCl organization
and was approved in 1993. In terms of technical features, it
added bi-directionally predicted frames (known as
B-frames) and half-pixel motion. (Half-pixel motion had
been proposed during the development of H.261, but was
apparently thought
to
be
too
complex at the time.)
It
pro-
vided superior quality than H.261 when operated at higher
bit rates. (At bit rates below, perhaps,
1
Mbit/s, H.261 per-
forms better, as MPEG-1 was not designed
to
be capable of
operation in this range.)
-
MPEG-2:
A step higher in bit rate, picture quality, and
popularity. MPEG-2 forms the heart of broadcast-quality
digital television for both standard-definition and
high-definition television (SDTV and HDTV)
[
7-91.
MPEG-2 video
(IS
13818-2/ITU-TH.262)
was
designed
to
encompass MPEG-1 and
to
also provide high quality with
interlaced video sources at much higher bit rates. Although
usually thought of as an
IS0
standard, MPEG-2 video was
developed as an official joint project of both the ISO/IEC
JTCl and ITU-T organizations, and was completed in late
1994. Its primary new technical features were efficient han-
dling of interlaced-scan pictures and hierarchical bit-usage
scalability. Its target bit-rate range was approximately 4-30
Mbit/s.
H.263:
The first codec designed specifically
to
handle
very low-bit-rate video, and its performance in that arena
is still state-of-the-art [lo,
111.
H.263 is the current best
standard for practical video telecommunication. Its orig-
inal target bit-rate range was about 10-30 Kbit/s, but this
was broadened during development
to
perhaps at least
10-2048 Kbit/s as it became apparent that
it
could be
su-
perior
to
H.261 at any bit rate. H.263 (version
1)
was a
project of the ITU-T and was approved in early 1996
(with technical content completed in 1995). The key new
technical features of H.263 were variable block-size mo-
tion compensation, overlapped- block motion compensa-
tion (OBMC), picture-extrapolating motion vectors,
three-dimensional run-level-last variable-length coding,
median MV prediction, and more efficient header infor-
mation signaling (and, relative
to
H.261, arithmetic cod-
ing, half-pixel motion, and bi-directional
prediction-but the first of these three features was also
found in JPEG and some form of the other two were in
MPEG-1). At very low bit rates (e.g., below 30 Kbit/s),
H.263 can code with the same quality as H.261 using
half or
less
than half the bit rate
[
121. At greater bit rates
(e.g., above
80
Kbit/s) it can provide a more moderate
degree of performance superiority over H.261. (See also
H.263
+
below.)
H.263+:
Technically a second version of H.263
[
10,
131. The H.263+ project added a number of new op-
tional features
to
H.263. One notable technical advance
over prior standards is that H.263 version 2 was the first
video coding standard
to
offer a high degree of error re-
silience for wireless or packet-based transport networks.
H.263+ also added a number of improvements in com-
pression efficiency, custom and flexible video formats,
scalability, and backward-compatible supplemental en-
hancement information. It was approved
in
January of
1998
by the ITU-T (with technical content completed in
September 1997). It extends the effective bit-rate range
of H.263
to
essentially any bit rate and any progres-
sive-scan (noninterlace) picture formats and frame rates,
and H.263+ is capable of superior performance relative
to
any existing standard over this entire range. The first
author was the editor of H.263 during the H.263
+
pro-
ject and is the Rapporteur (chairman) of the ITU-T Ad-
vanced Video Coding Experts Group (SG16/Q15),
which developed it.
NOVEMBER
1998
IEEE SIGNAL PROCESSING MAGAZINE
75
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