Information Technology Reference
In-Depth Information
Ghanbari et. al. in [2] gives the background and history of various coding
standards. The first video-coding standard developed by CCITT (now ITU-T) in
1984 was H.120. Shortly, afterwards, a new standard H.261 was developed and
then H.120 was abandoned. H.261 becomes the originator of all modern video
compression standards. Core of all the standards from H.261 onwards is almost
same and is as shown in Figure 1 and Figure 2. Some salient features of this codec
are the following:
•
16×16 MB motion estimation/compensation
•
8×8 DCT
•
Zig-zag scanning of DCT coefficients
•
Scalar quantization of those coefficients, and subsequent variable length
coding (VLC)
•
Loop filter
•
Integer-pixel motion compensation accuracy
•
2-D VLC for coding of coefficients
Later the H.261 was overtaken by H.263.
The first coding standard developed by ISO for motion pictures was MPEG-1
(1991). MPEG-1 provides the following features:
•
Bi-directional prediction for B-pictures
•
Half-pixel motion estimation
•
Better quality at higher bit rates
ISO and ITU-T jointly developed a coding standard MPEG-2 in the period
1994/95. MPEG-2 supports two new features that are as follows:
•
Interlaced scan pictures
•
Scalability
In 1995, H.263 was first developed to replace H.261 as the dominant video
conferencing codec. H.263 supports following features:
•
Half-pixel motion compensation
•
3-D VLC of DCT coefficients
•
Median motion vector prediction
•
Optional enhanced modes such as increased motion vector range,
advance prediction mode with Overlapped Block Motion Compensation
(OBMC)
•
Arithmetic entropy coding
•
Coding of PB frames to reduce coding overhead at low bit-rates.
MPEG-4 developed in early 1999 follows the H.263 design. Besides including all
prior features, it also has the following features:
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