Cryptography Reference
In-Depth Information
2.5.2 H.261
During the late 1980s study period, of the 15 block based videoconferen-
cing proposals submitted to the ITU-T, 14 were based on the Discrete Cosine
Transform (DCT) and only one on Vector Quantization (VQ). In the late 1980s
it was clear that the combination of Inter frame predictive coding and the Intra
frame DCT based coding greatly improved picture quality over H.120. This
effort led to the first practical digital video coding standard. The H.261 [18]
design was a pioneering effort, laid the framework for all subsequent interna-
tional video coding standards (MPEG-1/2/4, H.262/3/4). H.261 video coding
standard was designed and optimized for low target bit rate applications suit-
able for transmission of color video over ISDN at p64 kbits/s with low
delay; here p specifies an integer with values between 1 and 30, This allows
the transmission of H.261 video over more than one ISDN channel. H.261 uses
DCT based Intra frame coding to removes the spatial redundancy, and motion
compensation based Inter frame coding to remove the temporal redundancy.
Other details of the standards are listed in the following.
• Frame formats are CCIR 601 CIF (352288) and QCIF (176144).
• A macroblock spans a 1616 pixel area with 4 Y blocks and one C
r
block
and one C
b
block.,sothevideois4:1:1sub-sampled.
• I-frames use 88 DCT transform followed by entropy coding.
• P-frames use motion compensated predictive coding. The reference frame
is always the previous I-frame.
• No bi-directional predictive coding
• A uniform quantization is used instead of using a quantization table.
From the above list we can see clearly that almost all the techniques used
in H.261 can be found in late standards in one way or another. The continuous
efforts in improving coding e
ciency have resulted in significant improvements
in coding e
ciency, and a new standard, H.263, has been created. H.263 re-
tains most of the functions the H.261 has, and is backward-compatible to it.
This makes the H.261 essentially obsolete. H.261 is a major historical mile-
stone in the development of video coding standards.
2.5.3 MPEG-1
The ISO/IEC Moving Picture Experts Group (MPEG) was formed in 1988. Its
first video coding standard, MPEG-1 [19] was originally designed for typical
applications using non-interlaced video PAL system and NTSC system for
video quality comparable to that of Video Home System (VHS). The bit rate
ranges from 1.2 to 1.5 Mbits/s, which was the maximum data rate of the CD-
ROM at that time. Besides the multimedia CD-ROM applications, it is also
a popular video format on the Internet in early days. In addition, MPEG-1
is the standard of the video coding in Video CD, which is the most popular
video distribution format throughout Asia after VHS video cassette.
