H.264/MPEG-4 Part 10 Advanced Video Coding
H.264 represents a revolutionary advance in video compression
technology. It is now confirmed as the clear winner over the relatively
new MPEG-4 Part 2 (SP, ASP) standard, as well as promising performance advantages
over today's leading proprietary codecs.
Its significant advance in coding efficiency (e.g. good video quality
at bitrates below 2 Mbps) makes it an enabling technology for a
wide range of new applications, such as streaming mobile TV, HDTV
over IP, extended PVR and storage options for the home user. This
international standard has risen in importance with its recent adoption
by the 3G, the DVD Forum, and DVB, joining MPEG-2 as one of the
world's most common digital video formats.
The efficiency gains provided by H.264/AVC enable new business models
by allowing operators to offer video services in environments where
they previously were not possible, or by packing more video channels
into scarce and expensive transmission bandwidth. These technologies
are very likely to be adopted in traditional entertainment video
applications (DVD, satellite TV, digital terrestrial TV, VOD, cable
TV, video over xDSL) and video delivery applications, such as streaming
video, wireless and digital cinema.
Extremely low licensing fees from the MPEG-LA licensing body will
also ensure a fast market take-up of this new standard.
Major Features of the AVC/H.264 Standard
The H.264 standard provides advanced coding techniques within similar
scheme as used by previous MPEG standards. Higher efficiency and quality
are enabled by new tools, which include:
Improved Motion Estimation and Inter-Prediction
Motion estimation provides for the searching of sub-macro blocks of variable
size, from 16x16 down to 4x4 blocks. Motion vectors allow up to quarter
pixel accuracy for luminance, and up to 1/8th pixel for chrominance.
Spatial Intra Prediction and Transform
H.264 carries out intra-prediction for intra coded blocks before the
transform, performed on either 4x4 or 16x16 blocks and allowing up to
9 directional modes for direction dependent prediction. Residual data
transforms are executed on 4x4 blocks with modified integer discrete
cosine transform (DCT) which avoids rounding errors. The employment
of an adaptive in-loop filter increases subjective quality of video.
CAVLC and CABAC Encoding Algorithms
The standard provides two alternative and more efficient processes
of entropy coding. Context-adaptive variable length coding (CAVLC)
utilizes multiple variable length codeword tables for transform coefficient
encoding considering spatial neighborhood of the coded block. Context-adaptive
binary arithmetic coding (CABAC) in addition provides highly efficient
automatic adjustment for underlying probability model of encoded data.
H.264 Profiles
As part of a simplified system, the H.264 standard provides for seven
profiles:
- Baseline Profile is targeted towards real-time encoding and decoding
for CE devices. Supports progressive video, uses I and
P slices, CAVLC entropy coding.
- Main Profile is targeted mainly towards the broadcast market.
Supports both interlaced and progressive video with macroblock
or picture level field/frame mode selection. Uses I, P, B slices, weighted
prediction,
both CAVLC and CABAC for entropy coding.
- Extended Profile is targeted towards error prone channels
(such as mobile communication). Uses I, P, B, SP, SI slices,
supports both interlaced and progressive video, allows CAVLC coding
only.
- High Profile extends functionality of main profile for
effective coding of high definition content. Uses adaptive
8x8 or 4x4 transform, enables perceptual quantization
matrices.
- High 10 Profile is an extension of High profile for
10 bit component resolution.
- High 4:2:2 Profile supports 4:2:2 chroma format and
up to 10 bit component resolution. Suitable for video
production and editing.
- High 4:4:4 Profile supports 4:4:4 chroma format
and up to 12 bit component resolution. In addition
it enables lossless mode of operation and direct coding of RGB
signal. Targeted for professional
production and graphics.
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