Why
the buzz about H.264? It's the bitrate!
Concepts
H.264
is getting so much attention because it can encode video with approximately 3
times
fewer
bits than comparable MPEG-2 encoders. This opens new possibilities such as:
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Squeezing
more television programs into a given channel bandwidth;
Delivering
quality video over bandwidth-constrained networks (e.g., 3 and 4G Mobile),
Fitting
a high-definition movie feature onto a standard DVD.
Because
H.264 is up to twice as efficient as MPEG-4 Part 2 (natural video) encoding, it
has recently been welcomed into the MPEG-4 standard as Part 10 – Advanced Video
Coding. Many established encoder and decoder vendors are moving directly to
H.264 and skipping the intermediate step of MPEG-4 Part 2.
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If
you have some experience with video compression, the best way to appreciate the
buzz is to run a run a case yourself. An easy approach is to acquire our
Expert-H264 demo and interactively encode video content from a variety of
popular formats.
In
either case, the resulting .26L file will seem too small to be believed. When
you run the decoder and view the decoded file, you will be amazed at how good
the quality is at such a low bit rate.
Goals
& Approach of H.264
The
International Telecommunications Union (ITU) initiated the h.26L (for long
term) effort in 1998 as a continuation of work following the MPEG-2 and H.263
standards. The overriding goal was to achieve a factor-of-2 reduction in bit
rate compared to any competing standard.
Recall
that MPEG-2 was optimized with specific focus on Standard and High Definition
digital television services, which are delivered via circuit-switched head-end
networks to dedicated satellite uplinks, cable infrastructure or terrestrial
facilities. MPEG2's ability to cope is being strained as the range of delivery
media expands to include heterogeneous mobile networks, packet-switched IP
networks, and multiple storage formats, and as the variety of services grows to
include multimedia messaging, increased use of HDTV, and others. Thus, a second
goal for H.264 was to accommodate a wider variety of bandwidth requirements,
picture formats, and unfriendly network environments that throw high jitter,
packet loss, and bandwidth instability into the mix.
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The
H.264 approach is a strictly evolutionary extension of the block-based encoding
approach so well established in the MPEG and ITU standards. Key steps include:
Use
of Motion Estimation to support Inter-picture prediction for eliminating
temporal redundancies
Use
of spatial correlation of data to provide Intra-picture prediction.
Construction
of residuals as the difference between predicted images and source images.
Use
of a discrete spatial transform and filtering to eliminate spatial redundancies
in the residuals.
Entropy
coding of the transformed residual coefficients and of the supporting data such
as motion vectors.
Although
H.264 introduces carries several techniques that push the block based approach
up to the limits of its efficiency. It is not a fundamentally different
approach, but rather a significant refinement of well-established methods.
Major
Features of H.264
To
understand the new features of H.264, it is assumed that you are familiar with
the approach and terminology used in MPEG 1 & 2 – if not, there are many
fine books and reviews on this subject. For example, you can go to a useful
MPEG-2 tutorial for a summary of MPEG-2 video and systems.
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Now
we will summarize the key features and point you to some useful background
material. Note that some H.264 features not supported by initial profiles are
identified by an asterisk (*).
Improved
Inter-Prediction and Motion Estimation
First
recall the limitations of motion estimation in MPEG-2, which searches reference
pictures for a 16x16 set of pixels that closely matches the current macro
block. The matching set of pixels must be completely within the reference
picture. In contrast, H.264 provides:
Fine-grained
motion estimation. Temporal search seeks matching sub-macro blocks of variable
size as small as 4x4, and finds the motion vector to _ pel resolution. Searches
may also identify motion vectors associated with matching sub-macro blocks of
4x8, 8x4, 8x8, 8x16, 16x8, or the full 16x16. In future, even finer 1/8 pel resolution
will be supported.
Multiple
reference frames. H.264 provides additional flexibility for frames to point to
more than multiple frames – which may be any combination of past and future
frames. This capability provides opportunities for more precise
inter-prediction, but also improved robustness to lost picture data.
Unrestricted
motion search. Motion search allows for reference frames that may be partly
outside the picture; missing data can be spatially predicted from boundary
data. Users may choose to disable this feature by specifying a Restricted
Motion search.
Motion
vector prediction. Where sufficient temporal correlation exists, motion vectors
may be accurately predicted and only their residuals transmitted explicitly in
the bit stream.
Such
techniques not only provide for more accurate inter-prediction, but also help
to partition and scale the bitstream with priority given to data that is more
globally applicable. Thus, they not only improve compression but also
resilience to errors and network instabilities.
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CONTACT US
Company: Dingshengwei
Electronics Co., Ltd
Address: Bldg A, the
first industry park of Guanlong, Xili Town, Nanshan, Shenzhen, Guangdong, China
Tel: +86 0755 26909863
Fax: +86 0755 26984949
Mobile: 13410066011
Email : ken@soukacatv.com
Source:
pixeltools
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