sbs_video.txt

huffman编码压缩

at compressing video data than MPEG-2, this has not been enough of an advantage to
convert the whole broadcast industry to the MPEG-4 format.
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MPEG-4’s role will likely remain in lower-bandwidth applications in the desktop computer,
Internet, and cell phone worlds, and also in new applications where a 15% compression
improvement over MPEG-2 is desired and MPEG-2 compliance is not an issue.
MPEG-4 is actually a super-set of MPEG-2, so MPEG-4 players which decompress the video
stream can theoretically play both MPEG-2 and MPEG-4 formats. However, for secondary
reasons, this may not be true in practice. This initial MPEG4 standard is also called MPEG4
Part 2, which is also often confused with MPEG4 Part 10 below.
H.264/ AVC (also called MPEG4 Part 10 )
With MPEG-4 failing to considerably improve compression performance for full broadcast
signaling, another effort was initiated late in the 1990s. This new effort, H.264, is able to
achieve a 2:1 improvement over MPEG-2 on full quality SDTV and HDTV, and is expected to
come into wide use in satellite and cable TV over the next decade.
H.264/MPEG4-AVC is a jointly developed standard by the ITU-T Video Coding Experts Group
(VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG) and has been standardized
by the ITU under the H.264 name, and is also called MPEG-4 Part 10 AVC (Advanced Video
Compression) even though it is unrelated in operation to MPEG-4.
The main goals of the H.264/ MPEG4-AVC standardization effort are to provide significantly
enhanced compression performance and a network-friendly packet-based video
representation addressing conversational (i.e., video telephony) and non-conversational (i.e.,
storage, broadcast, or streaming) applications.
H.264 uses techniques fairly different from MPEG-2 and can match the best MPEG-2 quality
at up to half the data rate. H.264 also delivers excellent video quality across the entire
bandwidth spectrum — from 3G to HDTV and everything in between (from 40 Kbps to
upwards of 10 Mbps). Efficient encoders and decoders for H.264 are just coming into use in
2005.
The H.264 design incorporates a Video Coding Layer (VCL), which provides the core highcompression
of the video content, and a Network Abstraction Layer (NAL), which packages
that compressed content for delivery over networks. The VCL design has achieved a
significant improvement in rate-distortion efficiency, providing nearly a factor of two in bit-rate
savings against existing standards. The NAL designs are being developed to transport the
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coded video data over existing and future networks such as circuit-switched wired networks,
MPEG-2/ H.222.0 transport streams, IP networks, and 3G wireless systems.
H.264 contains a number of features that allow it to compress video much more effectively
than older codecs over a wide variety of network environments. Key H.264 features include:
· Multi-picture motion compensation using previously-encoded pictures as references
in a much more flexible way than in past standards, thus allowing up to 32 reference
pictures to be used in some cases (unlike prior MPEG standards, where the limit was
typically one or two in the case of conventional B pictures).
· Variable block-size motion compensation (VBSMC) with block sizes as large as
16×16 and as small as 4×4, enabling very precise segmentation of moving regions.
· An in-loop deblocking filter which helps to prevent the blocking artifacts common to
other DCT-based image compression techniques used in MPEG standards.
· A secondary Hadamard transform performed on Discreet Cosine coefficients of the
primary spatial transform to obtain even more compression in smooth regions.
· A network abstraction layer (NAL) definition allowing the same video syntax to be
used in many network environments, including features such as sequence parameter
sets (SPSs) and picture parameter sets (PPSs) that provide more robustness and
flexibility than provided in prior standards.
· Frame numbering, a feature that allows the creation of "sub-sequences" (enabling
temporal scalability by optional inclusion of extra pictures between other pictures),
and the detection and concealment of losses of entire pictures (which can occur due
to network packet losses or channel errors).
· Picture order count, a feature that serves to keep the ordering of the pictures and the
values of samples in the decoded pictures isolated from timing information allowing
timing information to be carried, controlled and changed separately by a system
without affecting decoded picture content.
JPEG 2000
The MPEG and H.264 standards relate primarily to motion video. For still pictures, the familiar
JPEG standard developed by the Joint Photographic Experts Group committee has been in
use for some years, and it is just now gradually being replaced by the JPEG committee's
improved JPEG 2000 standard, which was released in the year 2000.
JPEG 2000 is mentioned in this video overview because, even though JPEG 2000 is
designed for still picture use, Part 3 of the JPEG 2000 standard–Motion JPEG 2000–also
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provides for motion video. Motion JPEG 2000 adds a mechanism to the JPEG 2000 standard
for sending JPEG 2000 images in a video stream with support for associated audio. Since the
MJ2 (Motion JPEG 2000) format does not involve inter-frame coding and each frame is coded
independently, high-quality frame-based video recording and editing based on the high
quality of JPEG 2000 compression is possible.
JPEG2000 uses ‘wavelet’ compression technology rather the DCT technology used in the
MPEG and JPEG standards. DCT compresses an image into 8x8 pixel blocks and places
them consecutively in the file. The blocks are compressed individually, without reference to
the adjoining blocks, resulting in the blocky look associated with compressed JPEG files.
With high levels of compression, only the most important information is used to convey the
essentials of the image and much of the detail is lost, lowering the dynamic range of an
image.
In contrast, JPEG 2000 wavelet compression converts the image into a series of wavelets
that can be stored more efficiently than pixel blocks. Wavelet algorithms compress the entire
image with ratios of up to 300:1 for color and 50:1 for gray scale. Wavelet compression also
supports non-uniform compression, where specified parts of the image can be compressed
more than others.
JPEG 2000 is able to render pictures better by eliminating the blockiness that is a common
feature of DCT compression. Not only does JPEG 2000 exhibit smoother color toning and
clearer edges where there are sharp changes of color, JPEG 2000 also produces smaller
image file sizes than JPEG image files with the same level of compression.
The advantage of JPEG 2000 becomes apparent when high compression ratios are required;
when 2 bits per pixel are available, both standards provide a comparable image quality.
However, when this reduces to 0.5 bits per pixel available, JPEG 2000 still produces a usable
image, whereas JPEG does not.
The advantages of using Motion JPEG 2000 for video are:
· Low latency compared to MPEG streams which use ‘P’ and ‘I’ frames.
· For DVR applications, every image is self-contained and complete; no need
to reconstitute from P and I frames.
The disadvantages are:
· Lower compression ratios than MPEG algorithms.
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· Requires more computing power for decoding, so hardware-assisted
decoding via FPGAs, DSPs and ASICs are required. Software-only decoders
are not currently acceptable for full broadcast quality.
Digital Video Transmission Formats
When digital video is transmitted, a number of different formats are in use, as follows:
Broadcast Studios
SDI as mentioned above
· SMPTE259M for SD (Standard Definition) Video at 270 megabits/ second.
· SMPTE292M for HD (High Definition) at 1.54 gigabits/ second.
· SMPTE310M for sending MPEG-2 digitally in a broadcast studio.
Between Remote Computers, Over Ethernet
· Compressed formats: MPEG-2, MPEG-4, Motion JPEG 2000.
· These sent on RTP (Real Time Protocol) over UDP (User Datagram Protocol)
over IP over Ethernet.
· Typical data rates for compressed video are in the range of 0.1 megabits/ second
(low resolution teleconferencing and webcasts) to 4 mbits per second (DVD
quality video) on up to 19 megabits/ second (HDTV).
· With these rates, it is possible to send multiple streams of video over Ethernet
links (even 100BaseT).
It is also possible to send lower-end video over Internet links down to 1 megabit/
second rates.
Compressed Video to Home
Both of the following standards involve converting digital video back to analog to fit into
standard analog 6 MHz channels as available on cable TV and satellite TV systems.
Digital TV set-top boxes implement one of these standards:
ATSC (Advanced Television Systems Committee)
· Actually a set of standards from A/52 to A/90.
· For HD, includes Dolby Digital Audio Compression.
· Sends 19.4 Mbps of digital video data in a standard 6 MHz channel, allowing one
HD channel with full audio in standard 6 MHz channel.
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· Can send multiple SD digital channels on standard 6 MHz channel.
· U.S. origin.
DVB (Digital Video Broadcast)
· Actually a set of standards from ETSI EN 300 421 to ETSI EN 300 775.
· Similar to ATSC, but of European origin.
Digital Video Transmission Formats
The most prevalent file format for multimedia (video + audio) files on PC’s are:
· The Above MPEG Standards (.mpg or .mpeg).
· The AVI (Audio Video Interleave) standard from Microsoft (.avi). This is a very
flexible standard, allowing for virtually any compression method (it simply
searches for the specified decoder as referenced in the video stream), and also
provides similar high flexibility in the audio stream.
· Apple’s Quicktime Standard (.mov), which uses MPEG-4 internally.