H.261
Encyclopedia
H.261 is a ITU-T
video coding standard, ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the ITU-T Video Coding Experts Group (VCEG), and was the first video codec that was useful in practical terms.
H.261 was originally designed for transmission over ISDN
lines on which data rates are multiples of 64 kbit/s. The coding algorithm was designed to be able to operate at video bit rates between 40 kbit/s and 2 Mbit/s. The standard supports two video frame sizes: CIF
(352x288 luma with 176x144 chroma) and QCIF (176x144 with 88x72 chroma) using a 4:2:0 sampling scheme. It also has a backward-compatible trick for sending still picture graphics with 704x576 luma resolution and 352x288 chroma resolution (which was added in a later revision in 1993).
(which also underwent a revision in 1988 of some historic importance) as a digital video coding standard, H.261 was the first truly practical digital video coding standard (in terms of product support in significant quantities). In fact, all subsequent international video coding standards (MPEG-1 Part 2, H.262/MPEG-2 Part 2
, H.263
, MPEG-4 Part 2
, and H.264/MPEG-4 Part 10
) have been based closely on the H.261 design. Additionally, the methods used by the H.261 development committee to collaboratively develop the standard have remained the basic operating process for subsequent standardization work in the field (see S. Okubo, "Reference model methodology-A tool for the collaborative creation of video coding standards", Proceedings of the IEEE, vol. 83, no. 2, Feb. 1995, pp. 139–150). The coding algorithm uses a hybrid of motion compensated inter-picture prediction and spatial transform coding with scalar quantization, zig-zag scanning and entropy encoding
.
, and H.261 was the first standard in which the macroblock concept appeared. Each macroblock consists of a 16x16 array of luma samples and two corresponding 8x8 arrays of chroma
samples, using 4:2:0 sampling
and a YCbCr
color space
.
The inter-picture prediction reduces temporal redundancy, with motion vector
s used to help the codec compensate for motion. Whilst only integer-valued motion vectors are supported in H.261, a blurring filter can be applied to the prediction signal — partially mitigating the lack of fractional-sample motion vector precision. Transform coding using an 8x8 discrete cosine transform
(DCT) reduces the spatial redundancy. Scalar quantization is then applied to round the transform coefficients to the appropriate precision determined by a step size control parameter, and the quantized transform coefficients are zig-zag scanned and entropy coded (using a "run-level" variable-length code) to remove statistical redundancy.
The H.261 standard actually only specifies how to decode the video. Encoder designers were left free to design their own encoding algorithms, as long as their output was constrained properly to allow it to be decoded by any decoder made according to the standard. Encoders are also left free to perform any pre-processing they want to their input video, and decoders are allowed to perform any post-processing they want to their decoded video prior to display. One effective post-processing technique that became a key element of the best H.261-based systems is called deblocking filtering. This reduces the appearance of block-shaped artifacts caused by the block-based motion compensation and spatial transform parts of the design. Indeed, blocking artifacts are probably a familiar phenomenon to almost everyone who has watched digital video. Deblocking filtering has since become an integral part of the most recent standard, H.264 (although even when using H.264, additional post-processing is still allowed and can enhance visual quality if performed well).
Design refinements introduced in later standardization efforts have resulted in significant improvements in compression capability relative to the H.261 design. This has resulted in H.261 becoming essentially obsolete, although it is still used as a backward-compatibility mode in some video conferencing systems and for some types of internet video. However, H.261 remains a major historical milestone in the development of the field of video coding.
includes a H.261 encoder and decoder. It is supported by the free VLC media player
and MPlayer
multimedia players, and in ffdshow
and FFmpeg
decoders projects
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
video coding standard, ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the ITU-T Video Coding Experts Group (VCEG), and was the first video codec that was useful in practical terms.
H.261 was originally designed for transmission over ISDN
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
lines on which data rates are multiples of 64 kbit/s. The coding algorithm was designed to be able to operate at video bit rates between 40 kbit/s and 2 Mbit/s. The standard supports two video frame sizes: CIF
Common Intermediate Format
CIF , also known as FCIF , is a format used to standardize the horizontal and vertical resolutions in pixels of YCbCr sequences in video signals, commonly used in video teleconferencing systems. It was first proposed in the H.261 standard.CIF was designed to be easy to convert to PAL or NTSC...
(352x288 luma with 176x144 chroma) and QCIF (176x144 with 88x72 chroma) using a 4:2:0 sampling scheme. It also has a backward-compatible trick for sending still picture graphics with 704x576 luma resolution and 352x288 chroma resolution (which was added in a later revision in 1993).
History
Whilst H.261 was preceded in 1984 by H.120H.120
H.120 was the first digital video encoding standard. It was developed by COST 211 and published by the CCITT in 1984, with a revision in 1988 that included contributions proposed by other organizations...
(which also underwent a revision in 1988 of some historic importance) as a digital video coding standard, H.261 was the first truly practical digital video coding standard (in terms of product support in significant quantities). In fact, all subsequent international video coding standards (MPEG-1 Part 2, H.262/MPEG-2 Part 2
H.262/MPEG-2 Part 2
H.262 or MPEG-2 Part 2 is a digital video compression and encoding standard developed and maintained jointly by ITU-T Video Coding Experts Group and ISO/IEC Moving Picture Experts Group . It is the second part of the ISO/IEC MPEG-2 standard...
, H.263
H.263
H.263 is a video compression standard originally designed as a low-bitrate compressed format for videoconferencing. It was developed by the ITU-T Video Coding Experts Group in a project ending in 1995/1996 as one member of the H.26x family of video coding standards in the domain of the ITU-T.H.263...
, MPEG-4 Part 2
MPEG-4 Part 2
MPEG-4 Part 2, MPEG-4 Visual is a video compression technology developed by MPEG. It belongs to the MPEG-4 ISO/IEC standards. It is a discrete cosine transform compression standard, similar to previous standards such as MPEG-1 and MPEG-2...
, and H.264/MPEG-4 Part 10
H.264/MPEG-4 AVC
H.264/MPEG-4 Part 10 or AVC is a standard for video compression, and is currently one of the most commonly used formats for the recording, compression, and distribution of high definition video...
) have been based closely on the H.261 design. Additionally, the methods used by the H.261 development committee to collaboratively develop the standard have remained the basic operating process for subsequent standardization work in the field (see S. Okubo, "Reference model methodology-A tool for the collaborative creation of video coding standards", Proceedings of the IEEE, vol. 83, no. 2, Feb. 1995, pp. 139–150). The coding algorithm uses a hybrid of motion compensated inter-picture prediction and spatial transform coding with scalar quantization, zig-zag scanning and entropy encoding
Entropy encoding
In information theory an entropy encoding is a lossless data compression scheme that is independent of the specific characteristics of the medium....
.
H.261 design
The basic processing unit of the design is called a macroblockMacroblock
Macroblock is an image compression component and technique based on discrete cosine transform used on still images and video frames. Macroblocks are usually composed of two or more blocks of pixels. In the JPEG standard macroblocks are called MCU blocks....
, and H.261 was the first standard in which the macroblock concept appeared. Each macroblock consists of a 16x16 array of luma samples and two corresponding 8x8 arrays of chroma
Chrominance
Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...
samples, using 4:2:0 sampling
Chroma subsampling
Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance....
and a YCbCr
YCbCr
YCbCr or Y′CbCr, sometimes written or , is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the luma component and CB and CR are the blue-difference and red-difference chroma components...
color space
Color space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
.
The inter-picture prediction reduces temporal redundancy, with motion vector
Motion vector
In video compression, a motion vector is the key element in the motion estimation process. It is used to represent a macroblock in a picture based on the position of this macroblock in another picture, called the reference picture....
s used to help the codec compensate for motion. Whilst only integer-valued motion vectors are supported in H.261, a blurring filter can be applied to the prediction signal — partially mitigating the lack of fractional-sample motion vector precision. Transform coding using an 8x8 discrete cosine transform
Discrete cosine transform
A discrete cosine transform expresses a sequence of finitely many data points in terms of a sum of cosine functions oscillating at different frequencies. DCTs are important to numerous applications in science and engineering, from lossy compression of audio and images A discrete cosine transform...
(DCT) reduces the spatial redundancy. Scalar quantization is then applied to round the transform coefficients to the appropriate precision determined by a step size control parameter, and the quantized transform coefficients are zig-zag scanned and entropy coded (using a "run-level" variable-length code) to remove statistical redundancy.
The H.261 standard actually only specifies how to decode the video. Encoder designers were left free to design their own encoding algorithms, as long as their output was constrained properly to allow it to be decoded by any decoder made according to the standard. Encoders are also left free to perform any pre-processing they want to their input video, and decoders are allowed to perform any post-processing they want to their decoded video prior to display. One effective post-processing technique that became a key element of the best H.261-based systems is called deblocking filtering. This reduces the appearance of block-shaped artifacts caused by the block-based motion compensation and spatial transform parts of the design. Indeed, blocking artifacts are probably a familiar phenomenon to almost everyone who has watched digital video. Deblocking filtering has since become an integral part of the most recent standard, H.264 (although even when using H.264, additional post-processing is still allowed and can enhance visual quality if performed well).
Design refinements introduced in later standardization efforts have resulted in significant improvements in compression capability relative to the H.261 design. This has resulted in H.261 becoming essentially obsolete, although it is still used as a backward-compatibility mode in some video conferencing systems and for some types of internet video. However, H.261 remains a major historical milestone in the development of the field of video coding.
Software implementations
The LGPL-licensed libavcodecLibavcodec
libavcodec is a free software/open source LGPL-licensed library of codecs for encoding and decoding video and audio data. Same name but incompatible libraries are provided from both FFmpeg project and Libav project....
includes a H.261 encoder and decoder. It is supported by the free VLC media player
VLC media player
VLC media player is a free and open source media player and multimedia framework written by the VideoLAN project.VLC is a portable multimedia player, encoder, and streamer supporting many audio and video codecs and file formats as well as DVDs, VCDs, and various streaming protocols. It is able to...
and MPlayer
MPlayer
MPlayer is a free and open source media player. The program is available for all major operating systems, including Linux and other Unix-like systems, Microsoft Windows and Mac OS X. Versions for OS/2, Syllable, AmigaOS and MorphOS are also available. The Windows version works, with some minor...
multimedia players, and in ffdshow
Ffdshow
ffdshow is a media decoder and encoder mainly used for the fast and high-quality decoding of video in the MPEG-4 ASP and AVC formats, but it supports numerous other video and audio formats as well...
and FFmpeg
FFmpeg
FFmpeg is a free software project that produces libraries and programs for handling multimedia data. The most notable parts of FFmpeg are libavcodec, an audio/video codec library used by several other projects, libavformat, an audio/video container mux and demux library, and the ffmpeg command line...
decoders projects
See also
- Video compression
- CIF - Common Intermediate FormatCommon Intermediate FormatCIF , also known as FCIF , is a format used to standardize the horizontal and vertical resolutions in pixels of YCbCr sequences in video signals, commonly used in video teleconferencing systems. It was first proposed in the H.261 standard.CIF was designed to be easy to convert to PAL or NTSC...
External links
- H.261 (03/93): Video codec for audiovisual services at p x 64 kbit/s (ITUItuItu is an old and historic municipality in the state of São Paulo in Brazil. The population in 2009 was 157,384 and the area is 641.68 km². The elevation is 583 m. This place name comes from the Tupi language, meaning big waterfall. Itu is linked with the highway numbered the SP-75 and are flowed...
)