H.261
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H.261 is an ITU-T
video compression standard A video coding format (or sometimes video compression format) is a content representation format for storage or transmission of digital video content (such as in a data file or bitstream). It typically uses a standardized video compression algori ...
, first 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 Study Group 16 Video Coding Experts Group (
VCEG The Video Coding Experts Group or Visual Coding Experts Group (VCEG, also known as Question 6) is a working group of the ITU Telecommunication Standardization Sector (ITU-T) concerned with standards for compression coding of video, images, audio, ...
, then Specialists Group on Coding for Visual Telephony). It was the first video coding standard 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 (352×288 luma with 176×144 chroma) and QCIF (176×144 with 88×72 chroma) using a 4:2:0 sampling scheme. It also has a backward-compatible trick for sending still images with 704×576 luma resolution and 352×288 chroma resolution (which was added in a later revision in 1993).


History

The first digital video coding standard was
H.120 H.120 was the first digital video compression standard. It was developed by COST 211 and published by the CCITT (now the ITU-T) in 1984, with a revision in 1988 that included contributions proposed by other organizations. The video turned out not ...
, created by the
CCITT The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
(now ITU-T) in 1984. H.120 was not usable in practice, as its performance was too poor. H.120 was based on differential pulse-code modulation (DPCM), which had inefficient compression. During the late 1980s, a number of companies began experimenting with the much more efficient DCT compression for video coding, with the CCITT receiving 14 proposals for DCT-based video compression formats, in contrast to a single proposal based on vector quantization (VQ) compression. The H.261 standard was subsequently developed based on DCT compression. H.261 was developed by the
CCITT The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
Study Group XV Specialists Group on Coding for Visual Telephony (which later became part of ITU-T SG16), chaired by Sakae Okubo of NTT. Since H.261, DCT compression has been adopted by all the major video coding standards that followed. Whilst H.261 was preceded in 1984 by H.120 (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 H.263 is a video compression standard originally designed as a low-bit-rate compressed format for videotelephony. It was standardized by the ITU-T Video Coding Experts Group (VCEG) in a project ending in 1995/1996. It is a member of the H.26x fam ...
, MPEG-4 Part 2, H.264/MPEG-4 Part 10, and
HEVC High Efficiency Video Coding (HEVC), also known as H.265 and MPEG-H Part 2, is a video compression standard designed as part of the MPEG-H project as a successor to the widely used Advanced Video Coding (AVC, H.264, or MPEG-4 Part 10). In compari ...
) 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.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 Although H.261 was first approved as a standard in 1988, the first version was missing some significant elements necessary to make it a complete interoperability specification. Various parts of it were marked as "Under Study". It was later revised in 1990 to add the remaining necessary aspects, and was then revised again in 1993. The 1993 revision added an Annex D entitled "Still image transmission", which provided a backward-compatible way to send
still image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
s with 704×576 luma resolution and 352×288 chroma resolution by using a staggered 2:1 subsampling horizontally and vertically to separate the picture into four sub-pictures that were sent sequentially.


H.261 design

The basic processing unit of the design is called a
macroblock The macroblock is a processing unit in image and video compression formats based on linear block transforms, typically the discrete cosine transform (DCT). A macroblock typically consists of 16×16 samples, and is further subdivided into transform ...
, and H.261 was the first standard in which the macroblock concept appeared. Each macroblock consists of a 16×16 array of luma samples and two corresponding 8×8 arrays of chroma samples, using 4:2:0 sampling and a YCbCr color space. 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. The inter-picture prediction reduces temporal redundancy, with
motion vector Motion estimation is the process of determining ''motion vectors'' that describe the transformation from one 2D image to another; usually from adjacent frames in a video sequence. It is an ill-posed problem as the motion is in three dimensions b ...
s used to 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 8×8 discrete cosine transform (DCT) reduces the spatial redundancy. The DCT that is widely used in this regard was introduced by
N. Ahmed Nasir Ahmed (born 1940 in Bangalore, India) is an Indian-American electrical engineer and computer scientist. He is Professor Emeritus of Electrical and Computer Engineering at University of New Mexico (UNM). He is best known for inventing the ...
, T. Natarajan and
K. R. Rao Kamisetty Ramamohan Rao was an Indian-American electrical engineer. He was a professor of Electrical Engineering at the University of Texas at Arlington (UT Arlington). Academically known as K. R. Rao, he is credited with the co-invention of di ...
in 1974. 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 "
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-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 (such as their own motion estimation 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 A deblocking filter is a video filter applied to decoded compressed video to improve visual quality and prediction performance by smoothing the sharp edges which can form between macroblocks when block coding techniques are used. The filter aims ...
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 more recent standards H.264 and
HEVC High Efficiency Video Coding (HEVC), also known as H.265 and MPEG-H Part 2, is a video compression standard designed as part of the MPEG-H project as a successor to the widely used Advanced Video Coding (AVC, H.264, or MPEG-4 Part 10). In compari ...
(although even when using these newer standards, 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 (such as
H.323 H.323 is a recommendation from the ITU Telecommunication Standardization Sector (ITU-T) that defines the protocols to provide audio-visual communication sessions on any packet network. The H.323 standard addresses call signaling and control, m ...
) and for some types of internet video. However, H.261 remains a major historical milestone in the field of video coding development.


Software implementations

The
LGPL The GNU Lesser General Public License (LGPL) is a free-software license published by the Free Software Foundation (FSF). The license allows developers and companies to use and integrate a software component released under the LGPL into their own ...
-licensed libavcodec includes a H.261 encoder and decoder. It is supported by the free VLC media player and MPlayer multimedia players, and in
ffdshow ffdshow is an open-source unmaintained codec library that is mainly used for decoding of video in the MPEG-4 ASP (e.g. encoded with DivX or Xvid) and H.264/MPEG-4 AVC video formats, but it supports numerous other video and audio formats as we ...
and FFmpeg decoders projects.


Patent holders

The following companies contributed patents towards the development of the H.261 format: *
Hitachi () is a Japanese multinational corporation, multinational Conglomerate (company), conglomerate corporation headquartered in Chiyoda, Tokyo, Japan. It is the parent company of the Hitachi Group (''Hitachi Gurūpu'') and had formed part of the Ni ...
*
PictureTel Corp. PictureTel Corporation, often shortened to PictureTel Corp., was one of the first commercial videoconferencing product companies. It achieved peak revenues of over $490 million in 1996 and 1997 and was eventually acquired by Polycom in October ...
* Graphics Communication Technologies, Ltd. * Nippon Telegraph and Telephone (NTT) * BT Group * Toshiba * KDDI * Alcatel *
Compression Labs, Inc. Asure Software is a software company. Prior to September 13, 2007, the company was known as Forgent Networks. After rebranding as Asure Software, the company expanded into offering human capital management (HCM) solutions, including payroll, tim ...
* AT&T Corporation * GPT Data Systems ( GEC) * Philips * Sony * Sharp Corporation * Oki Electric Industry * Matsushita Communication Industrial Co., Ltd. *
Mitsubishi Electric , established on 15 January 1921, is a Japanese multinational electronics and electrical equipment manufacturing company headquartered in Tokyo, Japan. It is one of the core companies of Mitsubishi. The products from MELCO include elevators an ...
*
Fujitsu is a Japanese multinational information and communications technology equipment and services corporation, established in 1935 and headquartered in Tokyo. Fujitsu is the world's sixth-largest IT services provider by annual revenue, and the la ...
* Orange S.A. * NEC *
Electronics and Telecommunications Research Institute The Electronics and Telecommunications Research Institute () is a Korean government-funded research institution in Daedeok Science Town in Daejeon, Republic of Korea. Overview Established in 1976, ETRI is a non-profit government-funded research i ...


See also

*
Video compression In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
* CIF – Common Intermediate Format


References


External links


H.261 (03/93): Video codec for audiovisual services at p x 64 kbit/s
(
ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
) {{Compression formats Video codecs ITU-T recommendations ITU-T H Series Recommendations H.26x 1988 introductions