H.261

H.261
Video codec for audiovisual services at p x 64 kbit/s
Status	Published
Year started	1988
Latest version	(03/93)
Organization	video compression
Website	https://www.itu.int/rec/T-REC-H.261

H.261 is an

VCEG

, 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

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, created by the CCITT (now ITU-T) in 1984.^[3] H.120 was not usable in practice, as its performance was too poor.^[3] 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.^[4]

H.261 was developed by the

CCITT Study Group XV Specialists Group on Coding for Visual Telephony (which later became part of ITU-T SG16), chaired by Sakae Okubo of NTT.^[5] Since H.261, DCT compression has been adopted by all the major video coding standards that followed.^[4]

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 (

HEVC) 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.^[5]

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

subsampling horizontally and vertically to separate the picture into four sub-pictures that were sent sequentially.^[7]

H.261 design

The basic processing unit of the design is called a

entropy encoding

.

The inter-picture prediction reduces temporal redundancy, with

N. Ahmed, T. Natarajan and K. R. Rao in 1974.^[8] 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 (such as their own

HEVC

(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) 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-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 and FFmpeg

decoders projects.

Patent holders

The following companies contributed patents towards the development of the H.261 format:^[9]

Hitachi
PictureTel Corp.
Graphics Communication Technologies, Ltd.^[10]
Nippon Telegraph and Telephone (NTT)
BT Group
Toshiba
KDDI
Alcatel
Compression Labs, Inc.
AT&T Corporation
GPT Data Systems (GEC)
Philips
Sony
Sharp Corporation
Oki Electric Industry
Matsushita Communication Industrial Co., Ltd.
Mitsubishi Electric
Fujitsu
Orange S.A.
NEC
Electronics and Telecommunications Research Institute

References

^ "(Nokia position paper) Web Architecture and Codec Considerations for Audio-Visual Services" (PDF). H.261, which (in its first version) was ratified in November 1988.
^ ^a ^b ITU-T (1988). "H.261 : Video codec for audiovisual services at p x 384 kbit/s - Recommendation H.261 (11/88)". Retrieved 2010-10-21.
^ ^a ^b "The History of Video File Formats Infographic". RealNetworks. 22 April 2012. Retrieved 5 August 2019.
^
ISBN 9780852967102
.

^ ^a ^b 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
^ ITU-T (1990). "H.261 : Video codec for audiovisual services at p x 64 kbit/s - Recommendation H.261 (12/90)". Retrieved 2015-12-10.
^ ^a ^b ITU-T (1993). "H.261 : Video codec for audiovisual services at p x 64 kbit/s - Recommendation H.261 (03/93)". Retrieved 2015-12-10.
^ N. Ahmed, T. Natarajan and K. R. Rao, "Discrete Cosine Transform", IEEE Transactions on Computers, Jan. 1974, pp. 90-93; PDF file Archived 2011-11-25 at the Wayback Machine.
^ "ITU-T Recommendation declared patent(s)". ITU. Retrieved 12 July 2019.
^ "Patent statement declaration registered as H261-07". ITU. Retrieved 11 July 2019.

External links

H.261 (03/93): Video codec for audiovisual services at p x 64 kbit/s (
ITU
)

[1] "(Nokia position paper) Web Architecture and Codec Considerations for Audio-Visual Services" (PDF). H.261, which (in its first version) was ratified in November 1988.

[H2611988-2] ITU-T (1988). "H.261 : Video codec for audiovisual services at p x 384 kbit/s - Recommendation H.261 (11/88)". Retrieved 2010-10-21.

[history-3] "The History of Video File Formats Infographic". RealNetworks. 22 April 2012. Retrieved 5 August 2019.

[Ghanbari-4] 
ISBN 9780852967102
.

[Okubo-5] 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

[H2611990-6] ITU-T (1990). "H.261 : Video codec for audiovisual services at p x 64 kbit/s - Recommendation H.261 (12/90)". Retrieved 2015-12-10.

[H2611993-7] ITU-T (1993). "H.261 : Video codec for audiovisual services at p x 64 kbit/s - Recommendation H.261 (03/93)". Retrieved 2015-12-10.

[8] N. Ahmed, T. Natarajan and K. R. Rao, "Discrete Cosine Transform", IEEE Transactions on Computers, Jan. 1974, pp. 90-93; PDF file Archived 2011-11-25 at the Wayback Machine.

[9] "ITU-T Recommendation declared patent(s)". ITU. Retrieved 12 July 2019.

[10] "Patent statement declaration registered as H261-07". ITU. Retrieved 11 July 2019.

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