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H.263 is a video compression standard originally designed as a low-bitrate compressed format for videoconferencing
Videoconferencing is the conduct of a videoconference by a set of telecommunication technologies which allow two or more locations to interact via two-way video and audio transmissions simultaneously...

. It was developed by the 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 Experts Group (VCEG) 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 has since found many applications on the internet: much Flash Video content (as used on sites such as YouTube
YouTube is a video-sharing website, created by three former PayPal employees in February 2005, on which users can upload, view and share videos....

, Google Video
Google Video
Google Videos is a video search engine, and formerly a free video sharing website, from Google Inc. Before removing user-uploaded content, the service allowed selected videos to be remotely embedded on other websites and provided the necessary HTML code alongside the media, similar to YouTube...

, MySpace
Myspace is a social networking service owned by Specific Media LLC and pop star Justin Timberlake. Myspace launched in August 2003 and is headquartered in Beverly Hills, California. In August 2011, Myspace had 33.1 million unique U.S. visitors....

, etc.) used to be encoded in Sorenson Spark format (an incomplete implementation of H.263), though many sites now use VP6
On2 TrueMotion VP6 is a proprietary lossy video compression format and video codec. It is an incarnation of the TrueMotion video codec, a series of video codecs developed by On2 Technologies. This codec is commonly used by Adobe Flash, Flash Video, and JavaFX media files.- Overview :The VP6 codec...

 or H.264 encoding. The original version of the RealVideo
RealVideo is a suite of proprietary video compression formats developed by RealNetworks – the specific format changes with the version. It was first released in 1997 and is at version 10. RealVideo is supported on many platforms, including Windows, Mac, Linux, Solaris, and several mobile...

 codec was based on H.263 up until the release of RealVideo 8.

H.263 is a required video codec in ETSI 3GPP
The 3rd Generation Partnership Project is a collaboration between groups of telecommunications associations, known as the Organizational Partners...

 technical specifications for IP Multimedia Subsystem
IP Multimedia Subsystem
The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem is an architectural framework for delivering Internet Protocol multimedia services. It was originally designed by the wireless standards body 3rd Generation Partnership Project , as a part of the vision for evolving mobile...

 (IMS), Multimedia Messaging Service
Multimedia Messaging Service
Multimedia Messaging Service, or MMS, is a standard way to send messages that include multimedia content to and from mobile phones. It extends the core SMS capability that allowed exchange of text messages only up to 160 characters in length.The most popular use is to send photographs from...

 (MMS) and Transparent end-to-end Packet-switched Streaming Service (PSS). In 3GPP specifications, H.263 video is usually used in 3GP
3GP is a multimedia container format defined by the Third Generation Partnership Project for 3G UMTS multimedia services. It is used on 3G mobile phones but can also be played on some 2G and 4G phones....

 container format.

The codec was first designed to be utilized in H.324
H.324 is an ITU-T recommendation for voice, video and data transmission over regular analog phone lines. It uses a regular 33,600 bit/s modem for transmission, the H.263 codec for video encoding and G.723.1 for audio....

 based systems (PSTN
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

 and other circuit-switched
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...

 network videoconferencing and videotelephony), but has since also found use in H.323
H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

Real-time Transport Protocol
The Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...

/IP-based videoconferencing), H.320
H.320 is an umbrella recommendation by the ITU-T for running Multimedia over ISDN based networks. The main protocols in this suite are H.221, H.230, H.242, audio codecs such as G.711, and video codecs such as H.261 and H.263....

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...

-based videoconferencing), RTSP (streaming media
Streaming media
Streaming media is multimedia that is constantly received by and presented to an end-user while being delivered by a streaming provider.The term "presented" is used in this article in a general sense that includes audio or video playback. The name refers to the delivery method of the medium rather...

) and SIP
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

 (Internet conferencing) solutions.

H.263 was developed as an evolutionary improvement based on experience from H.261
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 , and was the first video codec that was useful in practical terms.H.261 was originally designed for...

, the previous ITU-T standard for video compression, and the MPEG-1
MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to 1.5 Mbit/s without excessive quality loss, making video CDs, digital cable/satellite TV and digital audio broadcasting possible.Today, MPEG-1 has become...

 and MPEG-2
MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission...

 standards. Its first version was completed in 1995 and provided a suitable replacement for H.261
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 , and was the first video codec that was useful in practical terms.H.261 was originally designed for...

 at all bitrates. It was further enhanced in projects known as H.263v2 (also known as H.263+ or H.263 1998), 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.263v3 (also known as H.263++ or H.263 2000). MPEG-4 Part 2 is H.263 compatible in the sense that a basic H.263 bitstream is correctly decoded by an MPEG-4 Video decoder.

The next enhanced codec developed by ITU-T VCEG
The Video Coding Experts Group or Visual Coding Experts Group is the informal name of Question 6 of Working Party 3 of Study Group 16 of the ITU-T. Its abbreviated title is ITU-T Q.6/SG 16...

 (in partnership with MPEG) after H.263 is the H.264
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...

 standard, also known as AVC and MPEG-4
MPEG-4 is a method of defining compression of audio and visual digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group under the formal standard ISO/IEC...

 part 10. As H.264 provides a significant improvement in capability beyond H.263, the H.263 standard is now considered a legacy design. Most new videoconferencing products now include H.264 as well as H.263 and H.261
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 , and was the first video codec that was useful in practical terms.H.261 was originally designed for...



Since the original ratification of H.263 in March 1996 (approving a document that was produced in November 1995), there have been two subsequent additions which improved on the original codec by additional optional annexes (extensions). For example, the H.263v2 added a deblocking filter via Annex J.

The original H.263 specified the following annexes:
  • Annex A – Inverse transform accuracy specification
  • Annex B – Hypothetical Reference Decoder
  • Annex C – Considerations for Multipoint
  • Annex D – Unrestricted Motion Vector mode
  • Annex E – Syntax-based Arithmetic Coding mode
  • Annex F – Advanced Prediction mode
  • Annex G – PB-frames mode
  • Annex H – Forward Error Correction for coded video signal

In March 1997, an informative Annex I describing Error Tracking — an encoding technique for providing improved robustness to data losses and errors, was approved to provide information for the aid of implementers having an interest in such techniques.

H.263 supports for a limited set of picture sizes:

  • 128x96
  • 176x144
  • 352x288
  • 704x576
  • 1408x1152

H.263v2 (H.263+)

H.263v2 (also known as H.263+, or as the 1998 version of H.263) is the informal name of the second edition of the ITU-T H.263 international video coding standard. It retained the entire technical content of the original version of the standard, but enhanced H.263 capabilities by adding several annexes which can substantially improve encoding efficiency and provide other capabilities (such as enhanced robustness against data loss in the transmission channel). The H.263+ project was ratified by the ITU in February 1998. It added the following Annexes:
  • Annex I – Advanced INTRA Coding mode
  • Annex J – Deblocking Filter mode
    Deblocking filter (video)
    A deblocking filter is a video filter applied to blocks in decoded video to improve visual quality and prediction performance by smoothing the sharp edges which can form between macroblocks when block coding techniques are used...

  • Annex K – Slice Structured mode
  • Annex L – Supplemental Enhancement Information Specification
  • Annex M – Improved PB-frames mode
  • Annex N – Reference Picture Selection mode
  • Annex O – Temporal, SNR, and Spatial Scalability mode
  • Annex P – Reference picture resampling
  • Annex Q – Reduced-Resolution Update mode (see implementors' guide correction as noted below)
  • Annex R – Independent Segment Decoding mode
  • Annex S – Alternative INTER VLC mode
  • Annex T – Modified Quantization mode
  • Annex X – Profiles and levels definition

H.263v2 also added support for flexible customized picture formats and custom picture clock frequencies. Previously the only picture formats supported in H.263 had been Sub-QCIF, QCIF, 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...

, 4CIF, and 16CIF, and the only picture clock frequency had been 30000/1001 (approximately 29.97) clock ticks per second.

H.263v2 specified a set of recommended modes in an informative appendix (Appendix II, since deprecated):
Level 1 Level 2 Level 3
Advanced INTRA Coding
Deblocking Filter
Supplemental Enhancement Information (Full-Frame Freeze Only)
Modified Quantization
Unrestricted Motion Vectors
Slice Structured Mode
Reference Picture Resampling (Implicit Factor-of-4 Mode Only)
Advanced Prediction
Improved PB-frames
Independent Segment Decoding
Alternate INTER VLC
Level 1 Level 2 Level 3

H.263v3 (H.263++) and Annex X

The definition of H.263v3 (also known as H.263++ or as the 2000 version of H.263) consisted of the addition of three additional annexes. These annexes and an additional annex that specified profiles (approved the following year) were originally published as separate documents from the main body of the standard itself. The additional annexes specified are:
  • Annex U – Enhanced reference picture selection mode
  • Annex V – Data-partitioned slice mode
  • Annex W – Additional supplemental enhancement information specification
  • Annex X (originally specified in 2001) – Profiles and levels definition

The prior informative Appendix II (recommended optional enhancement) was obsoleted by the creation of the normative Annex X.

In June 2001, another informative appendix (Appendix III, Examples for H.263 encoder/decoder implementations) was approved. It describes techniques for encoding and for error/loss concealment by decoders.

In January 2005, a unified H.263 specification document was produced (with the exception of Appendix III, which remains as a separately-published document).

In August 2005, an implementors guide was approved to correct a small error in the seldom-used Annex Q reduced-resolution update mode.


  • In countries without software patents, H.263 video can be legally encoded and decoded with the free LGPL-licensed libavcodec
    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....

     library (part of the 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...

     project) which is used by programs such as 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...

    , 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 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...


External links

  • The ITU-T specification for H.263
  • IETF AVT Working Group - Group that reviews codec packetizations for RTP
    Real-time Transport Protocol
    The Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...

    • RFC 4629 - RTP Payload Format for ITU-T Rec. H.263 Video
    • RFC 2429 - RTP Payload Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+) (Obsolete, upgraded spec in RFC 4629)
    • RFC 2190 - RTP Payload Format for H.263 Video Streams (Historic)