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Video coding for low bit rate communication
StatusIn force
Year started1996
Latest version3.0
January 2005
Base standardsH.261, H.262 (aka MPEG-2), MPEG-1
Related standardsH.264, H.265, H.266, H.323, H.324
DomainVideo compression
LicenseFreely available

H.263 is a video compression standard originally designed as a low-bit-rate compressed format for videoconferencing. 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 family of video coding standards in the domain of the ITU-T.

Like previous H.26x standards, H.263 is based on discrete cosine transform (DCT) video compression.[1] H.263 was later extended to add various additional enhanced features in 1998 and 2000. Smaller additions were also made in 1997 and 2001, and a unified edition was produced in 2005.

History and background[edit]

The H.263 standard was first designed to be utilized in H.324 based systems (PSTN and other circuit-switched network videoconferencing and videotelephony), but it also found use in H.323 (RTP/IP-based videoconferencing), H.320 (ISDN-based videoconferencing, where it was the most widely used video compression standard),[2] RTSP (streaming media) and SIP (IP-based videoconferencing) solutions.

H.263 is a required video coding format in ETSI 3GPP technical specifications for IP Multimedia Subsystem (IMS), Multimedia Messaging Service (MMS) and Transparent end-to-end Packet-switched Streaming Service (PSS).[3][4][5] In 3GPP specifications, H.263 video is usually used in 3GP container format.

H.263 also found many applications on the internet: much Flash Video content (as used on sites such as YouTube, Google Video, and MySpace) used to be encoded in Sorenson Spark format (an incomplete implementation of H.263[6][7][8]). The original version of the RealVideo codec was based on H.263 until the release of RealVideo 8.[9]

H.263 was developed as an evolutionary improvement based on experience from H.261 and H.262 (aka MPEG-2 Video), the previous ITU-T standards for video compression, and the MPEG-1 standard developed in ISO/IEC. Its first version was completed in 1995 and provided a suitable replacement for H.261 at all bit rates. It was further enhanced in projects known as H.263v2 (also known as H.263+ or H.263 1998) and H.263v3 (also known as H.263++ or H.263 2000). It was also used as the basis for the development of MPEG-4 Part 2.[10][11][12] MPEG-4 Part 2 is H.263 compatible in the sense that basic "baseline" H.263 bitstreams are correctly decoded by an MPEG-4 Video decoder.[10][13]

The next enhanced format developed by ITU-T VCEG (in partnership with MPEG) after H.263 was the H.264 standard, also known as AVC and MPEG-4 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 capabilities. An even-newer standard format, HEVC, has also been developed by VCEG and MPEG, and has begun to emerge in some applications.


Since the original ratification of H.263 in March 1996[14] (approving a document that was produced in November 1995), there have been two subsequent additions which improved on the original standard by additional optional extensions (for example, the H.263v2 project added a deblocking filter in its Annex J).

Version 1 and Annex I[edit]

The original version of 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

The first version of H.263 supported a limited set of picture sizes:

  • 128x96 (a.k.a. Sub-QCIF)
  • 176x144 (a.k.a. QCIF)
  • 352x288 (a.k.a. CIF)
  • 704x576 (a.k.a. 4CIF)
  • 1408x1152 (a.k.a. 16CIF)

In March 1997, an informative Appendix 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.263v2 (H.263+)[edit]

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
  • 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

H.263v2 also added support for flexible customized picture formats and custom picture clock frequencies. As noted above, the only picture formats previously supported in H.263 had been Sub-QCIF, QCIF, CIF, 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 Yes Yes Yes
Deblocking Filter Yes Yes Yes
Supplemental Enhancement Information (Full-Frame Freeze Only) Yes Yes Yes
Modified Quantization Yes Yes Yes
Unrestricted Motion Vectors No Yes Yes
Slice Structured Mode No Yes Yes
Reference Picture Resampling (Implicit Factor-of-4 Mode Only) No Yes Yes
Advanced Prediction No No Yes
Improved PB-frames No No Yes
Independent Segment Decoding No No Yes
Alternate INTER VLC No No Yes
Level 1 Level 2 Level 3

H.263v3 (H.263++) and Annex X[edit]

The definition of H.263v3 (also known as H.263++ or as the 2000 version of H.263) added three 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.

Open-source implementation[edit]

In countries without software patents, H.263 video can be legally encoded[15] and decoded with the free LGPL-licensed libavcodec library (part of the FFmpeg project) which is used by programs such as ffdshow, VLC media player and MPlayer.

See also[edit]


  1. ^ Davis, Andrew (13 June 1997). "The H.320 Recommendation Overview". EE Times. Retrieved 7 November 2019.
  2. ^ IEEE WESCANEX 97: communications, power, and computing : conference proceedings. University of Manitoba, Winnipeg, Manitoba, Canada: Institute of Electrical and Electronics Engineers. May 22–23, 1997. p. 30. ISBN 9780780341470. H.263 is similar to, but more complex than H.261. It is currently the most widely used international video compression standard for video telephony on ISDN (Integrated Services Digital Network) telephone lines.
  3. ^ ETSI (2009-04) ETSI TS 126 234 V8.2.0 (2009-04); 3GPP TS 26.234; Transparent end-to-end Packet-switched Streaming Service (PSS); Protocols and codecs Retrieved on 2009-06-02.
  4. ^ ETSI (2009-01) ETSI TS 126 140 V8.0.0 (2009-01); 3GPP TS 26.140; Multimedia Messaging Service (MMS); Media formats and codes Retrieved on 2009-06-02.
  5. ^ ETSI (2009-01) ETSI TS 126 141 V8.0.0 (2009-01); 3GPP TS 26.141; IP Multimedia System (IMS) Messaging and Presence; Media formats and codecs Retrieved on 2009-06-02.
  6. ^ Kaourantin.net (2005-08-13). "The quest for a new video codec in Flash 8". Archived from the original on 2009-02-06. Retrieved 2009-08-10. We went this route before with Sorenson Spark which is an incomplete implementation of H.263 and it bit us badly when trying to implement certain solutions.
  7. ^ Benjamin Larsson (2009-03-17). "h263-svq3 optimizations". FFmpeg-devel (Mailing list). Archived from the original on 2009-08-17. Retrieved 2009-08-09. Sorenson _Spark_ and H.263 are actually very similar. They differ mostly in header structure and ranges of the coefficients.
  8. ^ "Sorenson Spark". MultimediaWiki. Retrieved 2009-11-03. Video codec used in Flash Video files, based on H.263.
  9. ^ MultimediaWiki. "RealVideo G2". Retrieved 2009-11-02.
  10. ^ a b chiariglione.org (2006-08-10). "Riding the Media Bits, End of the Ride?". Archived from the original on 2011-01-22. Retrieved 2010-03-10.
  11. ^ Fernando Pereira. "MPEG-4: Why, What, How and When?". chiariglione.org. Archived from the original on 2011-10-18. Retrieved 2010-03-10.
  12. ^ ISO/IEC JTC1/SC29/WG11 (March 2000). "MPEG-4 Video - Frequently Asked Questions". chiariglione.org. Retrieved 2010-03-10.
  13. ^ chiariglione.org (2003-10-25). "Riding the Media Bits, Inside MPEG-4 - Part B". Archived from the original on 2011-01-22. Retrieved 2010-03-10.
  14. ^ ITU-T. "H.263 : Video coding for low bit rate communication". Retrieved 2009-11-02.
  15. ^ "7.3. Encoding with the libavcodec codec family". www.mplayerhq.hu. Retrieved 2021-05-08.

External links[edit]