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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
338:
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
320:(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
370:
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
486:
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
436:
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
363:) 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.
449:
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
474:
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
287:
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:
296:
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).
461:
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
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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
483:(although even when using these newer standards, additional post-processing is still allowed and can enhance visual quality if performed well).
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decoded video prior to display. One effective post-processing technique that became a key element of the best H.261-based systems is called
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491:) and for some types of internet video. However, H.261 remains a major historical milestone in the field of video coding development.
2279:
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280:, then Specialists Group on Coding for Visual Telephony). It was the first video coding standard that was useful in practical terms.
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Study Group XV Specialists Group on Coding for Visual
Telephony (which later became part of ITU-T SG16), chaired by Sakae Okubo of
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269:, first ratified in November 1988. It is the first member of the H.26x family of video coding standards in the domain of the
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391:, and H.261 was the first standard in which the macroblock concept appeared. Each macroblock consists of a 16×16 array of
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S. Okubo, "Reference model methodology – A tool for the collaborative creation of video coding standards",
316:(now ITU-T) in 1984. H.120 was not usable in practice, as its performance was too poor. H.120 was based on
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horizontally and vertically to separate the picture into four sub-pictures that were sent sequentially.
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335:. Since H.261, DCT compression has been adopted by all the major video coding standards that followed.
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702:"H.261 : Video codec for audiovisual services at p x 384 kbit/s - Recommendation H.261 (11/88)"
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828:"H.261 : Video codec for audiovisual services at p x 64 kbit/s - Recommendation H.261 (03/93)"
804:"H.261 : Video codec for audiovisual services at p x 64 kbit/s - Recommendation H.261 (12/90)"
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441:(DCT) reduces the spatial redundancy. The DCT that is widely used in this regard was introduced by
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678:"(Nokia position paper) Web Architecture and Codec Considerations for Audio-Visual Services"
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filtering. This reduces the appearance of block-shaped artifacts caused by the block-based
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324:(VQ) compression. The H.261 standard was subsequently developed based on DCT compression.
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with 704×576 luma resolution and 352×288 chroma resolution by using a staggered 2:1
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292:(352×288 luma with 176×144 chroma) and QCIF (176×144 with 88×72 chroma) using a
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H.261 (03/93): Video codec for audiovisual services at p x 64 kbit/s
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2012:
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N. Ahmed, T. Natarajan and K. R. Rao, "Discrete Cosine
Transform",
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H.261, which (in its first version) was ratified in
November 1988.
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includes a H.261 encoder and decoder. It is supported by the free
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The inter-picture prediction reduces temporal redundancy, with
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Standard Codecs: Image
Compression to Advanced Video Coding
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Video codec for audiovisual services at p x 64 kbit/s
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199:(then: Specialists Group on Coding for Visual Telephony)
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897:"Patent statement declaration registered as H261-07"
387:
The basic processing unit of the design is called a
283:
H.261 was originally designed for transmission over
49:. Unsourced material may be challenged and removed.
2785:
725:"The History of Video File Formats Infographic"
950:
792:, vol. 83, no. 2, Feb. 1995, pp. 139–150
616:Matsushita Communication Industrial Co., Ltd.
535:towards the development of the H.261 format:
395:samples and two corresponding 8×8 arrays of
494:
957:
943:
821:
819:
750:
748:
695:
693:
872:"ITU-T Recommendation declared patent(s)"
763:Institution of Engineering and Technology
719:
717:
549:Graphics Communication Technologies, Ltd.
109:Learn how and when to remove this message
754:
410:. The coding algorithm uses a hybrid of
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784:
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745:
690:
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339:international video coding standards (
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825:
801:
699:
779:
531:The following companies contributed
487:video-conferencing systems (such as
458:) to remove statistical redundancy.
47:adding citations to reliable sources
18:
252:https://www.itu.int/rec/T-REC-H.261
183:
13:
2748:Comparison of audio coding formats
318:differential pulse-code modulation
14:
2825:
918:
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658:CIF – Common Intermediate Format
23:
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382:
34:needs additional citations for
2804:ITU-T H Series Recommendations
2310:ISO/IEC base media file format
864:
848:IEEE Transactions on Computers
840:
670:
553:Nippon Telegraph and Telephone
1:
663:
7:
755:Ghanbari, Mohammed (2003).
646:
515:multimedia players, and in
327:H.261 was developed by the
10:
2830:
2753:Comparison of video codecs
1263:(GB/T 33475.2,GY/T 299.1)
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274:Video Coding Experts Group
267:video compression standard
16:Video compression standard
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439:discrete cosine transform
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850:, Jan. 1974, pp. 90-93;
495:Software implementations
415:inter-picture prediction
2577:Alliance for Open Media
790:Proceedings of the IEEE
425:, zig-zag scanning and
1497:MPEG-1 Layer III (MP3)
579:Compression Labs, Inc.
2799:ITU-T recommendations
2515:QuickTime File Format
611:Oki Electric Industry
306:video coding standard
2774:Compression software
2331:MPEG media transport
1408:Sorenson Video/Spark
584:AT&T Corporation
456:variable-length code
357:H.264/MPEG-4 Part 10
271:ITU-T Study Group 16
194:ITU-T Study Group 16
43:improve this article
2772:for techniques and
2770:Compression methods
1266:HDR Vivid(GY/T 358)
621:Mitsubishi Electric
523:decoders projects.
472:motion compensation
445:, T. Natarajan and
423:scalar quantization
345:H.262/MPEG-2 Part 2
322:vector quantization
123:
2814:1988 introductions
857:2011-11-25 at the
588:GPT Data Systems (
412:motion-compensated
304:The first digital
121:
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2473:DivX Media Format
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1336:Microsoft Video 1
1271:AVS3 P2(GY/T 368)
1257:(GB/T 20090.2/16)
653:Video compression
606:Sharp Corporation
463:motion estimation
312:, created by the
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242:video compression
204:Related standards
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2547:VOB, IFO and BUP
2321:Motion JPEG 2000
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2018:aptX Low Latency
1795:Chinese Standard
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1248:Chinese Standard
1020:Motion JPEG 2000
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765:. pp. 1–2.
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545:PictureTel Corp.
509:VLC media player
427:entropy encoding
419:transform coding
368:interoperability
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1810:(GB/T 33475.3)
1804:(GB/T 20090.10)
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1554:MPEG-H 3D Audio
1480:MPEG-1 Layer II
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859:Wayback Machine
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733:. 22 April 2012
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2758:List of codecs
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1098:Part 2 / LCEVC
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919:External links
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839:
826:ITU-T (1993).
815:
802:ITU-T (1990).
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700:ITU-T (1988).
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1069:Part 2 / HEVC
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1057:Part 33 / IVC
1055:
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1052:Part 10 / AVC
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350:
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341:MPEG-1 Part 2
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60: –
59:
55:
54:Find sources:
48:
44:
38:
37:
32:This article
30:
26:
21:
20:
2794:Video codecs
1824:(GB/T 22726)
1485:Multichannel
1341:MSU Lossless
1255:AVS1 P2/AVS+
1209:TrueMotion S
1147:H.265 / HEVC
1126:
1093:Part 1 / EVC
1081:Part 3 / VVC
1047:Part 2 / ASP
904:. Retrieved
900:
891:
879:. Retrieved
875:
866:
847:
842:
831:. Retrieved
807:. Retrieved
797:
789:
757:
735:. Retrieved
730:RealNetworks
728:
705:. Retrieved
684:
672:
530:
498:
485:
460:
431:
417:and spatial
386:
383:H.261 design
373:still images
365:
337:
326:
303:
282:
259:
258:
156:Organization
140:Year started
105:
96:
86:
79:
72:
65:
53:
41:Please help
36:verification
33:
2542:MOD and TOD
2483:Flash Video
2421:3GP and 3G2
2102:HEIC / HEIF
2054:compression
1814:Audio Vivid
1549:MPEG-D USAC
1544:MPEG-4 CELP
1539:MPEG-4 HVXC
1454:compression
1286:Apple Video
1202:and AOMedia
1152:H.266 / VVC
1142:H.264 / AVC
984:compression
969:compression
631:Orange S.A.
408:color space
377:subsampling
2788:Categories
2776:for codecs
2723:Daubechies
2698:Transforms
2600:Arithmetic
2267:Containers
1972:True Audio
1816:(GY/T 363)
1534:MPEG-4 DST
1529:MPEG-4 SLS
1524:MPEG-4 ALS
1398:SheerVideo
1372:QuickTime
1200:TrueMotion
966:Multimedia
833:2015-12-10
809:2015-12-10
707:2010-10-21
664:References
505:libavcodec
503:-licensed
468:deblocking
389:macroblock
168:PictureTel
69:newspapers
2525:RealMedia
2127:JPEG 2000
1930:RealAudio
1910:OptimFROG
1388:RealVideo
1376:Animation
1346:OMS Video
973:container
447:K. R. Rao
190:Committee
135:Published
2644:Lossless
2610:Modified
2503:Matroska
2382:Matroska
2293:MPEG-PES
1905:Musepack
1802:AVS1 P10
1381:Graphics
1331:Lagarith
855:Archived
852:PDF file
737:5 August
647:See also
559:BT Group
454:-level"
443:N. Ahmed
99:May 2023
2718:Wavelet
2659:DEFLATE
2605:Huffman
2595:Entropy
2587:Methods
2572:MPEG LA
2461:Smacker
2349:H.222.0
2305:MPEG-TS
2300:MPEG-PS
2288:MPEG-ES
2172:TIFF/IT
2167:TIFF/EP
2152:JPEG XT
2147:JPEG XS
2142:JPEG XR
2137:JPEG XL
2132:JPEG-LS
2013:aptX HD
1993:WavPack
1935:RTAudio
1875:Codec 2
1808:AVS2 P3
1735:GSM-EFR
1705:AMR-WB+
1639:G.729.1
1619:G.723.1
1609:G.722.2
1604:G.722.1
1403:Smacker
1393:RTVideo
1321:Huffyuv
1301:Cinepak
1261:AVS2 P2
975:formats
906:11 July
881:12 July
626:Fujitsu
596:Philips
574:Alcatel
564:Toshiba
540:Hitachi
533:patents
517:ffdshow
513:MPlayer
300:History
248:Website
180:Toshiba
164:Hitachi
151:(03/93)
83:scholar
58:"H.261"
2520:RatDVD
2414:Others
2181:Others
1983:VMR-WB
1978:TwinVQ
1843:Others
1730:GSM-FR
1725:GSM-HR
1715:EVRC-B
1700:AMR-WB
1672:Vorbis
1512:AAC-LD
1507:HE-AAC
1413:Theora
1356:ProRes
1351:Pixlet
1279:Others
1088:MPEG-5
1076:MPEG-I
1064:MPEG-H
1042:MPEG-4
1035:Part 2
1030:MPEG-2
1025:MPEG-1
769:
521:FFmpeg
403:and a
397:chroma
359:, and
262:is an
238:Domain
132:Status
85:
78:
71:
64:
56:
2809:H.26x
2741:Lists
2686:ADPCM
2681:µ-law
2676:A-law
2649:Lossy
2622:ACELP
2567:NETVC
2392:SMPTE
2354:T.802
2341:ITU-T
2317:(MP4)
2117:JBIG2
2080:ITU-T
2052:Image
1988:VSELP
1965:SVOPC
1955:Siren
1925:RCELP
1920:QCELP
1865:ATRAC
1850:ACELP
1667:Speex
1634:G.729
1629:G.728
1624:G.726
1614:G.723
1599:G.722
1594:G.719
1589:G.718
1582:µ-law
1577:A-law
1572:G.711
1564:ITU-T
1452:Audio
1326:Indeo
1306:Daala
1162:SMPTE
1137:H.263
1132:H.262
1127:H.261
1122:H.120
1110:ITU-T
1015:MJPEG
982:Video
681:(PDF)
555:(NTT)
489:H.323
477:H.264
421:with
405:YCbCr
349:H.263
329:CCITT
314:CCITT
310:H.120
294:4:2:0
264:ITU-T
260:H.261
228:H.320
224:H.266
220:H.265
216:H.264
212:H.263
208:H.262
160:ITU-T
122:H.261
90:JSTOR
76:books
2768:See
2713:MDCT
2691:DPCM
2637:WLPC
2627:CELP
2530:RIFF
2508:WebM
2498:M2TS
2488:HEIF
2456:Bink
2436:AIFF
2364:IETF
2253:WebP
2248:WBMP
2243:QTVR
2223:ICER
2218:FLIF
2208:DjVu
2198:AVIF
2188:APNG
2162:TIFF
2122:JPEG
2112:JBIG
2107:HEVC
2084:JPEG
2071:IETF
2038:LLAC
2033:LHDC
2028:LDAC
2008:aptX
1969:TTA
1950:SILK
1890:MELP
1885:Lyra
1880:iSAC
1870:CELT
1860:Asao
1855:ALAC
1833:ExAC
1828:L2HC
1758:AC-4
1753:AC-3
1745:ETSI
1710:EVRC
1687:3GPP
1677:FLAC
1662:iLBC
1657:Opus
1649:IETF
1472:MPEG
1438:YULS
1418:Thor
1366:4444
1316:FFV1
1296:Bink
1190:VC-6
1185:VC-5
1180:VC-3
1175:VC-2
1170:VC-1
1114:VCEG
1002:MPEG
971:and
908:2019
883:2019
767:ISBN
739:2019
601:Sony
569:KDDI
519:and
511:and
501:LGPL
499:The
481:HEVC
479:and
393:luma
361:HEVC
308:was
285:ISDN
278:VCEG
197:VCEG
184:etc.
143:1988
62:news
2728:DWT
2708:FFT
2703:DCT
2671:PCM
2664:LZW
2632:LSP
2617:LPC
2535:WAV
2493:IFF
2478:EVO
2468:BMP
2451:BPG
2441:AVI
2431:ASF
2426:AMV
2405:MXF
2400:GXF
2377:Ogg
2372:RTP
2280:IEC
2276:ISO
2238:QOI
2233:PGF
2228:MNG
2213:EXR
2203:BPG
2193:AV1
2157:PNG
2097:GIF
2076:W3C
2067:ISO
2063:IEC
2003:MQA
1998:WMA
1960:SMV
1945:SHN
1940:SD2
1915:OSQ
1900:MT9
1822:DRA
1786:LC3
1781:SBC
1763:DTS
1720:EVS
1695:AMR
1502:AAC
1467:IEC
1463:ISO
1433:XEB
1428:WMV
1361:422
1311:DVI
1291:AVS
1239:AV1
1234:VP9
1229:VP8
1224:VP7
1219:VP6
1214:VP3
997:IEC
993:ISO
929:ITU
901:ITU
876:ITU
636:NEC
590:GEC
452:run
333:NTT
290:CIF
172:NTT
45:by
2790::
2654:LZ
2446:AU
2278:,
2082:,
2078:,
2073:,
2069:,
2065:,
1465:,
1423:Ut
1112:,
1010:DV
999:,
995:,
899:.
874:.
818:^
781:^
761:.
747:^
727:.
716:^
692:^
683:.
429:.
355:,
351:,
347:,
343:,
230:,
226:,
222:,
218:,
214:,
210:,
182:,
178:,
176:BT
174:,
170:,
166:,
162:,
1469:,
958:e
951:t
944:v
931:)
927:(
910:.
885:.
861:.
836:.
812:.
775:.
741:.
710:.
592:)
450:"
276:(
112:)
106:(
101:)
97:(
87:·
80:·
73:·
66:·
39:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
