1、ITU-T RECMN*H.263 93 9 4862571 0583732 5-51 W INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU LINE TRANSMISSION OF NON-TELEPHONE SIGNALS H.261 (03193) VIDEO COBEC FOR AUDIOVISUAL SERVICES AT p x 64 kbits ITU-T Recommendation H.261 (P reviousiy “CCITT Recom
2、mendation“) ITU-T RECMN*H*2bL 73 4862571 0583733 49T FOREWORD The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the International Telecom- munication Union. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on t
3、hem with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, established the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. ITU-T Recom
4、mendation H.261 was revised by the ITU-T Study Group XV (1988-1993) and was approved by the WTSC (Helsinki, March 1-12, 1993). NOTES 1 As a consequence of a reform process within the International Telecommunication Union (ITU), the CCITT ceased to exist as of 28 February 1993. In its place, the ITU
5、Telecommunication Standardization Sector (ITU-T) was created as of 1 March 1993. Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector. In order not to delay publication of this Recommendation, no change has been made in the text to references c
6、ontaining the acronyms “CCITT, CCIR or IFRB” or their associated entities such as Plenary Assembly, Secretariat, etc. Future editions of this Recommendation will contain the proper terminology related to the new ITU structure. 2 telecommunication administration and a recognized operating agency. In
7、this Recommendation, the expression Administration is used for conciseness to indicate both a O ITU 1994 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in w
8、riting from the ITU. ITU-T RECMNtH.261 73 4862591 0583724 326 H CONTENTS 8 Scope Brief specification 2.1 Video input and output . 2.2 Digital output and input 2.3 Sampling frequency 2.4 Source coding algorithm . 2.5 Bit rate 2.6 Symmetry of transmission 2.8 Multipoint operation . Source coder . 3.1
9、Source format 3.2 Video source coding algorithm . 3.4 Forced updating Video multiplex coder 4.1 Data structure 2.7 Error handling . 3.3 Coding control 4.2 Video multiplex arrangement . 4.3 Multipoint considerations . Transmission coder 5.1 Bit rate 5.2 Video data buffering . 5.3 Video coding delay .
10、 5.4 Forward error correction for coded video signal Annex A . Inverse transform accuracy specification Annex B . Hypothetical reference decoder . Annex C . Codec delay measurement method Annex D - Still image transmission Page 1 1 2 2 2 2 2 3 3 3 3 3 3 6 6 7 7 7 18 19 19 19 20 20 21 22 23 24 Recomm
11、endation H.261 (03/93) ITU-T RECMN*H.ZbL 73 4862571 0583735 2b2 = Recommendation H.261 VIDEO CODEC FOR AUDIOVISUAL SERVICES AT p x 64 kbils (Geneva, 1990; revised ut Helsinki, 1993) The CCITT, considering (4 (b) multiples up to the primary rate or Hl 1/H12 rates; (c) B, Ho or Hl 1/H12 rate; (d) worl
12、d complicates the problems of specifying coding and transmission standards for international connections; (e some means of intercommunication among these terminals should be possible; that there is significant customer demand for videophone, videoconference and other audiovisual services; that circu
13、its to meet this demand can be provided by digital transmission using the B, Ho rates or their that ISDNs are likely to be available in some countries that provide a switched transmission service at the that the existence of different digital hierarchies and different television standards in differe
14、nt parts of the that a number of audiovisual services are likely to appear using basic and primary rate ISDN accesses and that (0 such intercommunication in the framework of Recommendation H.200; that the video codec provides an essential element of the infrastructure for audiovisual services which
15、allows (8) evolving series of Recommendations, that Recommendation H. 120 for videoconferencing using primary digital group transmission was the first in an appreciating that advances have been made in research and development of video coding and bit rate reduction techniques which lead to the use o
16、f lower bit rates down to 64 kbit/s so that this may be considered as the second in the evolving series of Recommendations, und noting that it is the basic objective of the CCITT to recommend unique solutions for international connections, recommends that in addition to those codecs complying to Rec
17、ommendation H. 120, codecs having signal processing and transmission coding characteristics described below should be used for intemational audiovisual services. NOTES 1 2 Codecs of this type are also suitable for some television services where full broadcast quality is not required. Equipment for t
18、ranscoding from and to codecs according to Recommendation H.120 is under study. 1 Scope This Recommendation describes the video coding and decoding methods for the moving picture component of audiovisual services at the rates of p x 64 kbit/s, where p is in the range 1 to 30. 2 Brief specification A
19、n outline block diagram of the codec is given in Figure 1. Recommendation H.261 (03/93) 1 ITU-T RECMN*H-ZbL 93 M 4862591 05837Lb LT9 Source Video multiplex Transmission coder coder buffer Transmission , coder b Video signal I Source Video multiplex decoder decoder a) Video coder Receiving Receiving
20、buffer decoder 4 Coded bit stream FIGURE l/H.26 1 Outline block diagram of the video codec 2.1 Video input and output To permit a single Recommendation to cover use in and between regions using 625- and 525-line television standards, the source coder operates on pictures based on a common intermedia
21、te format (CIF). The standards of the input and output television signals, which may, for example, be composite or component, analogue or digital and the methods of performing any necessary conversion to and from the source coding format are not subject to Recommendation. 2.2 Digital output and inpu
22、t The video coder provides a self-contained digital bit stream which may be combined with other multi-facility signals (for example as defined in Recommendation H.22 I). The video decoder performs the reverse process. 2.3 Sampling frequency Pictures are sampled at an integer multiple of the video li
23、ne rate. This sampling clock and the digital network clock are asynchronous. 2.4 Source coding algorithm A hybrid of inter-picture prediction to utilize temporal redundancy and transform coding of the remaining signal to reduce spatial redundancy is adopted. The decoder has motion compensation capab
24、ility, allowing optional incorporation of this technique in the coder. 2.5 Bit rate This Recommendation is primarily intended for use at video bit rates between approximately 40 kbith and 2 Mbitis. 2 Recommendation H.261 (03/93) ITU-T RECNNUH.2bL 93 m 48b259L 0583737 035 m 2.6 Symmetry of transmissi
25、on The codec may be used for bidirectional or unidirectional visual communication. 2.7 Error handling The transmitted bit-stream contains a BCH code (Bose, Chaudhuri and Hocquengham) (511,493) forward error correction code. Use of this by the decoder is optional. 2.8 Multipoint operation Features ne
26、cessary to support switched multipoint operation are included. 3 Source coder 3.1 Source format The source coder operates on non-interlaced pictures occurring 30 000/1 O0 1 (approximately 29.97) times per second. The tolerance on picture frequency is f 50 ppm. Pictures are coded as luminance and two
27、 colour difference components (Y, CB and CR). These components and the codes representing their sampled values are as defined in CCIR Recommendation 60 1. Black= 16 White = 235 Zero colour difference = 128 Peak colour difference = 16 and 240. These values are nominal ones and the coding algorithm fu
28、nctions with input values of 1 through to 254. Two picture scanning formats are specified. In the first format (CIF), the luminance sampling structure is 352 pels per line, 288 lines per picture in an orthogonal arrangement. Sampling of each of the two colour difference components is at 176 pels per
29、 line, 144 lines per picture, orthogonal. Colour difference samples are sited such that their block boundaries coincide with luminance block boundaries as shown in Figure 2. The picture area covered by these numbers of pels and lines has an aspect ratio of 4:3 and corresponds to the active portion o
30、f the local standard video input. NOTE - The number of pels per line is compatible with sampling the active portions of the luminance and colour difference signals from 525- or 625-line sources at 6.75 and 3.375 MHz, respectively. These frequencies have a simple relationship to those in CCIR Recomme
31、ndation 601. The second format, quarter-CIF (QCIF), has half the number of pels and half the number of lines stated above. All codecs must be able to operate using QCIF. Some codecs can also operate with CIF. Means shall be provided to restrict the maximum picture rate of encoders by having at least
32、 O, 1, 2 or 3 non-transmitted pictures between transmitted ones. Selection of this minimum number and CIF or QCIF shall be by external means (for example via Recommendation H.221). 3.2 Video source coding algorithm The source coder is shown in generalized form in Figure 3. The main elements are pred
33、iction, block transformation and quantization. The prediction error (INTER mode) or the input picture (INTRA mode) is subdivided into 8 pel by 8 line blocks which are segmented as transmitted or non-transmitted. Further, four luminance blocks and the two spatially corresponding colour difference blo
34、cks are combined to form a macroblock as shown in Figure 10. Recommendation H.261 (03/93) 3 ITU-T RECMN*H-263 93 4862573 05837L T7L x X!X x x x o!o O x xix x x x x xfx x x x ojo O x xfx x x x x x!x x X x 010 O x xlx x x x -.-.-.-.-I- .-.-.-.-.-.-.-.-.-. ! X Luminance sample O Chrominance sample -.-.
35、-.-. Block edge FIGURE 2m.26 1 Positioning of luminance and chrominance samples The criteria for choice of mode and transmitting a block are not subject to recommendation and may be varied dynamically as part of the coding control strategy. Transmitted blocks are transformed and resulting coefficien
36、ts are quantized and variable length coded. 3.2.1 Prediction The prediction is inter-picture and may be augmented by motion compensation (see 3.2.2) and a spatial filter (see 3.2.3). 3.2.2 Motion compensation Motion compensation (MC) is optional in the encoder. The decoder will accept one vector per
37、 macroblock. Both horizontal and vertical components of these motion vectors have integer values not exceeding f 15. The vector is used for all four luminance blocks in the macroblock. The motion vector for both colour difference blocks is derived by halving the component values of the macroblock ve
38、ctor and truncating the magnitude parts towards zero to yield integer components. A positive value of the horizontal or vertical component of the motion vector signifies that the prediction is formed from pels in the previous picture which are spatially to the right or below the pels being predicted
39、. Motion vectors are restricted such that ail pels referenced by them are within the coded picture area. 4 Recommendation H.261 (03/93) Video in - v Q- 1 T Q P F cc P t 9z 9 f V Transform Quantizer Picture memory with motion compensated variable delay Loop filter Coding control Flag for INTRNINTER F
40、lag for transmitted or not Quantizer indication Quantizing index for transform coefficients Motion vector Switching on/off of the loop filter FIGURE 3lH.261 Source coder 3.2.3 Loop filter The prediction process may be modified by a two-dimensional spatial filter (FIL) which operates on pels within a
41、 predicted 8 by 8 block. The filter is separable into one-dimensional horizontal and vertical functions. Both are non-recursive with coefficients of 1/4, 1/2, 1/4 except at block edges where one of the taps would fall outside the block. In such cases the l-D filter is changed to have coefficients of
42、 O, 1, O. Full arithmetic precision is retained with rounding to 8 bit integer values at the 2-D filter output. Values whose fractional part is one half are rounded up. The filter is switched odoff for all six blocks in a macroblock according to the macroblock type (see 4.2.3, MTYPE). Recommendation
43、 H.261 (03/93) 5 3.2.4 Transformer Transmitted blocks are first processed by a separable two-dimensional discrete cosine transform of size 8 by 8. The output from the inverse transform ranges from -256 to +255 after clipping to be represented with 9 bits. The transfer function of the inverse transfo
44、rm is given by: 77 f(x, y) = % c cC(u) C(v) F(u, v) COS n(2x + 1) u/16 COS (2y + 1) v/16 u=o v=o with u,v,x,y = 0,1,2 ,., 7 where x,y = spatial coordinates in the pel domain, u,v = coordinates in the transform domain, C(U) = i/ for u = O; otherwise 1, C(v) = 1/ for v = O; otherwise 1. NOTE - Within
45、the block being transformed, x = O and y = O refer to the pel nearest the left and top edges of the picture, respectively. The arithmetic procedures for computing the transforms are not defined, but the inverse one should meet the error tolerance specified in Annex A. 3.2.5 Quantization The number o
46、f quantizers is 1 for the INTRA dc coefficient and 3 1 for all other coefficients. Within a macroblock the same quantizer is used for all coefficients except the INTRA dc one. The decision levels are not defined The INTRA dc coefficient is nominally the transform value linearly quantized with a step
47、size of 8 and no dead-zone. Each of the other 31 quantizers is also nominally linear but with a central dead-zone around zero and with a step size of an even value in the range 2 to 62. The reconstruction levels are as defined in 4.2.4. NOTE - For the smaller quantization step sizes, the full dynami
48、c range of the transform coefficients cannot be represented. 3.2.6 Clipping of reconstructed picture To prevent quantization distortion of transform coefficient amplitudes causing arithmetic overflow in the encoder and decoder loops, clipping functions are inserted. The clipping function is applied
49、to the reconstructed picture which is formed by summing the prediction and the prediction error as modified by the coding process. This clipper operates on resulting pel values less than O or greater than 255, changing them to O and 255, respectively. 3.3 Coding control Several parameters may be varied to control the rate of generation of coded video data. These include processing prior to the source coder, the quantizer, block significance criterion and temporal sub-sampling. The proportions of such measures in the overall control strategy are not subject to recommendation. W
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