1、 ITU-T RECMN*H-L20 93 48b259L 0583544 2TL m ITU-T TELECOM MUN KATION STANDARDIZATION SECTOR OF ITU LINE TRANSMISSION OF NON-TELEPHONE SIGNALS H.120 (03/93) CODECS FOR VIDEOCONFERENCING USING PRIMARY DIGITAL GROUP TRANSMISSION ITU-T Recommendation H.120 (Previously “CCITT Recommendation” ITU-T RECMN*
2、H-120 93 m 4862591 0583545 138 m FOREWORD The IT 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 them with a view to st
3、andardizing 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 Recommendation H.120 was r
4、evised 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 Telecommunication Sta
5、ndardization 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 Similarly, in this reform process, the CCIR and the IFRB have been replaced by the Radiocommunication Sector.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 containing the acronym
6、s “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 this Recommendation,
7、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 writing from the ITU
8、. ITU-T RECMN*H.LZO 73 = 4862593 0583546 074 CONTENTS 1 A codec for 625.lines. 50 fieidds and 2048 kbit/s transmission for intra-regional use and capable of interworking with the codec of 2 1.1 Scope 1.2 Brief specification . 1.3 Video interface 1.4 Source coder . 1.5 Video multiplex coding 1.6 Tran
9、smission coding . 1.7 Error correction . Codecs not requiring separate television standards conversion when used on interregional connections . 2 A codec for 525-line, 60 fields/s and 1544 kbit/s transmission for intra-regional use and capable of interworking with the codec of clause 1 2.1 Introduct
10、ion 2.2 Brief specification . 2.3 Video interface 2.4 Source coder . 2.5 Video multiplex coding 2.6 Transmission coding . A codec for 525-lines, 60 field a version of the codec for 525-line, 60 fields and 2048 kbit/s operation capable of interworking with the codec described in 1. - Clause 3 describ
11、es a codec for intra-regional use in 525-line, 60 fields and 1544 kbit/s regions. The frame structures associated with the codecs described in this Recommendation are to be found in Recommenda- tion H. 130. As the codecs are complex items using combined intraframe and interframe picture-coding techn
12、iques which tend to be known only to specialists, Appendix I is provided giving a brief outline of the principles involved in the codecs of 1 and2. Recommendation H.120 (OY93) iii ITU-T RECMN*H-L20 73 H 4862591 0583549 883 m Recommendation H.120 CODECS FOR VIDEOCONFERENCING USING PRIMARY DIGITAL GRO
13、UP TRANSMISSION (Malaga-Torremolinos, 1984; amended at Melbourne, 1988 and at Helsinki, 1993) The CC, considering (a that there is growing evidence of a customer demand for a videoconference service; (b) that circuits to meet this demand can, at present, be provided effectively by digital transmissi
14、on using the primary digital group; (cl that the existence of different digital hierarchies and different television standards in different parts of the world complicates the problems of specifying coding and transmission standards for international connections; (dl that the eventual use of switched
15、 digital transmission networks should be taken into account, appreciating that rapid advances are being made in research and development of video coding and bit-rate reduction techniques which may lead to further Recommendations being proposed for videoconferencing at bit rates which are multiples o
16、r submultiples of 384 kbit/s during subsequent study periods so that this Recommendation may be considered as the first of an evolving series of Recommendations, and noting that it is a basic objective of the CCIT to recommend a unique solution for international connections as far as possible, recom
17、mends that the codecs having signal processing and interface characteristics described in clauses 1, 2 and 3, should be used for international videoconference connections. NOTE - Codecs of types other than those described in this Recommendation are not precluded. 1 A codec for 625-lines, 50 fieldsh
18、and 2048 kbih transmission for intra-regionall) use and capable of interworking with the codec of clause 2 1.1 Scope Clause 1 defines the essential features of a codec for the digital transmission, at 2048 kbit/s, of signals for videoconference or visual telephone service in accordance with Recommen
19、dation H. 100. The video input to the coder and output from the decoder in a 625-line, 50 fields/s signal, according to the “Class a” standard of Recommenda- tion H. 100, or alternatively, the 3 13-line, 50 fieldds signal of the “Class b” standard. Provision is also made for a sound channel and opti
20、onal data channels. A brief description of the operation of the codec is given in Appendix I. The term “intra-regional” is used here to describe connections within a group of countries which share a common television scanning standard and a common digital hierarchy, and may or may not be in geograph
21、ical proximity. The term “inter-regional” is used here to describe connections between groups of countries which have different television scanning standards andor different digital hierarchies. Recommendation H.120 (03193) 1 ITU-T RECMN*H=320 93 W 4862593 0583550 5T5 = The Recommendation starts wit
22、h a brief specification of the codec (see 1.2) and a description of the video interface. This is followed by details of the source coder (see 1.4) which provides analogue-to-digital conversion followed by recoding with substantial redundancy reduction in the face-to-face mode. Subclause 1.5 deals wi
23、th the video multiplex coder which inserts instructions and addresses into the digitized video signal to control the decoder so that it correctly interprets the signals received. Subclause 1.6 describes the transmission coder which arranges the various digital signals (video, sound, data, signalling
24、) into a form compatible with Recommendation G.732 for transmission over 2048 kbis digital paths. Subclause 1.7 describes optional forward error correction facilities. Provision is made in the digital frame structure for the inclusion of other optional facilities such as a graphics mode, encryption
25、and multipoint conferencing. Details of such facilities as are at present available are given in the annexes to this Recommendation. 1.2 Brief specification 1.2.1 Video inpuvoutput The video input and output are standard 625-line, 50 fields colour or monochrome television signals. The colour signals
26、 are in, or converted to, component form. Colour and monochrome operation are fully compatible. 1.2.2 Digital outputhiput The digital output and input are at 2048 kbit/s, compatible with the frame structure of Recommendation G.704. 1.23 Sampling frequency The video sampling frequency and the 2048 kH
27、z network clock are asynchronous. 1.2.4 Coding techniques Conditional replenishment coding supplemented by adaptive digital filtering, differential PCM and variable-length coding are used to achieve low bit-rate transmission. J.2.5 Audio channel An audio channel using 64 kbis is included. At present
28、, coding is A-law according to Recommendation G.711, but .provision is made for future use of more efficient coding. 12.6 Mode of operation The normal mode of operation is full duplex. 1.2.7 Codec-to-network signalling An optional channel for codec-to-network signalling is included. This conforms to
29、 emerging ideas in CCITT for switching 2-Mbit/s paths in the ISDN. 13.8 Data channels Optional 2 x 64 kbis and 1 x 32 kbi/s data channels are available. These are used for video if not required for data. 1.2.9 Forward error correction Optional forward error correction is available. This is required
30、only if the long-term error rate of the channel is worse than 1 in lo6. 1.2.10 Additional facilities Provision is made in the digital frame structure for the future introduction of encryption, a graphic mode and multipoint facilities. 2 Recommendation H.120 (03/93) ITU-T RECMN*H*320 93 4862593 05835
31、53 431 ! 1.2.11 Propagation delay When the coder buffer is empty and the decoder buffer full, the coder delay is less than 5 ms and the decoder delay is 130 f 30 ms at 2 Mbiih or 160 f 36 ms when only 1.5 Mbit/s are in us$). 1.3 Video interface The normal video input is a 625-line, 50 fields signal
32、in accordance with CCIR Recommendation 472. When colour is being transmitted, the input (and output) video signals presented to the analoguddigital convertors (and from the digitaanalogue convertors) are in colour-difference component form. The luminance and colourdifference components, E?y, (E?R -
33、E?y) and (E?B - E?Y) are as defined in CCIR Report 624. The analogue video input (and output) interface with the codec may be in the form of colour-difference components, colour components (R, G, B) or as a composite colour signal. The video interface is as recommended in CCIR Recommendation 656. Op
34、tionally, any other video standard which can be converted to give 143 active lines per field may be used. 1.4 Source coder 1.4.1 Luminance component or monocdrome 1.4.1.1 Analogue-to-digitai conversion The signal is sampled to produce 256 picture samples per active line (320 samples per complete lin
35、e). The sampling pattern is orthogonal and line, field and picture repetitive. For the 625-line input, the sampling frequency is 5.0 MHz, locked to the video waveform. Uniformly quantized PCM with 8 bitdsarnple is used. Black level corresponds to level 16 (OOOloooO). White level corresponds to level
36、 239 (1 1101 11 1). PCM code words outside this range are forbidden (the codes being used for other purposes). For the purposes of prediction and interpolation, the final picture element in each active line (i.e. picture element 255) is set to level 128 in both encoder and decoder. In all arithmetic
37、 operations, 8-bit arithmetic is used and the bits below the binary point are truncated at each stage of division. 1.4.1.2 Pre- and post-fdtenng In addition to conventional anti-aliasing filtering prior to analogue-tedigital conversion, a digital transversal filtering operation is carried out on the
38、 625-line signal to reduce the vertical definition of the picture prior to conditional replenishment coding. As a result of this process, 143 active lines per field are used instead of the 287% active lines of the 625-line signal, although the effective vertical definition is greater than one-half o
39、f that of a normal 625-line display. An interpolation process in the decoder restores the 625-line signal waveform. 1.4.1.3 Conditional replenishment coding A movement detector identifies clusters of picture elements which are deemed to be moving. The basic feature is a frame memory which stores 2 f
40、ields of 143 lines, each line containing 256 addressable points. The memory is updated at the picture rate and differences between the incoming signai and the corresponding stored values are used to determine the moving area in the coder. A similar frame memory must exist at the decoder and be simil
41、arly updated under the control of addressing information received from the coder. It is not necessary to specify the techniques used for movement detection because they do not affect interworking, although they do affect the resultant picture quality. Detected moving areas are transmitted by differe
42、ntial PCM with a maximum of 16 quantization levels. The first picture element in each moving area is transmitted by PCM. Variable-length coding is used on the DPCM code words. 2, These are typical figures. The delays depend on the detailed implementation used. Recommendation H.120 (03/93) 3 1TU-T RE
43、CMN*HlL20 93 4862571 0583552 378 W The first picture element of each cluster and the complete PCM lines, when they are transmitted to provide systematic or forced updating, are coded in accordance with 1.4.1.1. 1.4.13.1 DPCM prediction aigorithm The aigorithm used for DPCM prediction is: x=- A + , w
44、here X is the sample being predicted. 2 See Figure 1. For the purpose of prediction, line and field blanking are assumed to be at level 128 (out of 256). B C D - w w w Previous line Previous field FIGURE UH.120 Identification of saniples 1.4.1.3.2 Quantization law and variable-length coding 51 1 inp
45、ut levels are quantized to a maximum of 16 output levels, The quantizer does not assume the use of modulo 256 arithmetic. The quantization law and associated variable-length codes which are used for both luminance and colour-difference picture elements in moving areas which are not horizontally subs
46、ampled are given in Table 1. Error! Cannot open fue. The end-of-cluster code is 1 O O 1 and is designated as code number 1 1. The end-of-cluster code is omitted at the end of the last cluster in a line irrespective of whether it is a luminance cluster or a colour-difference cluster. 1.4.1.4 Subsampl
47、ing As the buffer fills, horizontal subsampling and fieldlfield subsampling are introduced. 1.4.1.4.1 Horizontal subsampling Horizontal subsampling is carried out only in moving areas. Normally, in this mode, only even elements are transmitted on even numbered lines and odd elements on odd numbered
48、lines. This gives rise to a line quincunx pattern in moving areas. Omitted elements are interpolated in the decoder by averaging the two horizontally adjacent elements. Interpolated picture elements are placed in the frame stores. A moving area cluster will always start with a PCM value and finish w
49、ith a transmitted DPCM picture element, even during subsampling. This means that in some instances, the transmitted cluster needs to be extended by one element in comparison with the moving area declared by the movement detector. At the end of the active line, however, this cannot occur as clusters must not extend into blanking, so cluster shortening by one element can be necessary. 4 Recommendation H.120 (03/93) ITU-T RECNN*H=32O 93 4862593 0583553 204 M TABLE 1A-I.120 Code table for non-horizontaly-subsampled moving areas Input levels -255 to -1 25 -1 24 to -95 -94 to -70 49 t
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