1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 ( TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU,).%G0G042!.3-)33)/./amended at Malaga- Torremolinos, 1984and at Melbourne, 1988)1 DefinitionThe visual telephone service is generally a two-way telecommunication service which uses a switchednetwork of broad
2、band analogue and/or digital circuits to establish connections among subscriber terminals, primarilyfor the purpose of transmitting live or static pictures.Special application one-way systems, e.g. surveillance and some information retrieval systems, or a non-switched videoconference service, can be
3、 regarded as degenerate cases of the visual telephone service.The visual telephone service also includes the associated speech.2 Facilities to be offeredThe design of the visual telephone service shall be such as to offer at least the following basic facilities:a) Transmission of live pictures such
4、as head and shoulders of one person or a small group of persons, withmoderate definition.b) Transmission of the associated speech.c) Transmission of graphics information such as drawings and documents with high definition (e.g. 625 linesor 525 lines).d) Video conference service, with or without the
5、use of split-screen techniques.The above-mentioned services shall, in general, be bi-directional, although uni-directional operation should bepossible. Also, some of the facilities can be omitted, if not required, in order to minimize costs.Note At the subscriber terminal, the use of ancillary equip
6、ments, e.g. for document reproduction, video taperecordings, etc., shall be possible.3 System parameters3.1 Picture standards3.1.1 The video standards of the subscriber sets shall be compatible with, readily convertible to, or identical to, thelocal broadcast television standards.3.1.2 Two classes o
7、f picture standards are recommended for the visual telephone system. They are given inTable 1/H.100.2 Fascicle III.6 - Rec. H.100TABLE 1/H.100Picture standardsThe region to which the figures should be appliedClass ItemsRegions where TV broadcastinguses 25 pictures per secondRegions where TV broadcas
8、tinguses 30 pictures per secondaNumber of horizontalscanning lines . . . . . . . . .Pictures per second . . . . . .Aspect ratio . . . . . . . . . .Video bandwidth. . . . . . . .62525(2:1 interleaved)4:35 MHz52530(2:1 interleaved)4:34 MHzbNumber of horizontalscanning lines . . . . . . . . .Pictures p
9、er second . . . . . . .Aspect ratio . . . . . . . . . .Video bandwidth . . . . . . .31325(2:1 interleaved)4:31 MHz26330(2:1 interleaved)4:31 MHzClass a standards are identical to the local broadcast video standards and will, in most cases, give sufficientdefinition for real-time picture transmission
10、 of a group of people (e.g. for conferencing) and of graphics material.Class b standards give sufficient definition for real-time transmission of a head and shoulder picture of oneperson or a small group. For the transmission of graphics information or other still pictures with high definition, aslo
11、w-scan technique has to be applied. For instance, a system using 625 or 525 horizontal scanning lines and 5, or less,pictures per second which gives a Class a definition in the 1 MHz bandwidth.Further study is required to define slow scanning parameters.4 Characteristics relating to split-screen tec
12、hniques for Class a television conference systems1)In television conference systems which use split-screen techniques to make more effective use of the picturearea, the following features for the terminals and transmitted signals are recommended. Preferred seating arrangementfor such systems are giv
13、en in Annex A.4.1 Picture formatThe transmitted picture should be 4 : 3 aspect ratio, split into upper and lower halves corresponding to thegroups of seats. Viewed from the camera system, the left-hand group should be in the upper half and the right-handgroup in the lower half.The split should occur
14、 at the end of lines 166 and 479 for 625-line television systems and at the end of line 142in Field 1 and line 141 in Field 2 for 525-line television systems, as shown in Figure 1/H.100.Before display, the receive equipment may discard half lines and first and last lines which are liable to beaverag
15、ed during standards conversion or vertical aperture correction of mixed signals.4.2 Identification signal for split-screen system4.2.1 Analogue video signalsThe identification signal for split-screen system should be inserted in the vertical blanking period, because thecontrol is required for each t
16、elevision frame or field._1)Split-screen techniques for systems using Class b standards require future study.Fascicle III.6 - Rec. H.100 3The line where the identification signal is inserted and its signal format are under study.4.2.2 Digital video signalsAn identification signal for split-screen sy
17、stem should be provided. In the case of codecs in RecommendationsH.120 and H.130 the format shall be that specified in Recommendation H.130.4.3 Compatibility with non-split-screen systemsThe simplest kind of a video telephone terminal is composed of a single camera and other equipments. Thesetermina
18、ls may be interconnected with split-screen system terminals. In that case, mechanical masks (if used) for the twosplit-screen displays (aspect ratio = 4 : 1.5) need to be removed, or if a display with 4 : 3 aspect ratio needs to beinstalled additionally.4.4 Cameras and displays arrangementThe entran
19、ce pupils of the TV camera optical system should be as near as possible to the centre of the TVdisplay showing remote conferees, in order to minimize errors in eye contact angle.Unless means are employed to place these pupils in line with the display, e.g. by use of half-silvered mirrors,the camera
20、system should be sited above the display and central to it.In order to keep the maximum horizontal errors as small as possible, the cameras used had better be in a cross-fire system, as for example in Figure A-1/H.100, and the camera/display assembly should be sited on the central axis ofthe termina
21、l. However, in some cases, adoption of parallel-fire system as shown also in Figure A-1/H.100 is necessarydue to a restriction in equipment arrangement.Whether the two cameras are arranged in cross-fire or parallel-fire is left open to each Administration since theselection does not affect the inter
22、connection of different systems.4.5 Picture processing methods at transmitting terminalsIn order to obtain the correct relationship between the signals from the two cameras for split-screen working,the cameras should be synchronized but the vertical drive pulses should be rephased. The drive to one
23、should beadvanced by one quarter of the vertical period while the drive to the other should be retarded by the same amount. Thiscauses a central strip of the target of each camera tube to be used and so minimizes the effects of distortions in thecorners of the targets. Figure B-1a)/H.100 illustrates
24、 the preferred method.Alternative methods which are not recommended although they do not give rise to problems of end-to-endcompatibility are compared in Annex B.4.6 Receiving equipmentThe receiving equipment should be capable of working with discontinuities in the received signal that may becaused
25、by switching between non-synchronous video sources.Note A split-screen device should be capable of working with a codec with the input and output frequencytolerances as specified in Recommendation H.120.4 Fascicle III.6 - Rec. H.100 Fascicle III.6 - Rec. H.100 5ANNEX A(to Recommendation H.100)Seatin
26、g arrangements when applying split-screen techniquesfor class a systemPreferred arrangements for video conferences using split-screen techniques are:A.1 The conference terminal accommodation should be for 6 primary seats in two adjacent groups of 3 as shown inFigure A.1/H.100.Provision for additiona
27、l seating behind may be made, so long as allowance is made for the central gap betweenthe two halves. For example, 4 additional persons may be seated on a second row as in the Figure.A.2 The chairmans position should be in the centre of the left-hand group of seats (viewed from the camera) withuser
28、controls accessible from both this position and the one of the chairmans left.Consequently, when split-screen pictures are displayed, stacked as received (i.e. shown as 3 over 3), thechairmans position is standardized as top centre.The suite of 3 chairs containing the chairmans position should also
29、be regarded as the primary position foroccasions when only half of a studio is in use. Such standardization is necessary for connection of 3 studios inconference using time-division multiplex of pairs of TV signals to share a common trunk between two studios.6 Fascicle III.6 - Rec. H.100ANNEX B(to R
30、ecommendation H.100)Picture processing methods in transmitting terminalsAlternative methods of obtaining the split-screen signal which are compatible with the recommended methodand which might be useful for experiments and demonstrations are shown in b) and c) of Figure B-1/H.100. In methodb), the t
31、wo cameras are directed upward and downward to pick up right and left halves of the conferencing room,respectively. Since circumferences of target and scanning areas are used, geometric and brightness distortions tend tooccur. In method c), vertical deflection currents are biased by the quantity corresponding to 1/4 of target height.Vertical deflection bias adjustement is needed every time cameras are exchanged. In method a), the vertical drivingpulses are phase-shifted by 1/4 V. The recommended method, a), avoids the problems of methods b) and c).Fascicle III.6 - Rec. H.100 7
