1、Rec. 812 1RECOMMENDATION 812SUBJECTIVE ASSESSMENT OF THE QUALITY OF ALPHANUMERICAND GRAPHIC PICTURES IN TELETEXT AND SIMILAR SERVICES(Question 119/11)(1992)Rec. 812The CCIR,consideringa) that there are systems which handle graphic and alphanumeric pictures and transmit them by means ofappropriate di
2、gital codes;b) that subjective tests often have to be used to assess the relative quality of the pictures and the effects producedon them by transmission errors;c) that methods for the subjective assessment of the quality of television pictures have been agreed (seeRecommendation 500);d) that alphan
3、umeric and graphic pictures have a specific character distinct from that of conventional televisionpictures, and the mental process involved in their subjective assessment may differ;e) that the results of subjective tests can be analysed and presented in many ways;f) that the results of subjective
4、tests can be interpreted in many ways,recommends1. that the general methods of test used for laboratory experiments and, whenever possible, for operationalassessments be as given in Recommendation 500, excepting, as appropriate, the viewing conditions and specificprocedures described in Annex 1;2. t
5、hat, in view of the importance of establishing the basis of subjective assessments of the quality ofalphanumeric and graphic pictures, the most complete descriptions possible of test configurations, test materials,observers, and methods should be provided in all test reports;3. that, in view of the
6、importance of ensuring that test outcomes relate well to the use of systems in practice, duenote should be taken of assessment context, as described in 3 of Annex 1.ANNEX 1The subjective quality of alphanumeric and graphicpictures in Teletext and similar servicesRecommendation 500 proposes methods t
7、o evaluate the subjective quality of pictures as contained in currenttelevision programmes. Studies are needed on the quality of alphanumeric and graphic pictures which are used forseveral new services transmitted via television channels and which use digital codes to describe alphanumeric andgraphi
8、c pictures. Some transmission parameters have an effect on the quality of displayed pictures: page resolution(number of rows per page and number of characters per row) in the case of alphamosaic coding of Teletext, charactercell resolution (number of pixels and lines per cell) in the case of DRCS (d
9、ynamically re-definable character set (seeRecommendation 653) coding and picture resolution in the case of broadcast audiography, facsimile or Teletext.Further, the effects of transmission errors which may affect the codes should also be considered. Thus, measurements ofquality and determinations of
10、 objective-to-subjective relationships for these parameters are necessary.2 Rec. 812Studies have shown that there are different aspects required for the quality assessment of these pictures whichmay have characteristics different from those of conventional television pictures. Parameters such as pix
11、el format,character cell resolution, spacings, colours and layout have effects on various quality attributes: legibility, quality,comfort, annoyance, effort of reading, fatigue and aesthetic considerations. Three main aspects are considered here: theviewing conditions, the assessment methods and the
12、 assessment context.1. Viewing conditionsRecommendation 500 defines viewing conditions for television pictures corresponding to low illuminationlevels in the room. It is likely that alphanumeric and graphic pictures would be viewed also in normal lightingconditions. Thus, a complementary set of view
13、ing conditions is suggested for study: illumination of 500 lux, screenmaximum luminance from 70 to 200 cd/m2, screen contrast ratio from 30 to 50 and a value of 1/4 for the ratio ofbackground luminance (from the walls of the room) to maximum screen luminance. Viewing distances from four toeight time
14、s picture height should also be considered.2. Assessment methodsA considerable number of studies have been made on typographical aspects. Most of them have used“performance measures” such as detection or recognition thresholds, recognition ratio, speed of reading, etc. Very fewhave used “subjective
15、measures” which are conventionally used in assessing the quality of television pictures. It isconsidered that new systems transmitted via television channels should have good performance (for example, percentageof good recognition of letters higher than 95%). The quality and impairment scales given
16、in Recommendation 500 couldthus be used efficiently although studies are needed to establish the way in which these scales can be related to legibility.A comparison with speech quality assessment methods (CCITT) has been tried and a 5-grade scale of “effort of reading”is suggested for further study.
17、Another method compares results of subjective assessments made using two different 5-grade scales given inTable 1.TABLE 1Legibility and reading effort scalesQuality of legibility scale Reading effort scaleExcellent legibility No reading effortGood legibility Attention necessary, but no appreciable r
18、eading effortFair legibility Moderate reading effortPoor legibility Substantial reading effortBad legibility Very substantial reading effortIt was found important to make the wording of each grade scale very explicit. The mean values of the scoresobtained with the reading effort scale are generally
19、higher than those obtained with the legibility scale and the range ofthe scores given by the observers is higher in the case of the reading effort scale.Rec. 812 3Another experiment used the quality scale described in 4.1.5.1 of Recommendation 500 to assess opinions ofboth overall quality and overal
20、l legibility of typescript transmitted by a television system of variable line standard andbandwidth. For each opinion, two models, one of greater complexity and accuracy, but both invoking the concept of“impairment-scale” addition were found that described the combined effects of limited horizontal
21、 and vertical definition.Legibility was also measured in terms of the proportion of characters correctly identified. However, legibility in suchterms remained high when quality was low, and it is evident that, usually, the former criterion is less useful.Another study carried out comparisons of perf
22、ormance and subjective methods on printed text material usingfixed-width and variable-width characters. Subjective methods were shown to be the more sensitive. The same type ofstudy was repeated using a cathode-ray tube display, applying this time only subjective methods. The use of thesesubjective
23、methods produced results dealing with the visually optimum sizes of fixed and variable matrices.At the moment, few experiments have been made on the subjective quality of alphanumeric and graphicpictures in the television field and further studies are required.3. Assessment contextA new approach to
24、service assessment considers the case where user activities in the service under study canbe defined accurately. Assessments are not made according to the conventional method of presenting images and simplyasking viewers for standard subjective assessments (e.g. Recommendation 500). Instead, viewers
25、 use the imagespresented as if they were using the service under study and all evaluations are performed in this context.Service-use emulation does not preclude the use of conventional subjective measures. However, it establishesa context for subjective evaluations that is more appropriate to the se
26、rvice under study. It also may permit the use ofobjective measures of viewer performance and the development of new subjective measures that are particularlyappropriate to the service and parameters under study. Finally, it establishes a more secure basis for generalizingassessments made in the laboratory to those made under service conditions._
