A new teletext character set with enhanced legibility

Citation for published version (APA): Nes, van, F. L. (1986). A new teletext character set with enhanced legibility. Proceedings of the SID, 27(3), 239- 242.

Document status and date: Published: 01/01/1986

Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)

Please check the document version of this publication:

• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication

General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal.

If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement: www.tue.nl/taverne

Take down policy If you believe that this document breaches copyright please contact us at: [email protected] providing details and we will investigate your claim.

Download date: 30. Sep. 2021 ' L. jl~. '2 , .. A New Teletext Charncter Set with Enhanced Legibility

FLORIS L. vAN NES

Abstract-Teletext is difficult to read, partly because of the letter be read by somebody with an average visual acuity, i.e., fonts employed. Present fonts are contained in a matrix of 6 (horizon· 1.0 (equivalent to 20/20), subtends 5 min of are. To avoid tal) x 10 (vertical) elements. Research on matrix characters of opti­ mum legibillty started in 1969 at the lnstitute for Perception Research. letters with a simHar contiguration being confused during Criteria resulting from this research have now been used to design al­ reading a text, it should be made up of letters that are phanumeric characters in a matrix of 12 x 10 elements for use in considerably higher, for example 12 min of are to quote Teletext. Several verslons of each character were designed and their a tigure from a human-factors handhook [2]. legibility tested in recognltion experiments. The legibility of the best For a large-screen TV display with Teletext letters of new version for each letter was compared with and shown generally to be greater than that of the presently used version. the regular size, this value corresponds to a viewing dis­ tanee of 1.6 m, i.e., less than half of that which is typical for viewing TV. Therefore, it is worth optimizing ebar­ l. INTRODUCTION aeter discriminability. OT-MATRIX characters are used for text display on We designed alphanumeric characters and punctuation Dconventional TV receivers in an increasing number marks on a matrix of 12 x 10 elements (horizontal x of consumer-electronics app1ications, such as Teletext, vertical, including gaps between letters and rows). Such videotex, electronk games, and personal computing. The a matrix allows more refined as well as more acceptable resolution of a TV display is rather limited because of configurations, compared to the 6 x 10 matrix now mostly bandwidth limitations of the TV channel and the video in use. The latter format presents minimal possibilities for amplifier, etc. Therefore, the dot matrix of which the designing upper- and lower-case letters. The resulting characters are composed is relatively coarse, implying that character configurations were judged by viewers as being they can only be schematic approximations of the elabo­ too square with too thin diagonal strokes. To counteract rate detailed fonts used in print. In order to ensure good such etfects, "character rounding" was introduced by legibility of such schematic letters and digits, they should adding half dots at the appropriate positions, close to the be designed with three criteria in mind [ 1]: acceptability, diagonal strokes [3]. The rounding rules are based on an identifiability, and discriminability. A character has high interlaced scan pattem; however, the use of two inter­ acceptability when its shape closely corresponds to a con­ laeed fields in one TV frame creates an annoying "line cept that observers have of this shape; it is highly iden­ flicker" effect when watching Teletext. Most present-day tifiable when its parts stand out clearly against the ebar­ European TV sets therefore do not interlace in the Tele­ aeter background; it has high discriminability when the text mode thus, unfortunately, obliterating character chances of it being confused with a similar character are rounding. In view of this outcome and, on the other hand, low. Such confusion may occur under difficult observa­ developments in the Oerman "Bildschermtext" (view­ tion conditions, such as low contrast between character data) service, a 12 x 10 matrix format has been recently and background or reading from a distance. A luminous adopted as the new videotex matrix standard by the Euro­ contrast that is too low occurs, for instance, when red or pean Conference of Posts and Telecommunications blue letters are used on a black background, or yellow Administrations (CEPT}. letters on a white background. With respect to viewing 11. DESIGN AND TESTS OF LOWER-CASE LETTERS distance, applications such as Teletext are commonly viewed from the same distance as normal TV programs. The first phase of this project consisled of designing Aowever, this di stance is too large for the si ze of Teletext four configurations for each lower-case character using the characters, which means that, especially for viewers with results of previous experiments on the acceptability as well a reduced visual acuity, Teletext is inherently difficult to as discriminability of another comparable character set as read. The following may illustrate this point: the height guidelines [I]. The new characters we re designed on a of the row of capitalletters on a Snellen chart, which can terminal screen by assembling matrix "dots" in a grapb­ ical representation of the character matrix that was mag­ nified approximately 15 times compared to the normal Manuscript received October 7, 1985; revised March 4, 1986. size. The resulting contiguration could subsequently be The author is with the lnstitute of Perception Research, IPO, Eindho­ ven, The Netherlands. observed on a TV screen at normal display size. In this IEEE Log Number 8608715. way, a stimulus set of 4 x 26 = 104 characters was ob-

0018-9383/86/0800-1222$0 I .00 © 1986 IEEE Proc~~din2s of the SID. Vol. 27/3. 1986 239 Rec:ogJIIzed .. : WORST VERSlONS • .. c d I I h I I k I M n o p Q r • u ' • ? • 4 1 1 • 1 10 7 2 3 1 1 2 .. 1 12 2 1 3 1 5 4 c 1 18 3 1 1 3 1 3 • 3 d 1 1 21 1 1 3 1 2 1 • 1 .2313 1121 2 ~ I • • 14 1 21 (a) (b) (c) (d) 3 7 1 3 1 10 1 1 3 3 f • 20 2 12 •I 19 5 1 10 12 Fig. I. Four configurations for the letter a that were used in the first two ! 1 2 •••• ~ J 33 ·-1 experiments. 16 • 2 10 1 11 1 4 2 1 2 .. I 2 5 13 8 2 9 5 12 m 1 3 1 • "i . 7 23 1 4 tained. Fig. l(a) to (d) shows the four different versions 0 7 2 1 5 13 1 1 1 i.. • 2 • 17 • of the letter a. 15. .• 1 1 ~ : 2031 1 2 2 1 • 8 3 1 1 1 1 1 1 • 1 These 104 eh araeters we re presented in random order I • 3 12 11 1 i • 18 1 ,. to two groups of 12 subjects each in two experiments. In iii . 1 1 241 • . • 14 14 the tirst experiment the characters were presented foveally . 7 1 1 3 5 5 2 2 '!_ 1 1 1 2 1 5 1 2 __ _1_~ for 2 s, on a 25-in color TV set (maximum horizontal • 5 1 2 • 2 5 3 screen dimension 53 cm, frame rate 50 Hz) at an abser­ vation distance of 8 m. At this distance the character box •• e 1 z:s 10 1 a te ze 11 ,. ze 11 •• '' 12 11 et a 13 :s• za a z • 10 11 of 12 x 10 dot-matrix elements, as shown in Fig. 1, sub­ Fig. 2, Confusion matrix for the worst versions of the investigated lower­ tended a viewing angle of 4.5 min of are horizontally and case letter configurations in a discriminability experiment with periph­ 6.5 min of are vertically. In the second experiment, the eral stimulus presentation. characters were presented peripherally for 0.1 s to the left or right (in random order) of a tixation cross that was gen­ erated in the center of the screen from the same TV set. Recoped as : BEST VERStONS •ttcdetghiJII.I 0 • • r o I u •• • r a '? In this experiment an observation distance of 4 m was • 1823 1 1 1 1 2 8 1 1 1 2 2 2 4 used, and the stimuli were presented at an eccentricity of : : 23 d 2 1 27 1 3 5 1 3 4 1 1 • 2 1 2 2 1 • plus or minus 2 degrees; the character box then subtend­ 1 15 1 1112 1 5 1 1 2 7 2 3 1 ing a viewing angle of 9 min of are horizontally and 13 125 25 3 1 8 min of are vertically. 36 The viewing distance in the tirst experiment and retina) ~ 1~ • 1~12 2 23. 2 eccentricity in the second one were chosen so that the 1 3 30 1 1 219123 11 average recognition score was around 50 percent. This 2 1 28 3 31 methad allows a clear separation between characters of 35 2 1 2 2 1 2 • 5 4 2 1 4 high discriminability, which then score considerably 1 2 1 1 21 1 • 1 higher than 50 percent, and characters of low discrimin­ 1222~ ~ • 3 26 ability. which then score much Iower. . 1 1 1 4 • 1 • ~ --~2516 Same of the results of the second experiment, in which •' 2 1 1 1 1 2 1 1 • each stimulus of the set was presented three times to each subject, are shown in Figs. 2 and 3. Fig. 2 represents a 20 3 • 11 1• 3 e 8 33 11 I 10 8 32 11 12 8 ... 1 22 14 18 21 2 I 4 Ie confusion matrix for the worst, i.e., least discriminabie Fig. 3. Confusion matrix for the best versions of the investigated lower­ versions (e.g., the a of Fig. l(a)); and Fig. 3 is for the case letter configurations in a discriminability experiment with periph­ best, i.e .• most discriminabie contigurations (e.g .• the a eral stimulus presentation. of Fig. l(d)). The main diagonals of Figs. 2 and 3 represent the cor­ representation of the character concemed and 2) titted best rect recognition scores. A comparison of these two diag­ in the complete alphabet, in the apinion of a few observ­ enals clearly shows that the differences between the best ers. Such a situation was obtained for the two letter a con­ and worst versions are not the same for all letters. With tigurations shown in Fig. l(c) and (d); that from Fig. l(c) respect to the confusion, it appears that the errors are more was considered to be more acceptable, so it was selected concentrated in particular cells for the least legible letter for the tinal set of optimally discriminabie and acceptable versions than for the most legible ones: there are 12 cells characters, named "IPO-Norrnal," and as such appears with a content of 10 or more in Fig. 2, and only two such in Fig. 5. cells in Fig. 3. The tirst experiment had yielded similar results. The 111. CoMPARATIVE EvALUATION OF LowER-CAsE correct scores from bath experiments were added for each LETTERS letter contiguration. Generally, the contiguration with the In the second phase of the project, the discriminability highest combined score was then taken for the tinal char­ of the IPO-Norrnal set was compared with that of three acter set. However, if there was only a small difference other sets in a new experiment. The other sets were: between the combined scores for two contigurations. ac­ ceptability criteria were taken into account to choose the 1) "IPO-Bold," with bold versions of the "IPO-Nor­ contiguration that 1) corresponded most to the intemal mal' • lower-case letters;

240 Van Nes/Teletext Character Set with Fnhanced l.ePihilitv %correct 100~------. 13 short letters S deseenders

75 IPO-Normol IPO-Bold

50

2S

Fig. 4. The recognition scores of ascenders, shon letters, and deseenders in a distance-reading experiment with lower-case letters from four al­ phabets: Presently used. i.e., 6 x 10, Eisenbeis. /PO-Normal, and /PO· Bold. The dashed line refers to the average recognition score for alllower­ case Jeuers from all four alphabets: 61 percent.

2) a German set of lower-case letters [4]; and itals in alphanumeric strings. The discriminability of the 3) the lower-case letters presently used in most teletext boldface numerals with the highest correct recognition decoders, but without "character rounding." scores was tested in a new experiment using only such bold digits as stimuli. Some numerals that had an unsat­ In this experiment, lower-case letters from the four al­ isfactorily low recognition score, viz. 5 and 6, were then phabets were presented centrally (at the same viewing dis­ redesigned, taking account of the particular confusion er­ tanee as previously used, i.e., 8 m) in random order; 13 rors of the subjects. The resulting set of numerals was subjects participated. Fig. 4 shows the results of the com­ again tested; this time the correct recognition scores were parative experiment, separately for the three types of more uniformly distributed among the numerals. lower-case letters: ascenders, short letters, and descend­ ers. Averaged over all the lower-case letters, the recog­ V. DISCUSSION AND CONCLUSIONS nition score of the "IPO-Normal" letters was 65 percent, that of "IPO-Bold" 63 percent, that ofthe German letters Finally, a complete set of 196 characters-alphanumer­ 59 percent, and that of the present set 57 percent. ics, punctuation marks, and supplementary symbols-was In judging the practical significanee of these results it obtained on a 12 x 10 dot matrix. The most important should be realized that when the characters of such sets characters are shown in Fig. 5. All alphanumeric ebar­ are used for representing nonredundant alphanumeric aeters of the set have a width of 9 or 10 matrix elements, strings, as may occur in codes of all sorts, the probability so the capita! si ze is (9 or 10) x 7 _ that the whole string is correctly recognized equals the The character design procedure described may be em­ product of the recognition probabilities for the symbols ployed in a variety of other applications. With its empha­ that constitute the string. Therefore, a difference in rec­ sis on discriminability, it is especially suited for the de­ ognition probability of a few percent at the level of single sign of characters to be read under poor observation symbols can become quite significant at the level of com­ conditions. plete codes. Bouwhuis [5] has shown that, in principle, Comparisons of the IPO-Normal character set with al­ the same multiplication rule holds for the recognition of phabets designed in other dot-matrix formats, for instanee three-letter words when the recognition probabilities of the ubiquitous VDT font with a capita! size of 7 x 9, are the component letters are known. difficult, at least as far as the respective mutual discrimi­ nations are concemed, because small differences in dot IV. CAPITALS AND NUMERALS configurations may entail substantial differences in rec­ Essentially the same two-phase procedure was used for ognition and confusion scores. For example: a horizontal upper-case letters. For numerals, however, a somewhat displacement of the ascending part of the numeral 6 over different route was followed. Three sets of numerals were a distance of one matrix element in the present experi­ designed: one set in which the numerals had the same ments caused a difference in correct score of more than stroke width as that of the upper- and lower-case letters 30 percent, viz. 47 versus 11 percent for the two different and two sets of boldface numerals with a larger stroke configurations, because the perceptual difference with the width. The discriminability of these numerals was tested other numerals, especially the 4, had been increased con­ in an experiment, with the numerals from the three sets siderably by the displacement. as stimuli. The boldface numerals scored as high as the One feature of the described character set, bold numeral others. 1t was then decided to use boldface nu merals in strokes, three elements wide-compared with two for the the final character set because the increased stroke width upper-case letters-is not found in the widely used 7 x 9 might facilitate the distinction between numerals and cap- fonts.

Proceedings of the SID, Vol. 27/3, 1986 241 \ ··--· ...... ,., __...... •;-: ~,-.,····::-. . :~:·::··::. - .....- ·-"'' :!..·-~~·-~ ... _T ~ ...... ~E E m ~'!=::_;:;.: :::::::~~- •--··-·~~-' ....._~. - ...... -. ::::=::..T •l;...,, •• ._.. ::7~~-~_:"~'

-~·.:::.··~~. ~; ·~ " - ·-··::···~· •~-7:.-ii ±::::::::::

Fig. 5. The basic set of IPO-Nonnal 12 x 10 dot-matrix characters (copy­ righted). The complete IPO-Nonnal set is now proteeled under the rules of the International Design Registration effected under the Geneva Pro­ tocol of 1975.

It facilitates the distinction between numeral-capital [6) H. F. Huddleston, "A comparison oftwo 7 x 9 matrix alphanumeric pairs such as 5-S, 0-0, 8-B in the IPO-Normal set. In designs for TV displays," Appl. Ergonomics, vol. 5, no. 2, pp. 81- 83, 1974. passing, it may be remarked that there appear to be few (7] H. L. Snyder and M. E. Maddox, "On the image quality of dot-matrix publisbed research results, if any, on the legibility of displays." Proc. S/D, vol. 21. no. I, pp. 3-7, 1980. lower-case dot-matrix letters, whereas there are at least some on the legibility of upper-case letters and numerals [6], [7]. * REFERENCES

[I) J. A. J. Roufs and H. Bouma, "Towards linking perception research Floris L. van Nes received the M.S. degree in and image quality," Proc. S/D, vol. 21, no. 3, pp. 247-270, 1980. electronic engineering from Delft University of {2) H. P. van Cott and R. G. Kinkade, Eds., Human Engineering Guide Technology, The Netherlands. in 1961, and the to Equipment Design, revised edition. Washington, DC: Amerkan Ph.D. degree in physics and rnathematics from the Institutes for Research, 1972, p. 107. University of Utrecht, The Netherlands, in 1968. {3) L Reynolds. "Teletext and viewdata-A new challenge for the de­ Currently, he is working as a research scientist signer," lnformation Design J., vol. I, pp. 2-14, 1979. at the Institute for Perception Research-IPO, [4) M. Eisenbeis, "Visual design ofinfonnation systems," Displays. pp. Eindhoven, where he is the coordinator of all ac­ 95-99, July 1980. tivities in infonnation ergonomics. His research is [5] D. G. Bouwhuis, "Visual recognition of words," Ph.D. dissertation, centered on the interaction of computers with Nijmegen University, 1979. nonexpert users.

242