Optimal Television Scanning Format For

OPTIMAL TELEVISION SCANNING FORMAT FOR CRT-DISPLAYS Erwin B. Bellers, Ingrid E.J. Heynderickx , Gerard de Haan , and Inge de Weerd Philips Research Laboratories, Briarcliff Manor, USA Philips Research Laboratories, Eindhoven, The Netherlands ABSTRACT In order to have a fair optimization of the television dis- At the time of introduction of the television, the video for- play format, i.e. comparing options with approximately 1 mat was optimized given both economical and technological the same cost , we may increase the number of scanning constraints. The resulting video format is not necessarily lines at the expense of a lower refresh rate, or a change optimal for the current display technology. Moreover, cur- in the interlace phase. Note that this recently obtained ad- rent consumer-level priced and state-of-the-art scan-rate ditional freedom enables a variety of pixel distributions in converters enable a spatio-temporal decoupling of the re- both space and time, for every chosen pixel rate. ceived video and the displayed video. This paper presents Section 2 focuses on the conducted experiments and se- the results of a subjective assessment indicating the pre- lected environment. Section 3 presents the results of the ferred CRT-display format. experiments. Finally, in Section 4 we draw our conclusions. 1 INTRODUCTION 2 THE EXPERIMENTS Interlaced video with 525 or 625 lines and a 50 or 60 Hz A video format is mainly characterized by the spatial reso- picture rate has been the television broadcast standard for lution, temporal frequency, i.e. update frequency, and the quite some time. Modern bright television screens, how- interlace phase. As explained above, a fair optimisation ever, require a modified display format to prevent annoying should compare alternatives at equal cost, which implies large area flicker and/or interline flicker. Moreover, matrix that we have to look for the right balance from a viewer’s displays require format conversion as they cannot directly point of view between the number of scanning lines, the re- cope with interlace. Finally, new image sources, such as the fresh frequency and the interlace factor at a given pixel fre- Internet, benefit from an increased spatial pixel density to quency. As far as we know, no objective metric yet is able improve the legibility of displayed textual and graphical in- to reliably predict the perceived quality resulting from com- formation. As such, the optimal scanning or video format binations of these three video format characteristics. There- requirements may differ per application. fore, subjective testing is the only available option for de- The techniques for high quality video format conversion termining the optimal video format. have recently reached a price / performance ratio that enables application in the consumer domain [1, 2, 3]. As a result, we can choose the displayed number of scanning lines, 2.1 Video formats the interlace factor and the picture rate at will. Given this From a viewer’s point of view the optimal video format new freedom, the question arises how to optimally choose will be the one that offers the best balance between verti- a display format for the current applications. Increasing the cal resolution, flickering and annoying interlace artefacts. number of scanning lines increases the vertical resolution. In those cases that subjects have to balance various aspects Modifying the interlaced scanning to the progressive for- of one stimulus (i.e. a still image or a sequence), the paired- mat eliminates any of the possible interlace artifacts like comparison technique is the preferred methodology. It im- line flickering and line crawl. Finally, an increase in the re- plies that a subject is shown a pair of stimuli, where each fresh rate reduces or eliminates the large area flicker. Con- stimulus of the pair is displayed in a different video format. sequently, one might expect optimal performance by using The subject is requested to indicate the stimulus that is most both the highest number of scanning lines and at the high- preferred. In this way, each subject has to compare for a set est refresh rate possible. Obviously, there is a cost increase associated with such an increase in overall quality. 1the same pixel and line frequency display format As a first format considered in the evaluation we increased the vertical number of lines to arrive at a progressive video signal indicated as 50p (for so-called ’50Hz- countries’). Similarly we selected 60p as a viable option. 625(1:1)@50Hz Instead of displaying the 625 lines of the 50p format with the same interlace phase (because it is progressive), we could also consider to display the same amount of lines per field but with a different interlace phase (50i). As such, we 525(1:1)@60Hz input format create the opportunity to further increase the vertical resolu- compare tion by a factor of 2! As nice as this might seem, in practice, Video we have to take the typical interlace artifacts, especially at Format Conversion a refresh rate of 50Hz, into account. And as a result, we need to balance the visibility of interlace artifacts versus the 1250(2:1)@50Hz e.g. 625(2:1)@50Hz potential double vertical resolution. The visibility of interlace artifacts can be reduced by an increase in the picture refresh rate. However, increasing the refresh rate comes at the cost of lowering the vertical resolu- 833(2:1)@75Hz tion. As such, an compromise is created by the 75i format. Finally, the 100i format is an existing format that is free of visible interlace artifacts and large area flicker. The disadvantage is that the vertical resolution has been reduced to 625(2:1)@100Hz the 625 lines again. Figure 1: Decoupling of the input video format and the dis- 2.2 Test set play format. Critical image material, originating from either television cameras, or obtained from Internet pages has been selected of images with various content all mutual combinations of for this subjective assessment. In total 4 different still pic- the different video formats in the study. tures were selected, i.e. Grapes, Text, Siena and Web, as The disadvantage of subjective testing in general is that shown in Figure 2. subjects are only able to assess a limited number of stimuli. The pictures Siena and Grapes are considered as typical Therefore, especially with the paired-comparison method- good quality scenes from a television camera, whereas the ology the number of video formats that can be evaluated on pictures Text and Web are more typical in a PC like envi- a selection of image material has to be restricted to about 5 ronment or the Internet. Although all scenes contain spatial formats. The 5 formats we carefully selected on availabil- high frequencies, particularly Text and Web uses the fre- ity and applicability differ in the number of scanning lines, quency spectrum up to the Nyquist frequency. the interlace phase and the refresh rate, but all use the same As we want to compare the various video formats with- pixel frequency (27 MHz2). These formats are (see also out having to rely on scan-rate conversion quality, we used Figure 1): still picture in these experiments only, i.e. we considered an ideal scan-rate conversion. • 50 Hz, progressive (1:1), 625 scanning lines (50p) • 60 Hz, progressive (1:1), 525 scanning lines (60p) • 50 Hz, interlaced (2:1), 1250 scanning lines (50i) 2.3 Test conditions • 75 Hz, interlaced (2:1), 833 scanning lines (75i) The characteristics of the display device often limits the • 100 Hz, interlaced (2:1), 625 scanning lines (100i) scanning or display format. Moreover, various display types like Cathode Ray Tube (CRT), Plasma Display Panel (PDP), As indicated, all these formats have an intrinsic higher Liquid Crystal Display (LCD), etc, differ in characteristics spatio-temporal resolution that the common standard defi- and do, therefore, not necessarily share the same ’optimal’ nition video format. As such these formats, therefore, po- display format. In our experiments we focused on optimiz- tentially result into an improved appreciation of the standard ing the display format for the CRT. definition video displayed nowadays, while at the same time Two 21” Philips computer monitor CRTs were used dur- the larger sampling frequency has proven to be a viable op- ing both sessions. Because of the high vertical resolution tion. that is required to evaluate some of the video formats, com- 2This is twice the pixel frequency for common interlaced video. puter monitor tubes instead of television tubes had to be (a) (b) Siena Grapes (c) (d) Text Web Figure 2: Snapshots of the test set. chosen. This, however, has the disadvantage that the over- 0.50 0.75 1.00 1.25 1.50 1.75 0.50 97.2 100.0 86.1 80.6 83.3 all luminance of the display is limited, and therefore, the 0.75 2.8 58.3 38.9 38.9 38.9 viewing distance had to be fixed to twice (because of the 1.00 0 41.7 36.1 33.3 22.2 emulated resolution (see Section 2.4)) the standard viewing 1.25 13.9 61.1 63.9 41.7 19.4 distance for computer monitor applications, which is 0.5 - 1.50 19.4 61.1 66.7 58.3 22.2 1.75 16.7 61.1 77.8 80.6 77.8 0.7 m. Thus, subjects assessed all pairs of images at two viewing distances: 1 m and 1.5 m. In the results we did not (a) 0.75 1.00 1.25 1.50 1.75 2.00 find a difference between these viewing distances.

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