Safe area for TVs

13. May 2013

Safe area is a term for the part of broadcasted picture, for which it’s guaranteed that it’s visible on all TV sets. Historically, the broadcasted picture displayed on older TVs was cut off on the edges and invisible (this part is called overscan).

It’s a shortcoming of both analog broadcast and image rendering on cathode ray tube (CRT) TVs, which are no longer produced today.

In practice, broadcasts didn’t even take into account that TVs would be ever able to display the parts beyond the edges, and these parts of the signal contained black bars, random data, etc. To prevent displaying errors on the edges of the picture, the central part of the picture was simply stretched over the surface of the TV. Part of the broadcasted picture (5 to 10%) was therefore not visible, but TV watching was at least not troubled by picture errors. The easiest way to imagine the overscan is to take a picture, cut off its edges and stretch the rest to the original size. If the broadcasted picture had any defects near the edges (bars, random noise, etc.), they would be removed by this process. Stretching of the picture can be clearly seen on the image below.

In the era of digital broadcasting and LCD TVs, and especially in case of applications, this should be an anachronism because applications are all tailored for the TV resolution. The only drawback is that the native resolution of TVs is either FHD (1920x1080px) or UHD, while application for smart TVs, with the exception of new models of Panasonic Viera, have resolution of 1280x720px.

The difference between native and application resolution leads to the fact that the TV converts everything from the native resolution to 1280x720px. This conversion results in partial imperfections in the final picture, however they are not visible with a naked eye and will be shown only on testing patterns. Importantly, the conversion has no effect on visible areas – the pixel that the application displays on position 0.0 will always be on the top left, and the pixel 1279x719px will always be on the bottom right in the rearmost position. From the above mentioned facts it would seem that the safe area is irrelevant today. We investigated the opinions of TV manufacturers. Samsung and Phillips have a clear standpoint, they confirmed that the entire area of the screen is usable. Colleagues from Samsung literally said: “There are no safe area limitation as those found on CRT devices,” technical support and documentation for Phlips NetTV similarly states: “Ensure all your content is within the safe area, i.e. the screen size defined by the Smart TV platform is 1280 x 720px.” Conversation with Panasonic went in a similar vein: However, we were informed that it’s good to count with the safe area in development of applications for Blu-Ray players: “This is because when Blu-ray devices are connected to displays with overscan, images at the edges may not be correctly displayed.” We also encountered similar problems in Blu-Ray players, as well as various set-top boxes or upgraders connected with HDMI or SCART cables to the TV. .

However, every TV until now offered a solution in the form of screen setting and adaptation to broadcasted picture – search for screen setting options like Screen Fit or similar. Considering TV settings, thanks to the fact that all devices have an aspect ratio of 16:9, and also because the applications for smart TVs have the same resolution as Blu-Ray player, we never detected any image cropping. The only problem in this respect is related to various set-top boxes with different aspect ratios (e.g. PAL 576i) but those do not belong in the category of smart TVs, it is a specific device used mostly for IPTV. Likewise, we didn’t have any problems with applications for OperaTV Store on the Sony BRAVIA platform. The more surprised we were by the request of LG, which was the motivation for writing this blog post: “Over-scan Area Rule: All selectable objects, text, and company branding logo must be placed within the Safe Area.” The surprise was all the greater as we never before encountered such comment during approval of applications for LG.

Out of curiosity, we went through each screen of our LG applications and we came to the conclusion that thanks to the graphic design, some important elements are not located at the edges of the screen, and it seems that other elements were not considered important by LG testers. We looked at other applications on LG SmartWorld, and in the end we agreed that we had never before noticed how much are these recommendations respected. We do not know the exact reason for this requirement of LG, however as it turned out, it took under two days to make the requested adjustment, so we didn’t tackle it further. We had verified in practice that offset of 30 pixels from each side is sufficient. It’s good to take it into account from the start, because even a small offset may influence the size and positions of a number of elements, fonts and overall composition. Therefore if you are preparing an application for LG and want to avoid problems during approval, do not forget the safe area during application design. For better understanding we prepared pictures comparing design of applications on Samsung and LG: More information on this topic can be found here.

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