Correcting/Adjusting Overscan In X.org/MythTV NOTE: I am not a video expert. I believe what follows is useful for someone trying to correct overscan in the context of running MythTV on a CRT. I don't own an LCD display, DFP, or projector. This info may or may not be useful to those trying to run MythTV on something other than a CRT. I'd welcome corrections to any of the info that follows.
What is Overscan and why is it a problem? Overscan is the solution to the problem of completely filling the screen edge to edge and avoiding distortions that appear in the video image at the edge of the screen. Essentially when you are watching TV there is some portion of the image that is not viewable by you, it's off the screen. When watching TV you probably don't notice this and in most cases it doesn't matter. TV show producers, movie-makers, set-top box designers, and even console video game makers are well aware that they cannot expect to use the edges of the display and expect viewers at home to be able to see it. So, they confine themselves to a rectangle in the middle of the screen. This is a problem when using software designed for display on a computer monitor. The application designer often makes the (perfectly reasonable) assumption that you can see every pixel. So, when traditional desktop applications collide with display on a TV, some required portion of the image gets cut off, often a menu bar, panel or set of icons. It can also be a problem when interfacing a video card to a TV. Sometimes the overscan is so unreasonable that even when playing video too much of the image is off the edge of the screen. So, how do you fix this overscan issue? Before we get to that I'll talk briefly about three elements that you will see on your TV while running MythTV and what if any amount of overscan might be appropriate. 1. Application UI This includes the general UI of MythTV but might also include interfaces for applications launched from within MythTV like xine or various game emulators, or possibly any other Linux application. For MythTV you can have some considerable overscan present and the MythTV UI is still quite usable. The exceptions are IMHO the Setup screens and the Guide. The setup menus are particularly hard to use with any overscan because you often can't tell if a particular checkbox is checked, or read the descriptions of various options, or see the Cancel, Back, Next/Finish buttons at the bottom of the screen. Now, MythTV has some easily used options to put the UI within the viewable area of the screen. They're great but there are some downsides to using them which we'll talk about later when we get to them. Other programs may or may not be adjustable in the same way. Normal desktop-type windowed applications can be resized to use only the viewable portion of your screen, if you can get a hold of the edge in the first place. Most applications that can be used in fullscreen mode (like the video game emulators) use the whole screen and that's it therefore you're stuck. For general application use you really don't want any overscan at all. Ideally the viewable portion of the display would fill your TV edge to edge with no overscan and no underscan. Some slight underscan (where there are black borders around the edge of the screen) would actually be preferable to any overscan for normal application use. 2. Video When watching video you could argue that having no over or underscan would be ideal. For various reasons, some slight overscan is probably preferable and inevitable. The reason that some overscan might be preferable is that you may find black borders around the edge of the screen to be distracting. Of course if you have no under and no over scan, there shouldn't be black borders but usually that's nor perfectly possible anyway. Also, some programming (TV shows, movies, etc.) don't use the whole display area. They include black bars as a portion of the video you are trying to play back. If your TV has no overscan at all noticeable black bands will appear when you play back video like this. You may or may not find this distracting. If your display is underscanned (like the default configuration of xbox xebian is on my TV) then playing back video which already contains some black bars can appear downright absurd. In addition most TVs exhibit some distortion at the edges which make the image appear bent or bowed at the edges, particularly in the corners. Sometimes you can correct this, depending on the TV. In other cases the only practical way to hide these is to push them off the edge of the viewable screen. For the reasons above I would prefer some slight amount of overscan when watching video, others may disagree. 3. OSD/Video Overlay When playing back video (whether via xine or MythTV or whatever) you can usually bring up some kind of On Screen Display or overlay (so-called because it's overlayed on top of the video you are watching.) In general you really want the whole OSD to fit on the screen. Many programs (like xine) assume that the whole display area is visible to the user and consequently will position it's OSD in such a way that it might be off the viewable edge of the screen. MythTV generally makes this assumption as well. (It might be possible to adjust this without decreasing the size of the gui and using the GUI size for playback, if it is I'd like to know about it. I'll talk more about this later.) In any case having the OSD off the edge of the screen may or may not be a problem. If it's subtitles it could be a big issue. If it's just the lower edge of the pause display in MythTV, it's probably no big deal. Mostly I dislike having the OSD off the edge of the screen because it looks terribly tacky. :) OK, now to the meat. You've got too much overscan or maybe not enough. You want to tinker with it but how do we change the amount of overscan? There are many methods. I'll cover them in the order in which you might want to try them. Some are easier than others, some may not be possible in all situations, because your TV doesn't allow it or because you're driver doesn't support it, etc. 1. Service Menu Adjustments You're all likely familiar with adjusting the size and position of the display on your PC monitor. Usually it's quite easy to enter an on-screen menu system and make adjustments to the height, width and position of the image. It might be possible to do the same thing with your TV. Often times making adjustments of this sort requires that you enter into a service menu. How you get into the service menu is highly brand and model dependent. My Sanyo requires that you hold down Volume- on the front panel while plugging the TV in. Most service menus are very not-user friendly. In order to have much of a chance of adjusting your TV via the service menu you would be wise to acquire the service manual for your TV. Sometimes they can be downloaded from the Internet (though often not directly from the manufacturer from the same location as the the regular user manual.) I'm not sure what proportion of TV's have such menus or how available manuals are for any given brand. Probably avsforum.com is a good place to start looking for information about your TV and of course google might come in handy also. Once you have a manual and are in the service menu you still may not be able to adjust overscan perfectly. You may not be able to directly affect the size and position of the display. Other parameters may influence it and even with the service manual you aren't likely to get clear instructions on how to achieve adjusting the overscan. My TV has hundreds of adjustable parameters mostly with very obscure names. The size/position parameters were fairly clear, others less so. Usually you can also adjust parameters relating to contrast, sharpness, brightness, etc although usually some of those are adjustable without going into the service menu. I highly suggest you take notes on what you changed, what the value was when you found it and what you changed it to. You're likely to regret it otherwise. So, what if you have size/position adjustments? Can we completely eliminate overscan? Probably... at the expense of some distortions at the edge of the screen. You may quickly find that adjusting the size/position will reveal or exacerbate image distortions at the edge of the screen. Often there are other adjustable parameters in the service menu that might help you correct them... if you can find them and understand how to use them. It's possible using them may fix some distortion in one part of the screen and create them in another. You may have to be willing to compromise when making these adjustments. What happens if you have no service menu, or no manual, or no desire to monkey with it? 2) Scan Converter Adjustments If you are using some kind of external converter to go from say, VGA to S/Video or VGA to Component it's possible that the device may have some over/under scan adjustments built in. I owned a cheap VGA->S/Video converter that had an overscan adjustment on it. It worked about as well as the converter did... which is not very well at all. The adjustments weren't very fine and there was no setting which worked well for me. Still, if you are using such a device it's worth trying out as it's likely to be quite easy to test. 3) Video Card/Driver Adjustments There are two parts to this. One is direct driver option support for adjusting overscan. The second is playing games with the video timings. 3a) Driver Options The nVidia corp. binary driver for Linux has an Overscan option called TVOverScan. My understanding is this increases the amount of overscan but cannot be used to decrease it. It also seems only work on Composite and S/Video outputs with supported cards, not every nVidia based card. It may also be possible to adjust overscan with the nvidia-settings tool included with the binary driver. SIS based cards also have support for adjusting overscan and position. I have no experience with it though so I direct you to http://www.winischhofer.at/linuxsisvga.shtml for more details. As for other brands, including ATI, I simply don't know one way or another. Look to the gatos (http://gatos.sourceforge.net/) or X.org documentation for what options may be available for your video driver. The xbox has (had?) a tool for making adjustments to over/underscan called xbv. No idea if it's still supported or if it's been replaced. Usually these options are worth looking into because it's relatively painless to experiment with them. 3b) Video Timings This route requires some experimentation and a little knowledge of how X modelines are constructed. In essence you are creating a video mode that uses timings which are the same (or substantially the same) as a mode your TV will support, but the resolution is reduced. This has the effect of decreasing the usable resolution of your screen but since the resolution as presented to X applications is also reduced they all naturally fall within (you hope) the viewable portion of your display. The drawback is that you usually end up using a very odd screen resolution. This may or may not be a problem. Some applications might not like a non-standard resolution, like windows games running under emulation. (They may not like HD resolutions anyway, so it may be moot.) MythTV has no problem with it though. Also, if you are running say 1902x1064 in order to be close to 1080i (1920x1080) that means that when playing back 1080i HDTV recordings they have to be scaled. You may never notice though, or it may bother you. In any case the definitive reference is the relevant, then largely irrelevant, then relevant again Video Timings HOWTO written by Eric S. Raymond in 2000. It's archived variously about the Internet, here's one link: http://en.tldp.org/HOWTO/XFree86-Video-Timings-HOWTO/ The Video Timings HOWTO is chock full of useful information on how video works from the perspective of X. It's informative even if you don't plan on trying to concoct your own modeline. 4) Application Adjustments MythTV has the ability to fix your overscan problems without resorting to any of the above messing about. It works really really well, especially if you only run MythTV and aren't worried about emulators, xine, etc. In fact, many people could be happy just adjusting some mythfrontend settings and forgetting all about the above. There are some drawbacks, but they aren't generally worse than 3b above and are far easier to achieve. In the Setup menu of mythfrontend enter the Appearance menu. The second page has options for adjusting the size and position of the Myth UI. Initially it's set to 0 for all which means to use the whole display. If you have overscan, start by setting the width and height to your screen resolution-20 or so. Then set the offsets to 10 to keep the image centered. When you're done finish your changes by skipping through the other menu options and mythfrontend will try out your new settings. Keep tweaking until everything is within the the viewable display area. Another option in this menu is to use the GUI size for TV playback. This may or may not be what you want. If your TV is only slightly overscanned you might switch it off. If your TV is badly overscanned turning it on might be a better choice. Since it's so relatively painless to change compared to everything we've discussed up until now you can experiment away until you get it like you want it. So why wouldn't you want to do it this way? In some ways you are casting away some resolution... choosing not to use it. However, if you can't get rid of the overscan via other methods then you can't use those pixels anyway so who cares, right? Plus these adjustments don't carry to other applications you may launch from MythTV, like game emulators, etc. There are other options which affect overscan/underscan during playback in MythTV. It's in Setup/TV/Playback several pages in. Here you can adjust the vertical and horizontal over/under scan. I don't know exactly how mythfrontend uses either of these values. Underscan might be implemented by using less of the screen during playback. How is overscan done? Clipping of the edges? Perhaps someone can comment on the way MythTV uses these options. Conclusion: There is actually quite a bit of info on the net about making adjustments to TVs and the tools to do it with. I found the following article which includes some useful links at the bottom in addition to discussing overscan and so on. Two of the links are to images designed to be displayed while making adjustments. http://www.mastersofcinema.org/reviews/03lookingbeyond.htm
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