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Old 06-11-03, 05:38 AM   #1
schizo
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Default Custom resolutions and TV Out using nvidias X11 drivers

Whilst in the process of setting up a freevo/mythtv box, I've ran into what I consider to be a weird and annoying limitation in the drivers. Out of the box, there are in principle only two modes that are usable on the TV and that's 800x600 & 640x480, both at 60Hz.

Now, a TV screen has 625 lines but the visible display of a TV consists of much fewer lines than this. In pratice a PAL TV screen can be said to have 576 visible lines, in other words 24 lines less than the 600 lines of the standard VGA mode. Given that the 600 lines are displayed in their entirety on a screen that only (at best) has 576 lines, some sort of interpolation must be done (I presume) to generate the actual lines that are displayed. Logically, this cannot improve the image quality any - and using the windows drivers, I can see a big improvement by going from 800x600 to 768x576.

Another problem with the standard modes is of course that they are all 60Hz modes, and not the 50Hz that the PAL-B TV I use would expect. How this is even possible is beyond me - an old television set might not be able to display this mode at all!

So , being something of a perfectionist, I set out to create what I consider to be proper TV modelines and also to fix the framerate to a more proper 50Hz vertical sync. I added two modes to my XF86Config - 768x576 and 720x540, both at 50Hz, calculated with a modeline tool I found.

So what happens? Both modes are rejected by the X server because, and I quote, "this is not a TV resolution" .. eh, come again?? These are _THE_ tv resolutions if you own a PAL tv, and everything else are VGA/VESA adaptations that almost but not quite match the TV spec.


Is this really what it appears to be, a hardcoded limitation in the drivers that do not allow any deviation from the preprogrammed semi-TV resolutions of 640 and 800, or is there some workaraound?

NVidia devs: Am I completely misunderstanding something fundamental here, or am I right in assuming that we're missing out on something rather important here? The windows drivers do support at least the 768 resolutions, and they DO look immensely better on my Sony 32"..
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Old 06-11-03, 07:31 AM   #2
Thunderbird
 
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Only a few tvout encoders support resolutions other than 640x480 and 800x600. If you use nvtv for tvout you can use real 768x576. Very likely the current 800x600 already is 768x576, since because of overscanning useally some lines are hidden.

The TV will display at 50Hz I think, when you set it to NTSC (which most European tv's support) it becomes real 60Hz. Likely when you set vrefresh in the XF86Config file from 60Hz to 50Hz it will work in 50Hz.
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Old 06-17-03, 06:12 AM   #3
notzed
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Having looked at some of the tv encoder specs, they usually have things like interpolation/scale settings that scale the 800x600 or 640x480 to the tv output size ... so surely this could be used to try to get 1:1 scaling (i.e. none) in vertical lines during output, even with some cropping of the video buffer, so that interlacing can work properly.

But basically as it is now, the output is very substandard, and there are no tuning options (hell, it doesn't even position itself in a very central location). 800x600 is scaled down to fit, and 640x480 is scaled up. So neither allows proper interlaced video output. An the nv-tv-out thing doesn't seem to handle my encoder chip properly, if it would help at all anyway.

Maybe the encoder chips aren't that flexible, but if it can be done under windows, then its obviously not a h/w limitation.
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Old 06-18-03, 02:03 PM   #4
schizo
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Yes, this is my point exactly. The available resolutions are either stretched or compressed in order to fit on a 'real' TV screen and this doesn't really help image quality any. The optimum would be a resolution where each line of pixels in the framebuffer correspond _EXACTLY_ to one scanline on the TV, and this is how your image is normally displayed on a CRT monitor.

Now; a TV screen normally has 620 scanlines, out of which 576 are used for picture data. There's no set resolution horizontally, but with square pixels you would set this to the same as your frame aspect ratio, ie 4:3, or 1.33333xscanlines. This comes out at 768 pixels, so your resolution then would be 768x576 for full overscan TV display, and this _IS_ the real TV resolution.

As has been pointed out this resolution is also available in the windows drivers, so there can't be any limitations in the hardware itself - and how could there be, given that the signal is almost certainly 576 lines or less when it's output to TV ?
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Old 06-18-03, 04:20 PM   #5
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Likely nvidia doesn't want to cause any confusion. All tvout encoders can do 640x480 and 800x600. Not all can be programmed to modes like 768x576, 720x576 and so on. If nvidia said "brooktree" chipsets can do all modes, chrontell (or is chronotel, I find both names ..) can only do 640x480 and 800x600 ... it would cause lots of confusion. The only way to find out the encoder is to check the card or to watch the xfree86 log file ...

Like I said before nvtv does support custom modes.
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Old 06-19-03, 09:44 AM   #6
AranC23
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Default 640x480 looks fine, 800x600 and above is squashed

I have a problem where 640x480 looks ok, but leaves black borders around the edge.

800x600 (and 1024x768) is squashed horizontally (leaving an inch or more or black space on the left and right.)

Any attempt to use xvidtune to fix it results in an out of range error. Any attempt to use nvtv to fix it (I have a booktree encoder chip) results in a scrambled display.

For now I'm just leaving it in 640x480 and dealing with the relatively small black borders. I find the menus in MythTV to be much more readable at higher resolutions though...

Since it's a GeForce2, the TVOverScan option for the XF86Config file won't help me (or so the README says.)

Would increasing the supposed refresh rates in the XF86Config file help? My TV is relatively new Sony Wega... I guess I'll have to go look for the manual.
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