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Old 07-14-07, 02:36 PM   #1
westis
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Default TV-out quality and overscan

Hi,

We're building a custom playout system for public access TV channels. It's built in Linux, on the gstreamer library.

The issue is that we don't get the picture quality expected. We playout fullscreen through the composite TV-out on the nVidia 7100GS graphics card to a Kramer switch with RCA inputs.

Our current system is a Windows XP box where we playout through a RealMagic Xcard, an mpeg-2 card that gives very good quality.

But through the nVidia 7100GS TV-out the picture is not as sharp, which is most noticeable for text. Anyone who might know why? Should the picture quality not be as good using a composite out on a graphics card as when using the same output on an mpeg-2 card?


The second issue is that we probably need to overscan the picture. On an old TV we see the entire picture, which we shouldn't since the are outside the safe area should be cut, as is done with our current system with the Xcard.

Does a fullscree through the graphics card underscan the picture? I haven't looked on an LCD TV, but I suspect that there are black borders around the picture, as we already see the entire picture on an old TV.

Hope I made myself clear enough... Would be grateful for any assistance!

Thank you.
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Old 07-14-07, 04:39 PM   #2
pe1chl
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Default Re: TV-out quality and overscan

What you are probably seeing here is the scaling between the screen that Linux is painting on, and the actual video output.
Unfortunately, the TV-OUT hardware is trying to be "helpful" by defining a "standard" video resolution like 640x480, 800x600 or 1024x768 and then scaling that picture to the actual resolution of the TV format. This introduces effects that are most clearly seen on sharp objects like text.

This was probably invented in the early Windows days when everyone had a "standard" screen resolution and using some arbitrary value like 720x576 would confuse certain programs. I think this is all long history (maybe games are still a problem? I never use games). For the rest of the users, the scaling is just a nuisance. It should go, at least the option should be available to get rid of it.

When you are using NTSC you could at least set the screen size for the TV-OUT screen to 640x480, as this happens to be the NTSC resolution. Probably there will still be scaling, because of the overscan. But it could be better.
For PAL users, I'm afraid there are no options.

I think there exist specialist cards for what you want to do. I.e. geared towards using a PC for broadcast-quality TV output. Those may serve you better.
When your equipment can accept RGB input instead of composite, you could also use the VGA output instead of TV-OUT, with a suitable modeline and maybe via some interface box to convert RGBHV to what your equipment expects.

For overscan, please read the driver README.txt
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Old 07-14-07, 05:45 PM   #3
westis
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Default Re: TV-out quality and overscan

Thanks a lot pe1chl for your informative reply!

So the mpeg-2-card RealMagic Xcard that we have now may then be a "specialist card for what you want to do. I.e. geared towards using a PC for broadcast-quality TV output." ?

Is there such a card that is Linux compatible? I don't think that the Xcard is...

I'll see if I can find some kind of RGB-to-RCA converter, if that may produce better results. Why would that be better than sending the video through the composite output?

/Daniel
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Old 07-14-07, 06:38 PM   #4
pe1chl
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Default Re: TV-out quality and overscan

Quote:
Originally Posted by westis
So the mpeg-2-card RealMagic Xcard that we have now may then be a "specialist card for what you want to do. I.e. geared towards using a PC for broadcast-quality TV output." ?
I don't know that card. Probably you can find suitable cards via Google etc.
Those cards also have features like genlock.

Quote:
Originally Posted by westis
I'll see if I can find some kind of RGB-to-RCA converter, if that may produce better results. Why would that be better than sending the video through the composite output?
When you convert to composite video (RCA), the results will not be much better. The only advantage will be that you skip a hard-to-control scaling step. The VGA output still can be controlled with specially tweaked modelines, while the TV-OUT cannot.
Of course you will have best results when not converting to composite but keeping the signal as RGB or YPbPr.
But it depends what kind of broadcast standard you are using too (analog or DVB).
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Old 07-14-07, 08:02 PM   #5
smp5
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Default Re: TV-out quality and overscan

By the way I've noticed the same extra border around the TV-Out on a Toshiba M45 laptop with its S-Video connector. Theres a huge convenience factor with such output from a laptop however the quality is very low!

I've had a little better luck using a TView Micro by Focus enhancements, which simply takes your VGA out and converts it to composite. That gives the expected full screen output and I drive it at 1024x768 and let it deal with converting all that information to the best possible for composite.

Granted thats still an inexpensive product. Our public access TV center here has an Extron VSC 200 Computer To Video Scan Convereter rack mounted with all their other equipment, and I could probably run a 25foot VGA cable from my computer to it as the next step up. This would just replace my inexpensive TView Micro.

The ultimate approach would be the NVIDIA Quadro FX based SDI cards, as pe1chl was hinting at to get the true RGB YPbPr output, pixel for pixel matched for broadcasting for routing through the video switching equipment. Much more expensive.

Which public access provider are you with? At some level, producers from your establishment and ours should keep in touch on such approaches. And particularly, management from each should know how other outfits are addressing this. The public access station I'm at as a producer is http://www.mnn.org/ in New York
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Old 07-14-07, 09:40 PM   #6
westis
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Default Re: TV-out quality and overscan

Thanks for your replies!

smp5: I'm the local manager at the Open Channel i Vxj, Sweden (www.okv.se) and also a member of the board for the National Association of Open Channels in Sweden.

Indeed, cooperation with these kind of things is essential for public access TV channels! As a newly eastablished channel (we started broadcasting on May 1st 2006) we are in need of knowing how others have solved these kinds of issues. Especially as we're on a very low budget (no government support to public access TV stations in Sweden....).

And we probably could contribute a bit ourselves too. For example, we're currently building a very nice Linux-based broadcasting/playout system called Ticasto, which is the reason I'm looking for ways to get the video from that to the TV in best possible quality. But maybe that's a topic to be discussed elsewhere...

Sounds interesting with the solutions you mention. The last option is way out of our budget though. And maybe one should not mention this on an nVidia forum, but the Matrox Parhelia series looks interesting too, in ways of outputting s-video in native PAL format.

smp5, I'll PM you with more details of how to get in touch with me to discuss this more outside this forum.

Thanks for your help guys!
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Old 07-15-07, 05:35 AM   #7
phelin
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Default Re: TV-out quality and overscan

Quote:
Originally Posted by westis
So the mpeg-2-card RealMagic Xcard that we have now may then be a "specialist card for what you want to do. I.e. geared towards using a PC for broadcast-quality TV output." ?

Is there such a card that is Linux compatible? I don't think that the Xcard is...
Take a look in here: http://dxr3.sourceforge.net/about.html

That solution is the most commonly used one with self-made Linux HTPCs with old hardware and the cards themselves cost about 10 euros when bought second-hand.
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Old 07-15-07, 09:01 AM   #8
westis
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Default Re: TV-out quality and overscan

Quote:
Originally Posted by phelin
Take a look in here: http://dxr3.sourceforge.net/about.html

That solution is the most commonly used one with self-made Linux HTPCs with old hardware and the cards themselves cost about 10 euros when bought second-hand.
phelin, would you suggest such a card rather than RGB from the VGA output or s-video out from a graphics card that has inherit support for the PAL format?

The computer itself is quite good, an AMD Athlon 64 X2 4000+ 2.1GHz with 1Gb RAM.
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Old 07-15-07, 11:47 AM   #9
phelin
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Default Re: TV-out quality and overscan

Quote:
Originally Posted by westis
phelin, would you suggest such a card rather than RGB from the VGA output or s-video out from a graphics card that has inherit support for the PAL format?

The computer itself is quite good, an AMD Athlon 64 X2 4000+ 2.1GHz with 1Gb RAM.
I have not seen one in action myself, but I have been told that the quality is as good as it is possible when connecting with composite or svideo, much better than from typical graphics cards. I have used composite out of a Technotrend DVB-C card, which is supposed to be the same quality as the DXR3 cards, and it was really good.

Of course RGB should be better, but it is quite hard to take such out. If you are thinking about Parhelia, well, the support is not that good.

Perhaps you should check this: http://www.realh.ukfsn.org/tvhowto
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Old 07-15-07, 12:13 PM   #10
westis
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Default Re: TV-out quality and overscan

phelin: Do the Technotrend DVB-C, DXR3 or Hollywood Plus take still images drawn with GTK too?

The "streamer" or whatever you'd call the bulletin board kind-of system with information between shows is using GTK to display still images. That's why I suspect it needs to be shown fullscreen on the Linux computer.

The Tview computer-to-TV scan concerters that smp5 mention maybe then are better options, as they also takes the VGA output and converts it to good quality TV format?

/Daniel
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