X works but is very soft / unsharp (nvidia) [SOLVED]

[pvh1987]

I just installed Gentoo on my computer. Everything seems to work like it should except for the X server.

Although the X server works and displays twm on my 1920x1200 monitor when typing startx, the image is very soft, like if the wrong resolution was chosen. However, it looks like it actually is 1920x1200 but very unsharp. How is this possible? My monitor, a Samsung SyncMaster, is connected to my GeForce 8800 GT using DVI.

I have followed both the X Howto and the Nvidia Howto from the Gentoo documentation resources.

In order to start the X server, I had to put at least these lines into my xorg.conf:

[Kaso_da_Zmok]

did you try booting some linux live usb with nvidia or nouveau for comparison?

[pvh1987]

No I did not. But before I had Ubuntu 10.10 (I think) and it used the "commercial" nvidia drivers and it worked just fine.

[NeddySeagoon]

pvh1987,

Please put your /var/log/Xorg.0.log on a pastebin site so we can see what Xorg is actually doing.

Most of your xorg.conf is redundent and will be ignored.
Only the

[pvh1987]

OK, I reduced my xorg.conf and started X again (same results) and here is the ouput of my xorg log:

http://pastebin.com/PAQ1bejQ

[NeddySeagoon]

pvh1987,

Xorg says

[pvh1987]

Looks strange. Shouldn't the DPI be equal for both horizontal and vertical resolution? Maybe that is the problem.

I use the exact same DVI cable, graphics card and monitor as a few days ago, when I used Ubuntu on the same computer. Never had this problem before.

Since I have two DVI-outlets on my graphics card, I just tried to connect to the monitor using the other outlet. Rebooted. Same thing -- soft, unsharp picture.

It must be a driver and/or configuration problem. Any other ideas?

[NeddySeagoon]

pvh1987,

It does not follow that it must be a driver issue because Ubuntu operates differently.
The driver only draws the screen pixels in the pixel buffer. They cannot be blurred there. Therfore its in the transmission or display of the bixel buffer.
We know that Xorg is operating in the native resolution of the display, so there is a 1:1 mapping between the pixel buffer ang the display. i.e. every pixel in the pixel buffer in the graphics card maps to exactly one pixel on the screen. No scaling is required, which would reduce image quality.

Please compare the wikipedia images of DVI plugs and tell which one you have.
You can try a different nvidia-drivers if you wish.
We can force a single link display mode by writing a modeline, if you like, too. This probably fix the issue at the cost of a reduced refresh rate. The pixel clock must be under 165 MHz.

The differences in DPI is due to the relationships between the horizontal pixel timing and the vertical pixel timing. Yes they should be identical but I've never seen them better than very close.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[pvh1987]

Ok, thank you for pointing that out for me.

So, I figured out that my graphics card have two DVI-I (Dual Link) ports (at least, they will be able to take a fully configured DVI-cable with all possible pins). However, the DVI cable I am using is a DVI-D (Single Link). These observation are based on the pins available at the connectors compared to the figure from Wikipedia: http://en.wikipedia.org/wiki/File:DVI_Connector_Types.svg

I assume that my monitor's native resolution is 1920x1200@60Hz (that is the setting I've always used on Ubuntu and Windows).

[wcg]

A little off-topic, but I can no longer resist asking this question.

What is with these digital tvs/monitors that refresh at the same
frequency as flourescent lights? (Back before digital monitors
were common, this was considered a source of perceived
flicker. If your monitor only worked at 60hz with your gpu,
you needed to change your room lighting or suffer for it.)

Now I am seeing all of these monitors (and hd tvs) advertised
at X by Y at 60hz. What's up with that?
_________________
TIA

[gorkypl]

The referesh rate of LCD means something very different than in CRT. In CRT the whole screen was redrawed, and flickering could be seen for frequencies under 85Hz.
In LCD the pixels are lit constantly, and the refresh rate means only how often the pixel can change its state. So LCDs do not flicker in the same manner* as CRTs did, and 60Hz is a high enough value that you cannot see individual frames.

*OK, almost. There is a flickering of LCD backlight, because the brightness is controlled by PWM. So when you lower the brightness, the LCD starts to flicker. The frequency of this flickering is independent of monitor refresh rate, and is determined by PWM module. In better monitors (Eizo) the rate is so high (300Hz IIRC) that cannot be seen, in lower quality monitors (most other than Eizo and proffesional NEC ) it can be a problem when the brightness is low.
See for example:
http://www.youtube.com/watch?v=YnlnbQfDb8w
_________________
BTW, TWM FTW!

[Jaglover]

**There is no such problem with LED monitors. In cinema 24 frames per second does not flicker because the whole frame is switched instantly, not drawn dot by dot as in CRT.
_________________
My Gentoo installation notes.
Please learn how to denote units correctly!

[pvh1987]

Any new ideas that might solve this problem?

[pvh1987]

I think that I after all might have a solution myself. Going into the on-screen menu on my monitor, I chose "Image Reset". After that, the screen is not soft anymore. I still think this is strange, but it seems to work.

[NeddySeagoon]

wcg,

With a CRT monitor, only a small part of a single line is ever lit at a time. The lillusion of a complete image on a CRT display depends on a feature of vision called "persistance of vision"
Sensitive individuals can pereceive the beats between flourecent lighting, 100Hz or 120Hz depening on your region and the frame/field blanking of a CRT display, the vertical refresg rate,
Percistance of vision is also what makes interlaced display modes work. Most people can see interlace flicker once its pointed out to them and its very annoying. Its gone away with CRT displays.

In LCDs and LED displays the image is retained for meany seconds (not a few microseconds) even if the view source is removed. You can't see this as displays blank the disply when there is no input.
It means that while the image can be changed at the refresh rate, the pixels are always lit until they are changed, so there is always a complete image presented to the eye. Thus no flicker.

As others have said, the LCD backlight flickers as brightness control is by pulse width mdulation. However, this is applied a a frequency of 200 or 300 kHz, which is well oustide the range of human perception.
Care has to be taken in the design of the display so that beats between the PWM and the pixel clock on the display do not cause visible banding. This is always a compromise as the pixel clock and its deriviatives vary depending on the display mode selected. Hence some video modes look worse than others.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

[wcg]

OK, thanks for the info on lcd/led pixel clocks.
(Back to the actual topic, which seems to be
solved in this case.)
_________________
TIA