Blur and blue tint on DVI signal from dock

[pimpinainteasy]

I have a Dell 2709WFP 27" LCD that I use without any problems with the VGA outputs of my Thinkpads and docking stations.

But I've tried connecting the monitor by DVI to two different docking stations (one mini dock, one Dock II, both with the same T42 inside them), and each time, I get an image that is readable, but tinted blue, and a bit blurry.

I am guessing that this is simply because the docking stations don't produce enough current for the 27" LCD, and there's no way around it (other than possibly an external video card in the Dock II). But am I overlooking any other possible solutions?

Thanks for any tips.

[dr_st]

I am guessing that this is simply because the docking stations don't produce enough current for the 27" LCD

Of all the possibilities, what made you think of this strange idea?

Since when must a docking station produce current for the LCD?

[RealBlackStuff]

In the IBM/Lenovo specs:

Support for a DVI monitor is through the ThinkPad Dock II option and is limited to a resolution of 1280 x 1024 at 60 Hz.

[pimpinainteasy]

Since when must a docking station produce current for the LCD?

What else is a DVI signal made of, if not electrical current?

My impression -- gleaned secondhand from online discussion -- was that the DVI signal coming out of the Dock II doesn't have enough amperage to drive bigger monitors.

[dr_st]

That's the first time I've heard such weird assumptions. I'd appreciate if you could provide the source.

The DVI signal is data only. The monitor does not rely on the amplitude of the signal. The only power passed over DVI is +5V standby power.

[pimpinainteasy]

Even a digital signal is made of current. See for example this online reference:

http://www.datapro.net/techinfo/dvi_info.html

The official DVI specification mandates that all DVI equipment must maintain a signal at 5 meters (16 feet) in length. But many manufacturers are putting out much stronger cards and bigger monitors, so the maximum length possible is never exact.
Although the mandated DVI spec is 5 meters, we do carry cables up to 25 feet, and have succesfully extended them even longer than that (although results do vary depending on hardware). For guaranteed signal quality on long runs, you should consider using a powered DVI signal booster.

Despite common belief, there is such thing as signal loss in digital pictures. When a DVI run is unstable, you may see artifacts and "stuck" pixels on your display; further degredation tends to flicker out or shake, and the ultimate sign of loss is a blank display. In-house tests on varying equipment have produced strong signals up to 9 and 10 meters long. Tests at 12 meters generally resulted in signal loss and an unusuable image on the display, and anything longer rendered no image at all.

And there seems to be a common assertion that bigger monitors require a stronger minimum signal strength.

Again, on both of these points, I'm only repeating what I've read, and it makes sense to me -- but I'm not an engineer or technician.

I tried lower resolutions, all the way down to 800*600, and it's still blue on DVI, perfect on VGA. Just for fun (mine even if not yours), here is a photo of my screen in DVI:

http://picasaweb.google.com/lh/photo/nq ... directlink

Using VGA isn't so bad, but still, this blue tint is an interesting problem I'd like to figure out.

[wap32]

Well, DVI still has seperate signals for red, green and blue, so perhaps a faulty cable or plug?

[dr_st]

Even a digital signal is made of current.

There is no argument about this.

http://www.datapro.net/techinfo/dvi_info.html

Despite common belief, there is such thing as signal loss in digital pictures. When a DVI run is unstable, you may see artifacts and "stuck" pixels on your display; further degredation tends to flicker out or shake, and the ultimate sign of loss is a blank display.

There is no argument about this either.

And there seems to be a common assertion that bigger monitors require a stronger minimum signal strength.

This is the claim that I find peculiar. Probably just a myth. There is no logical reason for that to be so. It's quite possible that particular monitors are more finicky than others, but no reason for there to be a correlation between that and size. Larger monitors require more power for the matrix and the backlight, but the DVI signal is not the source of that power.

Well, DVI still has seperate signals for red, green and blue, so perhaps a faulty cable or plug?

This is a more plausible explanation, as far as I see it.

[pimpinainteasy]

This is the claim that I find peculiar. Probably just a myth. There is no logical reason for that to be so. It's quite possible that particular monitors are more finicky than others, but no reason for there to be a correlation between that and size. Larger monitors require more power for the matrix and the backlight, but the DVI signal is not the source of that power.

I also find this claim peculiar (to say the least), along the lines of "maybe the modem overheated because you were using it to call long-distance." (Priceless advice from a Radio Shack employee ca. 1992.) But it's (the DVI thing, not the modem thing!) been repeated so often, in this forum and others, that I began to believe it. The prevailing wisdom (in my understanding) seemed to be that Thinkpad docks often can't drive very big monitors, because they were designed in the time of smaller monitors, and thus they provide lower-current signals.

Cable or plug: that could be the culprit, but it's a brand new Dell 2709WFP monitor, and I really doubt those would ship with such a blatant defect. (Can't be the Thinkpad Dock plug, as I tried three different ones, with same results.) Maybe I'll try a different DVI cable sometime, but for now, I'll just use it with the VGA cable and be happy with that.

Thank you again for engaging me.

[pimpinainteasy]

Problem identified: it was the Thinkpad itself. I tried external DVI with a different Thinkpad, using the same monitor, cable, and docking station, and no blue tint. I guess it was something in the DVI chip, as the original Thinkpad has no blue tint problem with internal display nor with external VGA.

[wap32]

A blue tint still makes me think one of the color signals (red) is not getting through.

Has the laptop ever been opened? I guess an accidental tool slippage or similar could have knocked one of the smd components in the signal path.

I'm just speculating here, but I have had a similar problem with a graphics card where one of the colors was missing as well (causing a tint) and turned out to be a torn apart surface mount component.

[pimpinainteasy]

I'm no longer using that T42 with an external monitor, so I'm not concerned about the problem practically, but am still somewhat interested in it theoretically.

Poking around a different forum, I found someone who was having a similar problem on a Thinkpad with an external monitor, but with VGA output (only) and a yellow tint:

http://superuser.com/questions/25721/th ... dvi-output

I have a Thinkpad T60 that I use at work. When I plug in an external monitor to the VGA port of the machine (either the VGA port in the laptop's docking station, or the VGA port built into the laptop itself), the image on that monitor appears tinted yellow. (White appears as yellow; gray appears as light red; blue, green, and yellow are mostly unchanged.)

This behavior does not occur on an external monitor plugged in via the docking station's DVI port -- colors appear normally on an external monitor plugged in via DVI. The laptop's built-in screen also displays colors normally.

I hope this thread might help someone with a similar problem.

[sjthinkpader]

Yes, my T60 will give a yellow tint on VGA port also. Everybody notices that during a meeting.