Signal degradation with Dual-Link DP-DVI adapter

[Timur]

When I connect a display via a Dual-Link adapter (tested original Apple and Dr. Bott) to the external Mini-Displayport of my Macbook Pro 17" 2009 I don't get full 8 bit per channel color depth (or at least it's seriously messed up with heavy banding), neither in native resolution (2560x1600) nor in non-native (1920x1200).

Running the same display via the same cable over a passive Single-Link adapter works perfectly well (all 256 shades of grey present).

I tried two different cables (both a Dual-Link and a Single-Link cable, both of which work with the Single-Link adapter). So either both Adapters are bugged the very same, or the 9600M GT does something wrong when a Dual-Link adapter is connected. It could also be a driver issue, but at least the internal display doesn't show any banding when running in Clone mode.

Same happens on OS X with both the 9600M and 9400M. So either both adapters ain't working properly or there's something wrong with the combination of NVidia 9x00M and Dual-Link DVI.

And I tested three more single-link displays (1x 1920x1200, 2x 1680x1050) and only one works (the 1920x1200 Viewsonic VP2330), the other two show the same problem with both Mini-Displayport to Dual-Link DVI adapters.

Then I tested connecting a current ATI based IMac 21.5" to one of the not working displays (a Samsung) and there worked with both adapters.

So it seems that the 9400/9600M in the Macbook Pro output a signal of lower strength (compared to the ATI based Imac), which leads to signal degradation once coupled with a Dual-Link adapter. The reason why the VP2330 works probably is because it can work with weaker signals. That doesn't help me at all with my 30" display though and only 1 working display out of 4 is rather disappointing.

 

[dr. shdw]

Digital signal doesn't really degrade, it's more all or none. It's most likely a compatibility issue..

 

[Timur]

Digital signal still has to be transferred as analog voltages. And if you say "it's all or nothing" then there cannot be a "compatibility issue", because either both the display and GPU port talk the same protocol or they do not.

And since these issues not only happen at full 2560x1600 resolution, but also at 1920x1200 over both a dual and single link cable I don't see where that "incompatibility" should stem from? The active Dual-Link adapter should pass on the very same signal that the passive Single-Link adapter squeezes through (same GPU port, same cable, same display, just different adapter).

Even the pure white background in this forums flickers slightly when running over the Dual-Link adapter (at full 2560 resolution atm). Tomorrow I should get another adapter (Club 3D branded, but I think it's the Accell one).

So the answer that seems to make most sense is that the "active" (as in needs external USB/5V power to function) adapter degrades the signal. And since the Imac worked nevertheless I guess that it outputs a higher power/voltage/quality signal to begin with.

These adapters cost between 80 and 110 Euro, one should expect them to work premium. But seeing how the Imac output works better I am inclined to say: You cannot make gold out of straw (aka if bad goes in then bad comes out).