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jmiddel Feb 14, 2013 01:54 AM
Dual Link Digital DVI-D input??
I am thinking of getting a souped up Mini to replace my 27" iMac that's going out of Applecare. I found a 27" monitor for under $400, which requires the Dual Link Digital DVI-D input. Which is expensive ($99) and gets lousy reviews. Apple's 27" LED (not TB) does not require this, so does anyone know why a monitor would require this DVI/USB monstrosity? Especially, USB??
P Feb 14, 2013 08:30 AM
Not sure I understand the question, but... You are talking about the Apple adapter, right?

What happens is this: DVI is based upon VGA, so it is a quite inefficient way to drive a display, which means that the maximum bandwidth is quite limited. If you want to drive a 2560*1600 (or 2560*1440) display, that is more than a single DVI connection can handle. The workaround is to basically drive two displays and stitch them together so you don't notice - DVI-DL.

The problem is that DVI, despite being a digital connection, requires various analog circuitry to function, and this circuitry needs to be duplicated for each link. These days, GPU makers usually don't bother adding too many sets of the analog circuitry, so they are quite limited in how many VGA/DVI connections they can drive at once. If you look closely at the specs of modern GPUs, you will see that you are quite limited in which ports you can use together.

The regular DisplayPort to DVI adapter is really just a thing to tell the GPU to start outputting DVI over the DisplayPort and wire up the pins correctly. The Dual Link adapter cannot do that, because the mini doesn't have the analog circuitry to output DVI-DL. The only way to do it is to do some sort of active conversion, and this requires power (hence the USB) and costs money (hence the price).

AFAIK, the only Apple 27" displays are DisplayPort and Thunderbolt, neither of which have these limitations. The Apple 30" Cinema Display does require a DVI-DL connection.
mduell Feb 14, 2013 04:58 PM
I think you're associating cause and effect backward. You need the DLDVI adapter because your iMac doesn't have DLDVI output. The DLDVI adapter needs USB for power. There are tons of computers out there with DLDVI output, both Mac Pros and PCs.

You can get a non-Apple 27" monitor with native DisplayPort, then you just need a Mini Displayport/ThunderBolt to DisplayPort cable/adapter which are cheap.
jmiddel Feb 14, 2013 09:05 PM
P, thank you for clarifying that so clearly. And mduell, that's what I'll have to do. I had found this monitor For only $385.02 each when QTY 50+ purchased - 27" IPS LED CrystalPro Monitor WQHD 2560x1440 for $380, but it only works with the DLDVI input, so I found another one for $530 which accepts DisplayPort and HDMI. Given the price for the DLDVI adapter, not much more. It's this Nixeus WQHD 27" S-IPS Ultra High Resolution 2560x1440 PC/MAC Monitor NX-VUE27: Computers & Accessories Thank you both again :)
ibook_steve Feb 15, 2013 07:46 PM
I agree with everything except saying that "DVI is based upon VGA." A DVI-I connector includes VGA pins so it supports VGA, but like you say, DVI is all digital, so it isn't really based on VGA.

P Feb 16, 2013 11:53 AM
DVI is sort of based on VGA - at the very least, the signaling was designed with VGA very much in mind. DVI is based on the same transmission method as VGA, and as such, it requires DACs - one per link. Basically it takes the digital signal and transmits it as if it were a rasterized analog image. DisplayPort uses packetized data transmission - like most digital signals - which is why it is cheaper to implement, allows higher resolutions and update frequencies, and can be implemented for as many displays per GPU as the manufacturer wants to bother adding connections for.
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