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2560x1440 single link and adapters




I have 2 Dell U2515H (2560x1440) monitors, both of them connected to an Asus ROG Matrix 780Ti  which as 1xDVI-I, 1xDVI-D, 1xHDMI, 1xDP

The connections are as following: 1 monitor connected DP - DP, the TV is connected to HDMI, and for the other monitor I have those 2 DVI ports.

Now the story:


I read all over that for quadHD 60Hz resolution you need a dual link dvi cable or adapter, else it will work at 30Hz or won't work at all.

1. The first strange thing is that the first day I had an adapter (DVI single link - HDMI) which came with an older nvidia card, and when I coupled the 2nd monitor it worked just fine. 2560xx1440@60Hz, how comes that when it was supposed not to work because of the bandwidth limitations ?


2. I bought a dual link dvi - hdmi connector just to be sure, but with this connector my second monitor when I put my computer to sleep, it wouldn't go to sleep. But instead would had display a message "no hdmi cable connected". I tried with the nvidia adapter single link dvi - hdmi and it was going to sleep properly.

Ok, so I bought a second dual link dvi - hdmi adapter from another manufacturer, and guess what, with this one it would go to sleep without any problems.

Why is that ? Wasn't supposed that those adapters to be the same ? Why do they differ ?


3. The third strange fact for which I would like an explanation:

While were the above tests were made on the first DVI-I port, I said to try on the second port DVI-D to see what happens.

With the nvidia adapter single link dvi - hdmi: it still working 2560x1440@60Hz in windows, it's going to sleep properly but when booting the 2nd monitor display a message that "the resolution or frequency not supported. Please set it to default 2560x1440 60Hz" something like that.

With the 2nd dual link dvi - hdmi adapter which is ok going to sleep, resolution ok ... but same message when booting as with the single link dvi - hdmi adapter.

While with the 1st adapter dual link dvi - hdmi which is not going to sleep, but instead display that message no hdmi cable connected, is booting ok without any issues or messages displayed.

All these boot issues are on the 2nd DVI-D port, because on the first port DVI-I all of them are working just fine when booting.


Can someone explain me what is going on ? And why is this behaviour, what is different from one to another or from port to port ?





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While single-link DVI doesn't technically have the bandwidth for 2560x1440 at 60Hz (one link at 165MHz, which is the DVI standard, is only enough for 1920x1200 at 60Hz) HDMI is based on the exact same signalling as DVI. On revision 1.3 HDMI increased its pixel clock to 340MHz which gives it support for 2560x1440 at 60Hz.


Because DVI and HDMI are based on the same signal, internally all your graphics cards' DVI and HDMI ports are connected to the same controller which handles both, and it is actually capable of 340MHz pixel clock to handle HDMI. Single-link DVI and HDMI both have 19-pins and are electrically compatible, so as far as the graphics cards' controller is concerned they are all basically the same ports, and it will happily output 340MHz HDMI signals through a DVI port if it's connected to an HDMI device at the other end with an adapter cable. Usually it's a single-link DVI device/monitor on the receiving end which causes the limitation, whose DVI port will only take up to 165MHz as per the DVI standard. When you used a DVI-to-HDMI adapter your DVI output essentially became an HDMI output, and since the HDMI/DVI controller and monitor were both capable of the 340MHz pixel clock, you are able to get 2560x1440 at 60Hz. You can test this behavior also by plugging in speakers or headphones into your monitor (and making sure your computer is set to output audio through the correct device). You will get sound through the monitor via your single-link DVI-to-HDMI cable, even though the DVI standard does not carry sound. It's being used as an HDMI port in this case.


Note that again, HDMI has 19-pins which is electrically compatible with the 19-pin single-link DVI. Further HDMI revisions have increased bandwidth by increasing the pixel clock from 165MHz (DVI) to 340MHz (HDMI 1.3) to 600MHz (HDMI 2.0). However these are all still 19 pins. Dual-Link DVI doubles the bandwidth like HDMI 1.3, but it does it by keeping the pixel clock the same and physically adding 6 more signalling pins. HDMI does not have these extra pins, so it is not inherently compatible. Any proper dual-link DVI to HDMI will be a signal converter box of some kind, it is not possible with a simple adapter cable. Converter boxes are finicky and it is not uncommon to have issues with sleep or simply not work altogether. There is no standard for conversion between standards, so it's sort of a toss-up. Some companies do it better than others.

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Thanks, it's much clearer now regarding the working single-link adapter.


The only thing which puzzle me now is why the adapters behave different on the 2nd dvi port of the video card. Might be related to videocard ?

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