U2515H, DVI to HDMI, resolution 2048x1152

Hi folks

I've just bought a Dell UltraSharp U2515H and I'm having a similar problem, as are many other people, I see, both with the U2515H and the U2715H. Can't get the monitor to do 2560x1440 over HDMI.

I bought the monitor to use with my primary desktop PC (an Asus Core i7-920 motherboard running Windows 7 Ultimate 64-bit), and 2560 resolution was a major factor in my choice and the high price. So getting this to work is a priority!

On the desktop PC I am using an Asus EAH4890/HTDI/1GD5/A video card, which is capable of 2560x1440 resolution and has twin DVI-D video output ports (no HDMI). The DVI ports are both DVI-D Dual which is needed to support the 2560 resolution, as DVI-D Single only supports up to 1920x1080. So no problem there.

Since the monitor only has HDMI ports, I am trying to connect it in two ways:

1) Using a DVI-D to HDMI converter cable between video card and monitor.

2) Using a DVI-D to HDMI converter dongle, and then a conventional HDMI cable from there to the monitor.

Method (2) causes the monitor to issue an error message saying that current timing mode isn't supported and suggests using 2560x1440 resolution.

Method (1) works, but the maximum resolution offered under Windows is 2048x1152, which is what I'm running it at now. Both Windows and the video card driver are fully updated and I am using Asus EAH4890 video card driver v8.850.0.0 dated 2011/04/19.

I suspected a problem with the video card on the desktop PC, so I tried the monitor with my laptop (a Dell Latitude E6530 Core i7 running Windows 7 Ultimate 64-bit), using the laptop's HDMI port and a plain HDMI-to-HDMI cable (different cable). Lo and behold, exactly the same problem : the maximum resolution mode offered by Windows 7 as being compatible is the same : 2048x1152.

From reading other folk's submissions on this problem, I gather that using the monitor with a DisplayPort cable does make it possible to drive it at its native resolution. But that is not an option for me - it must work with HDMI (as the published specifications say it does).

It seems to be some limitation of the monitor when driven with HDMI.

Has anyone cracked this problem properly, ie not with a work-around ?