agent_x007 wrote on 2023-06-03, 15:08:
To point out few things : You are being very optimistic about what everyone has available to them (OP has modern 1440p monitor, so no CRT magic of not having native resolution for him here).
Connecting VGA output to any modern digital only monitor, will result in reduction of max res/refresh to whatever adapter is cable of outputting (usually 1080p60).
Optimistic or not, I am talking about the actual physical limits that most video cards with actual VGA output have and which are the primary limitations of what can come out the port. It is inaccurate to say 2048 x 1536 @ 85Hz being the limit. While it does reach 388MHz pixel clock with CVT standard, and could be seen as a higher end due to being near the real limit of 400MHz pixel clock of most video cards and why it is in the VESA standard as the highest thing, and highest you can choose in a typical driver, but in reality, without any optimistic factors, any resolution that fits into the 400MHz limit can be output although one will have to create custom resolutions to do so in many cases. Some drivers allow this to be done without additional tools, many do not and alternative methods are required such as PowerStrip or Modelines etc.
It certainly is true the monitor's own input can be a limiting factor but saying VGA tops out at x resolution, is not accurate. One needs to work down from pixel clock limit of a video card, you take the maximum pixel clock which is usually 400MHz, then divide it with your wanted framerate, line count (including blanking and sync lines) and then you're left with maximum horizontal pixel count that includes blanking and sync pixels. It is as simple as that. Old Radeon 7000 in my test computer has documentation say 300MHz RAMDAC but testing shows it has upper limit of 400MHz as most other things do from that time and beyond.
Baoran wrote on 2023-06-03, 14:30:
Is it an artificial limitation when a manual of a motherboard for example says that maximum output resolution from d-sub in the I/O panel is 1920x1200@60Hz?
This limit can be due to the signal conversion from DVI/HDMI(and sometimes DP) to VGA with some cheap chip, of which almost all have a 165MHz pixel clock limit which is just enough to do this resolution with reduced blanking (which will often not work with a CRT and causes geometry problems at edges of screen) but an LCD that has much smaller blankng needs will be fine with that.
Similarly, using one of the numerous dongles sold on ebay etc. will be similarly limited (since they also use same kind of chips). I am aware of one dongle that actually has max pixel clock specified as 500MHz but it was expensive (couple hundred €) and I don't remember what its name was either but I learned of it from this forum a while ago.
The 165MHz pixel clock limit is something that is relevant for DVI output in particular, it cannot be broken and with DVI to HDMI dongles you are still bound to it. Dual link DVI can double the effective bandwidth though and that 1440p monitor in question absolutely needs such a cable to work at the native resolution. I would expect it to be able to receive its native resolution over VGA too, but only from a real VGA port of a video card and not through a conversion dongle due to previously mentioned aspects.