3

I'm responsible for several conference rooms and have setup 1080p projectors and I provide both HDMI and VGA connectivity. HDMI for DisplayPort and Mini-DisplayPort, and VGA as a fallback, universal option. Contrary to what I expected, people seem to have much more trouble with the HDMI than VGA, so VGA gets used a lot more than you'd think (even as most workstation laptops made in the last 3-4 years have DisplayPort or Mini-DisplayPort...). Also to my surprise, VGA outputs over 1080p on a 50ft cable run with very minimal degradation on certain laptops - other laptops just don't offer 1080p as a resolution choice and top out at 1600x1200 or something else. Specific example: a ThinkPad W530 will do 1080p, a W520 won't, over VGA. (both do 1080p over displayport/mini-DP)

What determines what resolutions a laptop is willing to output over VGA?

I'm thinking this will come down to either a video driver that says it supports only certain resolutions for output, or limitations of the RAMDAC (which wouldn't be in play, at least DAC wise, on a digital output, but WOULD on VGA, an analog output).

The basic reason for the question is that I noticed, say, a ThinkPad W520 with 1080p built in display, will output 1080p fine over DisplayPort to a 1080p projector, but will cap out at 1600x1200 (practically the same pixel count, just a little shy) on VGA. Now, this wouldn't be surprising at all except SOME laptops have no issue outputting 1080p over VGA, even with lower native resolutions.

Why do I care? Well if there's some way I could enable it... for situations where my users end up using VGA anyway, it's preferable for display mirroring if they can output their laptop's native resolution, which, you guessed it, is very often 1080p on 15" models.

DISCLAIMER: This is primarily a curiosity, I'm not claiming 1080p over VGA is ideal by any means, but hey, if it works. I've seen HDMI start artifacting more over same-length, same gauge cabling (up to 50' run in certain rooms). If you think this is better suited to SuperUser, please move it, but this is framed from an IT standpoint of something that affects a real pool of users in a multiple conference room, 50+ deployed laptop scenario.

4
  • Not at my pc to research it at the moment but answers would probably be found on home theatre PC sites as they have to deal with that same issue. For some video cards modified drivers allow them to output 1080p
    – Grant
    Aug 9, 2013 at 2:24
  • Curious, do you think the reason it caps out at 1600x1200 is because it thinks it is connected to a 4:3 monitor through the VGA port?
    – TheCleaner
    Aug 9, 2013 at 2:26
  • @TheCleaner possibly, or that for some reason VGA resolutions the videocard is allowing are predominantly 4:3... however, 1600x900 is usually presented on these as well. I'll verify on the laptop that tops out at 1600x1200 what some of the other options are ...but the laptops themselves are widescreen, making 1600x1200 a poor choice for mirroring. Aug 9, 2013 at 2:36
  • looks like a known issue: google.com/…
    – TheCleaner
    Aug 9, 2013 at 2:49

1 Answer 1

1

I think you guessed it, what happens is the driver that checks the monitor using the DDC which is a simple serial connection (most often still not shown in VGA connector diagrams!)

Wikipedia has a quite good page on the DDC and the different protocols:

http://en.wikipedia.org/wiki/Display_Data_Channel

Since you mentioned using a rather long cable, I'm actually thinking that the serial connection may not work right. In general, the driver will use a safe fallback if it cannot determine the resolution of the display device.

Since it works with some computers, they may have a better signal (higher voltage that can carry further) than the computers that fail.

Having a driver to which you can teach what you are connecting to and force the output to X/Y/Z is probably the easiest solution in your case, although every person would have to install that driver. Otherwise, it would be a repeater that increases the signal midpoint (or maybe 2 of those at 1/3rd each...)

The one last thing, but I would be surprised that is the problem, would be that the protocol used by your display device is not known by the computer...

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .