Hello,
I just bought a “Plugable USB to VGA Adapter”, and I have it plugged into a 17" CRT monitor. I’m running Windows 7 on my laptop, and the device is working fine, but I can’t get the the device to put out a refresh rate higher 60Hz.
My CRT screen flickers at 60Hz, so I want to get it up to 75Hz. I know that the adapter itself is only updated 60 times a second, but I think I should be able to have my screen update itself at 75Hz.
Is this a limitation of the adapter?
(I couldn’t find my adapter on the list…the one I bought is: http://plugable.com/products/USB-VGA-…)
Hi Nathan,
Thanks for posting! I can give some background, but whether a higher refresh rate can be offered, depends on a combination of factors.
The DisplayLink chip itself can definitely do a 75Hz update rate on a 17" CRT (I assume your monitor is at most 1600x1200 resolution? The adapter can handle much higher pixel clocks)
But the default resolution and refresh rate is determined by what modes the monitor tells the OS (and DisplayLink driver) through the Extended Display Identification Data (EDID) it returns. By default, the DisplayLink driver will tell the DisplayLink chip to set the monitor’s one native preferred mode, and leave it at that (whether it’s 60Hz, 75Hz or other).
Then other refresh rates offered in the EDID may also be offered in Windows, plus a fixed set of Windows modes (1024x768,…). For example, running a Plugable adapter on a system with an Intel GPU, and connecting with a Samsung SyncMaster 941BW LCD monitor running at 1440x900 the default refresh rate is 60Hz. I’m able to open advanced properties on the display and change the refresh rate setting to 75Hz.
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Now on an LCD, I can’t notice a difference, so it would be interesting if we could make a similar change on your CRT and notice the refresh rate difference.
What do you see when you open the advanced properties dialog of the monitor?
Thanks for your patience here on Presidents’ Day. 
Best wishes,
Bernie
Thanks for the reply, Bernie.
When my 17" is plugged into the Plugable device and I open the advanced properties dialog, I only see 60Hz as an option.
However, when I plug that monitor into the external VGA port on my laptop, and I go to advanced properties, I have options all the way to 120Hz.
Any ideas?
Hi Nathan,
Unfortunately, there’s not going to be a way to force the DisplayLink adapter to any higher refresh rates than what the monitor offers in it’s EDID.
Here’s what’s happening: there’s kind of a philosophical difference between what your main GPU’s driver is doing and what DisplayLink driver is doing.
The main GPU driver is offering modes/refresh rates that are beyond what the monitor specifically advertises. These may work (they’re likely based on standard VESA timings), but it’s a kind of “buyer beware” thing – for example, you may notice when connected to your laptop VGA that you can set higher refresh rates like 120, but you may notice the picture isn’t properly centered, etc. for the monitor. That’s because it’s not the native timing recommended by the monitor. It’s probably also possible to set modes that come up completely out of sync.
Rather, DisplayLink drivers philosophically stick to what the monitor advertises for its preferred timings in its EDID - it uses exactly those timings. So it should come up good every time on the monitor, without adjustment.
But if the monitor doesn’t explicitly offer alternatives modes/refresh rates in its EDID, DisplayLink won’t add them to Windows’ list of choices. Thus the difference you’re seeing in terms of refresh rates offered.
So, sorry, this is an intentional design decision/tradeoff of the DisplayLink drivers – one that simplifies things and avoids trouble for many users, but limits what power users could do in terms of overriding defaults.
Wikipedia has good gory details on EDID if you want to read more: http://en.wikipedia.org/wiki/Extended…
Also note that LCDs don’t have the issues CRTs used to in terms of refresh rate and visible flicker. It’s nice to put old monitors to use, though.
Hope that helps to at least explain what’s going on, sorry we don’t have a happier answer. Let us know if we can do anything for you.
Best wishes!
Bernie
