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Difference between 55hz and 60hz on a 2K screen

User avatar tobik
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This is not a kwin question strictly speaking but that's the software I'm using right now and I hope there will be smart people able to answer my question :)

I hooked a 2k (2560x1440) monitor over HDMI to my laptop running OpenSuse with KDE Plasma 5 (and the new kwin). My laptop is ~2 years old and has an Intel HD 4000 graphics card. Apparently, a laptop with this configuration is not able to push a full 2k signal through the HDMI cable. I found a trick using xrandr how to do that but I had to reduce the refresh rate from 60hz to 55hz.

I thought that sacrificing 5hz wouldn't make much of a difference but apparently it does. My "wobbly" windows feel somehow less fluent and even the cursor is leaving a "trail" behind. I turned on the fps monitor and the kwin fps is still locked to the monitors refresh rate, i. e. it runs at 55fps without any drops (therefore it doesn't seem like a performance problem). I did some more experiments. I tried running a webgl Quake 3D demo. With FullHD resolution at 60fps/hz it ran absolutely smoothly, with 2k resolution at 55fps/hz there was an apparent stutter.

Does 5fps really make so much difference? It's hard to believe... Is there anything that can be done about that?
luebking
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You'll need hdmi 1.3 which was specified in 2006 - I assume the HD4000 supports that and the culprit is the cable (because it's the cable in 99% of all cases)
The capable cables are branded "High Speed", but you'll actually have to try (or read reviews of the cable - it is *not* a passive device)

The problem is that you cannot simply run the monitor at 55Hz (they've some fixed rates they support, eg. but not necessarily 24Hz, 50Hz, 60Hz, 100Hz, 120Hz) - you're *feeding* it at 55Hz, but it probably repaints at 60Hz.

So you're sending a frame every 18.18ms, but the screen updates every 16.66ms - depending on how the screen reacts, you'll effectively run 30Hz or the image stalls for 2 frames every 8th or 9th frame or similar (judder)
User avatar tobik
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luebking wrote:You'll need hdmi 1.3 which was specified in 2006 - I assume the HD4000 supports that and the culprit is the cable (because it's the cable in 99% of all cases)
The capable cables are branded "High Speed", but you'll actually have to try (or read reviews of the cable - it is *not* a passive device)


I think I have the "High Speed" cable but anyway, that's no the issue. It's really a problem of the chipset. "Officially" only 1080p output is supported, with this little neat trick you can get up to 2k@55Hz but I wasn't able to find anybody that would go beyond that.

See for example this http://www.notebookcheck.net/2560x1440-or-2560x1600-via-HDMI.92840.0.html.

luebking wrote:The problem is that you cannot simply run the monitor at 55Hz (they've some fixed rates they support, eg. but not necessarily 24Hz, 50Hz, 60Hz, 100Hz, 120Hz) - you're *feeding* it at 55Hz, but it probably repaints at 60Hz.

So you're sending a frame every 18.18ms, but the screen updates every 16.66ms - depending on how the screen reacts, you'll effectively run 30Hz or the image stalls for 2 frames every 8th or 9th frame or similar (judder)


Oh, I kind of expected something like that. 30hz vs 60hz seems very likely given the difference in smoothness. Too bad. Anyway, do I have any options to improve the stutter? Like let's say... if my monitor supported 50hz, then it would make sense to feed it with 50fps to get smoother picture. But it somehow seems to me that all lcd displays just start at 60hz as the native refresh rate and only go up for the player's needs.
luebking
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Multiples of 25Hz are supported by (several, not all) devices for the EU market, because of PAL - the more the panel is also used in TVs, the more likely you'll get 50Hz support and yes: that should improve things.

Here's a modeline generator, where you can enter the values from your displays tech sheet:
http://xtiming.sourceforge.net/cgi-bin/xtiming.pl

This should allow you to generate a 50Hz modeline.

If the monitor should support interlacing you could use that (60Hz should be no problem since every frame only carries half the resolution)
User avatar tobik
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Thank you for the link. It looks kind of complicated but hopefully a bit of googling will help me fill the form :)

Unfortunately the refresh rate is usually not listed among the monitor specifications so it's hard to find those that would support 50hz.
mschultz
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I connected with hdmi and it gives error: "no signal" can you help me please?

Moderator edit: removed quotes that do not help to understand the question.
User avatar Mamarok
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mschultz wrote:I connected with hdmi and it gives error: "no signal" can you help me please?.


You write to a 4 year old post which is very likely not relevant anymore today.

You need to be more specific about your system (distribution, version) and hardware.


Mamarok, hanging out in the Amarok forum since August, 2007
Currently running Kubuntu 19.04, Plasma 5.15.5

 
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