LG OLED with a Linux computer: Getting that pitch black

This post was written by eli on June 27, 2020
Posted Under: cinnamon,Linux

Introduction

So I got myself an LG OLED65B9. It’s huge and a really nice piece of electronics. I opted out the bells and whistles, and connected it via HDMI to my already existing media computer, running Linux Mint 18.1. All I wanted was a plain (yet very high quality) display.

However at some point I noticed that black wasn’t displayed as black. I opened GIMP, drew a huge black rectangle, and it displayed as dark grey. At first I thought that the screen was defective (or that I was overly optimistic expecting that black would be complete darkness), but then I tried an image from a USB stick, and reassured myself that black is displayed as complete darkness. As it should. Or why did I pay extra for an OLED?

Because I skipped the “play with the new toy” phase with this display, I’m 100% it’s with its factory settings. It’s not something I messed up.

I should mention that I use plain HD resolution of 1920x1080. The screen can do much better than that (see list of resolutions below), and defaults at 3840x2160 with my computer, but it’s quite pointless: Don’t know about you, I have nothing to show that goes higher than 1080p. And the computer’s graphics stutters at 4k UHD. So why push it?

I have a previous post on graphics modes, and one on the setup of the Brix media center computer involved.

So why is black displayed LCD style?

The truth is that I don’t know. But it seems to be a problem only with standard 16:9 graphics modes. When switching to modes that are typical for computers (5:4 and 4:3 aspect ratios), the image was stretched to the entire screen, and black areas showed as pitch black. I’m not sure about this conclusion, and even less do I have an idea why this would happen or why a properly designed display would “correct” a pixel that arrives as RGB all zeros to something brighter.

Also, the black level on text consoles (Ctrl-Shift-F1 style) is horrible. But I’m not sure about which resolution they use.

An idea that crossed my mind is that maybe the pixels are sent as YCbCr in some modes or maybe the computer goes “Hey, I’m a TV now, let’s do some color correction nobody asked for” when standard HDTV aspect ratios are used. If any, I would go for the second possibility. But xrandr’s verbose output implies that both brightness and gamma are set to 1.0 for the relevant HDMI output, even when black isn’t black.

The graphics adapter is Intel Celeron J3160′s on-chip “HD Graphics” processor (8086:22b1) so nothing fancy here.

The fix

This just worked for me, and I didn’t feel like playing with it further. So I can’t assure that this is a consistent solution, but it actually seems that way.

The idea is that since the problem arises with standard 16:9 modes, maybe make up a non-standard one?

Unlike the case with my previous TV, using cvt to calculate the timing parameters turned out to be a good idea.

$ cvt 1920 1080 60
# 1920x1080 59.96 Hz (CVT 2.07M9) hsync: 67.16 kHz; pclk: 173.00 MHz
Modeline "1920x1080_60.00"  173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync
$ xrandr -d :0 --newmode "try" 173.00  1920 2048 2248 2576  1080 1083 1088 1120 -hsync +vsync
$ xrandr -d :0 --addmode HDMI3 try
$ xrandr -d :0 --output HDMI3 --mode try

At this point I got a proper 1920x1080 on the screen, with black pixels as dark as when the display is powered off. The output of xrandr after this was somewhat unexpected, yet functionally what I wanted:

$ xrandr -d :0 --verbose
  1280x720 (0x4b) 74.250MHz +HSync +VSync +preferred
        h: width  1280 start 1390 end 1430 total 1650 skew    0 clock  45.00KHz
        v: height  720 start  725 end  730 total  750           clock  60.00Hz
  1920x1080 (0x141) 173.000MHz -HSync +VSync *current
        h: width  1920 start 2048 end 2248 total 2576 skew    0 clock  67.16KHz
        v: height 1080 start 1083 end 1088 total 1120           clock  59.96Hz
  1920x1080 (0x10c) 148.500MHz +HSync +VSync
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  67.50KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  60.00Hz
 [ ... ]
  try (0x13e) 173.000MHz -HSync +VSync
        h: width  1920 start 2048 end 2248 total 2576 skew    0 clock  67.16KHz
        v: height 1080 start 1083 end 1088 total 1120           clock  59.96Hz

So the mode in effect didn’t turn out the one I generated (“try”), but a replica of its parameters, marked as 0x141 (and 0x13a on another occasion). This mode wasn’t there before.

I’m don’t quite understand how this happened. Maybe Cinnamon’s machinery did this. It kind of gets in the way all the time, and at times it didn’t let me set just any mode I liked with xrandr, so maybe that. This whole thing with graphics modes is completely out of control.

I should mention that there is no problem with sound in this mode (or any other situation I tried). Not that there should be, but at some point I thought maybe there would be, because the mode implies a computer and not a TV-something. But no issues at all. Actually, the screen’s loudspeakers are remarkably good, with a surprisingly present bass, but that’s a different story.

As for making this special mode permanent, that turned out to be a problem in itself. This post shows how I eventually solved it.

List of graphics modes

Just in case this interests anyone, this is the output of a full resolution list:

$ xrandr -d :0 --verbose
[ ... ]
HDMI3 connected primary 3840x2160+0+0 (0x1ba) normal (normal left inverted right x axis y axis) 1600mm x 900mm
	Identifier: 0x48
	Timestamp:  -1469585217
	Subpixel:   unknown
	Gamma:      1.0:1.0:1.0
	Brightness: 1.0
	Clones:
	CRTC:       0
	CRTCs:      0
	Transform:  1.000000 0.000000 0.000000
	            0.000000 1.000000 0.000000
	            0.000000 0.000000 1.000000
	           filter:
	EDID:
		00ffffffffffff001e6da0c001010101
		011d010380a05a780aee91a3544c9926
		0f5054a1080031404540614071408180
		d1c00101010104740030f2705a80b058
		8a0040846300001e023a801871382d40
		582c450040846300001e000000fd0018
		781e871e000a202020202020000000fc
		004c472054560a20202020202020012d
		02035af1565f101f0413051403021220
		212215015d5e6263643f403209570715
		07505707016704033d1ec05f7e016e03
		0c001000b83c20008001020304e200cf
		e305c000e50e60616566eb0146d0002a
		1803257d76ace3060d01662150b05100
		1b304070360040846300001e00000000
		0000000000000000000000000000008b
	aspect ratio: Automatic
		supported: Automatic, 4:3, 16:9
	Broadcast RGB: Automatic
		supported: Automatic, Full, Limited 16:235
	audio: auto
		supported: force-dvi, off, auto, on
  3840x2160 (0x1ba) 297.000MHz +HSync +VSync *current +preferred
        h: width  3840 start 4016 end 4104 total 4400 skew    0 clock  67.50KHz
        v: height 2160 start 2168 end 2178 total 2250           clock  30.00Hz
  4096x2160 (0x1bb) 297.000MHz +HSync +VSync
        h: width  4096 start 5116 end 5204 total 5500 skew    0 clock  54.00KHz
        v: height 2160 start 2168 end 2178 total 2250           clock  24.00Hz
  4096x2160 (0x1bc) 296.703MHz +HSync +VSync
        h: width  4096 start 5116 end 5204 total 5500 skew    0 clock  53.95KHz
        v: height 2160 start 2168 end 2178 total 2250           clock  23.98Hz
  3840x2160 (0x1bd) 297.000MHz +HSync +VSync
        h: width  3840 start 4896 end 4984 total 5280 skew    0 clock  56.25KHz
        v: height 2160 start 2168 end 2178 total 2250           clock  25.00Hz
  3840x2160 (0x1be) 297.000MHz +HSync +VSync
        h: width  3840 start 5116 end 5204 total 5500 skew    0 clock  54.00KHz
        v: height 2160 start 2168 end 2178 total 2250           clock  24.00Hz
  3840x2160 (0x1bf) 296.703MHz +HSync +VSync
        h: width  3840 start 4016 end 4104 total 4400 skew    0 clock  67.43KHz
        v: height 2160 start 2168 end 2178 total 2250           clock  29.97Hz
  3840x2160 (0x1c0) 296.703MHz +HSync +VSync
        h: width  3840 start 5116 end 5204 total 5500 skew    0 clock  53.95KHz
        v: height 2160 start 2168 end 2178 total 2250           clock  23.98Hz
  1920x1080 (0x1c1) 297.000MHz +HSync +VSync
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock 135.00KHz
        v: height 1080 start 1084 end 1089 total 1125           clock 120.00Hz
  1920x1080 (0x1c2) 297.000MHz +HSync +VSync
        h: width  1920 start 2448 end 2492 total 2640 skew    0 clock 112.50KHz
        v: height 1080 start 1084 end 1094 total 1125           clock 100.00Hz
  1920x1080 (0x1c3) 296.703MHz +HSync +VSync
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock 134.87KHz
        v: height 1080 start 1084 end 1089 total 1125           clock 119.88Hz
  1920x1080 (0x16c) 148.500MHz +HSync +VSync
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  67.50KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  60.00Hz
  1920x1080 (0x1c4) 148.500MHz +HSync +VSync
        h: width  1920 start 2448 end 2492 total 2640 skew    0 clock  56.25KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  50.00Hz
  1920x1080 (0x16d) 148.352MHz +HSync +VSync
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  67.43KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  59.94Hz
  1920x1080i (0x10c) 74.250MHz +HSync +VSync Interlace
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  33.75KHz
        v: height 1080 start 1084 end 1094 total 1125           clock  60.00Hz
  1920x1080i (0x10d) 74.250MHz +HSync +VSync Interlace
        h: width  1920 start 2448 end 2492 total 2640 skew    0 clock  28.12KHz
        v: height 1080 start 1084 end 1094 total 1125           clock  50.00Hz
  1920x1080 (0x1c5) 74.250MHz +HSync +VSync
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  33.75KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  30.00Hz
  1920x1080 (0x1c6) 74.250MHz +HSync +VSync
        h: width  1920 start 2448 end 2492 total 2640 skew    0 clock  28.12KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  25.00Hz
  1920x1080 (0x1c7) 74.250MHz +HSync +VSync
        h: width  1920 start 2558 end 2602 total 2750 skew    0 clock  27.00KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  24.00Hz
  1920x1080i (0x10e) 74.176MHz +HSync +VSync Interlace
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  33.72KHz
        v: height 1080 start 1084 end 1094 total 1125           clock  59.94Hz
  1920x1080 (0x1c8) 74.176MHz +HSync +VSync
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  33.72KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  29.97Hz
  1920x1080 (0x1c9) 74.176MHz +HSync +VSync
        h: width  1920 start 2558 end 2602 total 2750 skew    0 clock  26.97KHz
        v: height 1080 start 1084 end 1089 total 1125           clock  23.98Hz
  1280x1024 (0x1b5) 108.000MHz +HSync +VSync
        h: width  1280 start 1328 end 1440 total 1688 skew    0 clock  63.98KHz
        v: height 1024 start 1025 end 1028 total 1066           clock  60.02Hz
  1360x768 (0x4b) 85.500MHz +HSync +VSync
        h: width  1360 start 1424 end 1536 total 1792 skew    0 clock  47.71KHz
        v: height  768 start  771 end  777 total  795           clock  60.02Hz
  1152x864 (0x1ca) 81.579MHz -HSync +VSync
        h: width  1152 start 1216 end 1336 total 1520 skew    0 clock  53.67KHz
        v: height  864 start  865 end  868 total  895           clock  59.97Hz
  1280x720 (0x110) 74.250MHz +HSync +VSync
        h: width  1280 start 1390 end 1430 total 1650 skew    0 clock  45.00KHz
        v: height  720 start  725 end  730 total  750           clock  60.00Hz
  1280x720 (0x111) 74.250MHz +HSync +VSync
        h: width  1280 start 1720 end 1760 total 1980 skew    0 clock  37.50KHz
        v: height  720 start  725 end  730 total  750           clock  50.00Hz
  1280x720 (0x112) 74.176MHz +HSync +VSync
        h: width  1280 start 1390 end 1430 total 1650 skew    0 clock  44.96KHz
        v: height  720 start  725 end  730 total  750           clock  59.94Hz
  1024x768 (0x113) 65.000MHz -HSync -VSync
        h: width  1024 start 1048 end 1184 total 1344 skew    0 clock  48.36KHz
        v: height  768 start  771 end  777 total  806           clock  60.00Hz
  800x600 (0x115) 40.000MHz +HSync +VSync
        h: width   800 start  840 end  968 total 1056 skew    0 clock  37.88KHz
        v: height  600 start  601 end  605 total  628           clock  60.32Hz
  720x576 (0x116) 27.000MHz -HSync -VSync
        h: width   720 start  732 end  796 total  864 skew    0 clock  31.25KHz
        v: height  576 start  581 end  586 total  625           clock  50.00Hz
  720x576i (0x117) 13.500MHz -HSync -VSync Interlace
        h: width   720 start  732 end  795 total  864 skew    0 clock  15.62KHz
        v: height  576 start  580 end  586 total  625           clock  50.00Hz
  720x480 (0x118) 27.027MHz -HSync -VSync
        h: width   720 start  736 end  798 total  858 skew    0 clock  31.50KHz
        v: height  480 start  489 end  495 total  525           clock  60.00Hz
  720x480 (0x119) 27.000MHz -HSync -VSync
        h: width   720 start  736 end  798 total  858 skew    0 clock  31.47KHz
        v: height  480 start  489 end  495 total  525           clock  59.94Hz
  640x480 (0x11c) 25.200MHz -HSync -VSync
        h: width   640 start  656 end  752 total  800 skew    0 clock  31.50KHz
        v: height  480 start  490 end  492 total  525           clock  60.00Hz
  640x480 (0x11d) 25.175MHz -HSync -VSync
        h: width   640 start  656 end  752 total  800 skew    0 clock  31.47KHz
        v: height  480 start  490 end  492 total  525           clock  59.94Hz
  720x400 (0x1cb) 28.320MHz -HSync +VSync
        h: width   720 start  738 end  846 total  900 skew    0 clock  31.47KHz
        v: height  400 start  412 end  414 total  449           clock  70.08Hz

So it even supports fallback mode with a 25.175 MHz clock if one really insists.

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