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I'm attempting to use a TV as an external monitor for my laptop, connecting with a HDMI cable. When I plug the cable into HDMI 1 (on the TV, the laptop only has a single HDMI port) both the laptop and the TV recognize the connection and connect immediately.

My problem is that the TV will periodically blink / go blank. When I "Join" the displays, the TV displays properly, as long as there are no windows open on it, just the static desktop. Within a few seconds of moving a window onto the TV (from the laptop screen), horizontal blue lines start flickering at seemingly random locations on the screen, then the TV goes blank for a few seconds, then comes back on. Then the cycle repeats - a few seconds good, blue lines, blank. Once I move the window back over to the laptop screen, things settle down, and the TV displays the desktop, with blue lines flashing every 4-9 seconds, and only occasional blanking. The TV does not report a loss of signal an all in this mode.

When I set my displays as "Mirrored", the TV displays properly for a few seconds, and then goes blank or says that it has no signal. After a 5-20 seconds it will show up again, and then the cycle repeats.

I have a short video showing these behaviors at https://www.dropbox.com/s/l3ksukabm3lgeyg/20210127_200918.mp4?dl=0. (For anyone checking this question once it has been fixed, I might have removed the video.)

Hardware specs from "Settings|About": the laptop is a Dell Vostro 3560 (Intel i5-3210M CPU, 6GB RAM, AMD Turks/Intel HD Graphics 4000 [IVB GT2]) running Ubuntu 20.04 64 bit, Gnome version 3.36.8, using Wayland for the windowing system. The TV is a Samsung UN32M5300 smart TV (software version T-KTSAKUC-1290.3).

Things I have tried/checked:

  1. I swapped to a brand new HDMI cable (I don't know what version it is, but is connector has "High Speed" molded into it). No difference.
  2. I (ugh) booted over to Windows 10. The TV worked as an external monitor (in both mirrored and joined modes) flawlessly. Edit: I wanted to test changing the HDMI port on the TV under Windows (#5 below), so I booted back over. It's now exhibiting the same blanking behavior, but with no flashing.
  3. The refresh rate is set at 60.00 Hz, and I verified that that is the rate specified for the TV by Samsung. Just to check, I did try 59.94 Hz and 40.00 Hz, and there wasn't any change in the problem, so I went back to 60.00 Hz.
  4. I switched the windowing system from xorg to wayland after seeing it suggested on here (Ubuntu screen flickering). No change.
  5. I moved the cable to the second HDMI port on the TV [it's labeled "HDMI 2 (ARC)"]. This makes things worse. Even when the screens are "Joined", the TV will only display the desktop for a split second, then it goes blank for 20+ seconds, then reports no signal for a while, then blanks again, ad nauseam. When I moved the cable back to port 1, the original behavior resumed. I tried this a couple of times to verify that it wasn't a one off.

I think that the problem is somewhere in how the screen is refreshed, but I'm not sure, and don't know what to try next. Any suggestions?

Edit: To make things even more confusing, I noticed that when at the Ubuntu login screen, everything behaves properly. I waited ~20 minutes, and not one flicker, just a steady purple display.

Greg Shoults
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  • I have seen similar issues caused by cheap/low-spec HDMI cables before but only when using high res outputs from games consoles and the like (4k/HDR etc). I can see you are using HD res and that should not be a problem for most HDMI cables and you said you tried a couple anyway. What graphics driver/version are you using? Go to Ubuntu Software Updater and click on Settings then Additional Drivers tab. If proprietary I would try the Nouveau (or vice-versa) to rule out driver issues. – codlord Jan 28 '21 at 08:22
  • Today I took the laptop and connected it to a different TV, and it worked fine. At this point I think that it's the TV that's the problem. – Greg Shoults Jan 29 '21 at 04:29
  • Only thing I can suggest to try is adding a custom resolution using cvt -r "reduced blanking" mode. I have seen some monitors cannot handle certain clock timings and adding a reduced blanking mode can make it work. All information is here, see the troubleshooting part about the cvt -r flag and the section on adding custom resolutions (and about how to add the mode permanently if it fixes the issue): https://wiki.archlinux.org/index.php/Xrandr#Troubleshooting – codlord Jan 29 '21 at 11:17
  • @codlord Thanks for the suggestion. I tried it, and it seemed to be working for a few minutes, and then the TV started blanking again. I may have to bite the bullet and buy a monitor - then I will need to figure out how to cast to it from an Android phone without purchasing a Chromecast... – Greg Shoults Feb 01 '21 at 00:52

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