Brad Q
Posts: 105
Joined: Mon Aug 12, 2019 12:10 am

Hdmi->DVI back light issue. Pi4

Tue Dec 17, 2019 7:59 pm

Monitor is a Samsung SyncMaster 940BW. While it is quite old it has been and is a good 1440X900 monitor with VGA and DVI inputs. I had been using the monitor with my Xenon box with a Displayport to DVI cord which worked fine. Now I have moved to the Pi4-4GB as my daily driver but still need to use the Xenon for a couple of hours a week. So I bought a DVI to female HDMI adapter, a Dp to HDMI cord (Xenon) and a Micro (or whatever the pi4 is)HDMI to HDMI cord. Everything works and I do not have to worry about wear on the DVI port.

However, when computer sets idle (really only noticeable in a dark room) the screen does go blank (as normal) but the back light (or whatever you call it) is still on(throwing some light). Which it does not do when I switch back to the DP to DVI cable(on Xenon). It does this on both computers. My concern is that over time this will "burn out" the back light. Since the Xenon is used so little now, it really drops out of the equation. Is there a setting (overlay?) that I need to change in the Pi4? Is this an issue with the DVI to HDMI adapter I bought?

Running Raspbian Buster without any modifications and Pi4 is not overclocked.

Thanks

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 30979
Joined: Sat Jul 30, 2011 7:41 pm

Re: Hdmi->DVI back light issue. Pi4

Tue Dec 17, 2019 8:22 pm

You need to turn on 'DPMS' power support. Easiest to install xscreensaver and then turn it on via the GUI in that.
Principal Software Engineer at Raspberry Pi Ltd.
Working in the Applications Team.

Brad Q
Posts: 105
Joined: Mon Aug 12, 2019 12:10 am

Re: Hdmi->DVI back light issue. Pi4

Tue Dec 17, 2019 9:04 pm

It is already installed and DPMS is on.

Brad Q
Posts: 105
Joined: Mon Aug 12, 2019 12:10 am

Re: Hdmi->DVI back light issue. Pi4

Tue Dec 17, 2019 9:33 pm

It looks like it may be a conflict between the gnome flashback screen saver and the Xscreensaver. Preferences->Desktop sessions settings-> uncheck Screensaver(Gnome Flashback). I will not know for sure for a few hours (after dark).

Thanks Jamesh. I think I started to fix this once before but got side tracked somewhere along the way.

klricks
Posts: 8343
Joined: Sat Jan 12, 2013 3:01 am
Location: Grants Pass, OR, USA

Re: Hdmi->DVI back light issue. Pi4

Tue Dec 17, 2019 9:44 pm

The screen blanking does not turn off the backlight on a RPi.
There is a config.txt command that will turn off the HDMI on older RPi when blanking times out, but this does not work on the RPi4:

Code: Select all

hdmi_blanking=1
Note that DPMS is enabled by default.
As a test this command is supposed to blank in 10 sec, suspend in 20 sec, and turn off the display in 30 sec:

Code: Select all

xset dpms 10 20 30
But it does not work correctly only blanking happens..... no suspend or shutoff.

If your backlight is LED then I would not worry too much. If florescent tube then time to get a new monitor.
Unless specified otherwise my response is based on the latest and fully updated RPi OS Bullseye w/ Desktop OS.

jamesh
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 30979
Joined: Sat Jul 30, 2011 7:41 pm

Re: Hdmi->DVI back light issue. Pi4

Tue Dec 17, 2019 11:24 pm

klricks wrote:
Tue Dec 17, 2019 9:44 pm
The screen blanking does not turn off the backlight on a RPi.
There is a config.txt command that will turn off the HDMI on older RPi when blanking times out, but this does not work on the RPi4:

Code: Select all

hdmi_blanking=1
Note that DPMS is enabled by default.
As a test this command is supposed to blank in 10 sec, suspend in 20 sec, and turn off the display in 30 sec:

Code: Select all

xset dpms 10 20 30
But it does not work correctly only blanking happens..... no suspend or shutoff.

If your backlight is LED then I would not worry too much. If florescent tube then time to get a new monitor.
I fixed this a whole back so if you have the latest firmware it should work correctly. Might need rpi-update
Principal Software Engineer at Raspberry Pi Ltd.
Working in the Applications Team.

Brad Q
Posts: 105
Joined: Mon Aug 12, 2019 12:10 am

Re: Hdmi->DVI back light issue. Pi4

Wed Dec 18, 2019 1:01 am

It did not function on the "stock" (9/25?) firmware. However after a rpi=update it works as expected. Thanks

Brad Q
Posts: 105
Joined: Mon Aug 12, 2019 12:10 am

Re: Hdmi->DVI back light issue. Pi4

Fri Jan 10, 2020 10:01 pm

In the roughly one month since I used that rpi-update I have had two machine "lock ups" both when coming back from a screen time out. The first time it came back to the normal desktop but unresponsive, forcing a reboot. The second it came back to a black screen with an non flashing (just a white underscore at the VERY top left of screen) cursor, again forcing a reboot. These were at least a week apart and the machine runs pretty much 24/7. No other unusual symptoms or changes (normal updates). Nothing drawing power from the board. Using a Canakit power supply.

klricks
Posts: 8343
Joined: Sat Jan 12, 2013 3:01 am
Location: Grants Pass, OR, USA

Re: Hdmi->DVI back light issue. Pi4

Tue Jan 21, 2020 4:27 pm

This fix has now been rolled out to a general update / upgrade.

Code: Select all

sudo apt update
sudo apt upgrade
Some notes:
hdmi_blanking=1 in /boot/config.txt is no longer required. (This means there is no longer any screen blanking by making all screen pixels black)

On timeout the RPi shuts off HDMI signal instead of just turning the pixels black. This leaves the monitor to its own power-saving function(s) if any. With my monitor/TV, on timeout, I get a no signal banner for about 20 min. Then the monitor shuts itself off. After the monitor shuts off I have to use the monitors power button or remote to turn the screen back on. (During the 20 min no signal period I can wake the monitor by mouse/keyboard activity).
The xset s commands no longer do anything...... So to disable screen blanking only one command is needed in autostart:

Code: Select all

xset -dpms
Or

Code: Select all

xset dpms 0 0 0
As before there is no difference with the STANDBY, SUSPEND and OFF parameters associated with the xset dpms command. They all 3 function the same on matter which parameter is used. They all just turn off HDMI output as described above.
For example this command causes HDMI to blank after 10 seconds (Standby). Nothing else happens after 20 (Suspend) or 30 (OFF) seconds:

Code: Select all

xset dpms 10 20 30
Unless specified otherwise my response is based on the latest and fully updated RPi OS Bullseye w/ Desktop OS.

Return to “Troubleshooting”