RPi 4b hdmi signal only works from if plugged in from system start up

Question

It seems like I can never get the screen to show anything through HDMI unless it is plugged in from system boot. I find it hugely frustrating not being able to just switch HDMI in and out. Is that to be expected, or do I need to do something to enable HDMI to function with live switching?

Answer 1

If you look in /boot/config.txt you will see a line commented out that states:

hdmi_force_hotplug=1

Make this active and reboot. Now when you plug the HDMI monitor in (or put the mains plug in), the Pi will recognise it and use it BUT it has a very low default resolution.

To address the resolution issue you will need to add a few more lines to set the default resolution you want the Pi to use based on the tables here on the official site

For example, for 1080p at 50Hz I would add:

hdmi_group=1
hdmi_mode=31

You can also set the resolution from the GUI or raspi-config at the command line.

Note: Just check where you put these lines as the config file can be split into sections depending on the Pi you are using.

Answer 2

The Pi determines screen resolution by interrogating the display, so this is as designed.

Earlier models defaulted to NSTC video without a monitor (which is pretty useless).

If you need to boot without a monitor set a default resolution.

Read the instructions in config.txt (although it is easier using raspi-config).