Since Catalina my iMac is blinking when 2nd monitor is off

[flowam]

Hello,
The screen of my iMac is blinking when my 2nd monitor (Sony TV) is switching off but keeps connected. I have to unplug the HDMI adaptor (or the HDMI cable) to stop the blinking. This problem exists since I upgraded to Catalina from El Capitan. All equipment (hardware, cables, adaptors...) are strictly the same as previously.
What can I do to solve it ? Is there a terminal command to write or a pref setting to modify ?

I already empty the PRAM and SMC. nothing changes !
thanks by advance

my setup :
Imac 21.5" 2013 w/ SSD & 16Go RAM
MacOS Catalina 10.15.7
thunderbolt/HDMI adaptor + câble HDMI to connect the TV
TV/moniteur Sony

 

[chscag]

Welcome to our forums.

The first step to troubleshooting problems that involve a second monitor or multiple monitors, is to try a second set of cables.

I know you said that the problem has occurred since you upgraded from El Capitan to Catalina. That upgrade involved a big jump in macOS versions. As you probably are aware, 32 bit apps will no longer run in Catalina and that includes drivers (kexts).