iMac is blinking since Catalina upgrade

Jan 9, 2021
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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


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