Thank you for your reply. I have connected my 520 ST to an old Sony Trinitron CRT. Now it looks pretty cool
It really depends on the screen, and yes it could be a pattern too.

Hi all, I wonder if someone could help me. I have an Atari 520 STM with 1MB RAM upgrade and TOS 1.04, it is connected via SCART cable to a SAMSUNG LED TV / Monitor. The green background colour of the desktop has a wavy pattern, all games show "jail bars". The black and white image looks go...