So I found a new thing that was poorly documented on the Atari STE
There are faulty shifters!
(this is the video chipset)
The C300588, found in most STEs, is okay
The C301712 found in Mega STE and maybe later STEs is mostly faulty and generate audio cracks on PCM DMA audio when blitter transfers are in progress.
There is at least 2 revisions of the C301712, the later one seems to be fixed.
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Since PCM audio DMA wasn't really common on STE back then, this issue was not at all documented and most people didn't even know they could ask for an IC exchange (which was possible).
So far all STEs that I tested are okay, but not my Mega STE.
So at last it explains why I had those nasty audio clicks when I coded my VGA intro on Mega STE.
