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Old 05 January 2020, 23:21   #1
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Join Date: Sep 2009
Location: Norway
Posts: 1,346
Strange Paula quirk allows for infinite playback lengths

Before I explain this, I just want to mention that this trick doesn't work on WinUAE, so you need a real Amiga for this to work.

I was playing around with a random .mod I found, and I noticed that one specific sample would play random junk at the end every time you played it, even though the length/replen/repeat attributes were sane. I played around a bit with the length and loop length, and suddenly it played infinite data after the sample end, every time it was triggered.

In other words, I found a trick that allows Paula to just keep on playing whatever is in chipmem forever, never caring about the LEN register. This, however, only seems to work if the period is just about too low for the DMA to be stable. This causes cracks/pops every now and then, and doesn't make this trick very useful in practice. Still quite funny though!

I made an example .mod that demonstrates this. Load it up on an Amiga with 2mb chipmem and see the magic. ProTracker is preferred, so you can see that it's just one note trigger. This might not always work, it can playback random junk in memory instead, depending on where the .mod samples end up with the AllocMem() call in the .mod loader. So yeah, maybe play back the song at low volume first and see. Loading up a very small Workbench environment should increase the chances of the .mod sounding perfect.

Maybe it's something Toni should look into regarding WinUAE? Probably not though, but still an interesting quirk.
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