Playing Atari ST Music on the Amiga with Zero CPU

Audience

This post is for anyone who loves the technical and historical aspects of chiptune music, as well as enthusiasts of the Amiga PAULA and Atari YM2149 audio chips.

The Context

In my Cycle-Op demo, I showcased a classic sin-dots effect that rendered 6405 dots at 50 FPS on the Amiga 500. Two years later, Amiga legend Hannibal released the long-awaited 3D Demo 3, packed with a lot of impressive effects. Among them was his own sin-dots record, beating mine with 6682 dots.

To top it all off, and in the classic demo scene tradition of playful roasting, Hannibal left me this message:

“Hi Leonard, you optimized your dots well for an Atari programmer. But there were hundreds of dots left if you optimize like an Amiga expert.”

That was too good to ignore. I had to respond by finding a way to beat his new sin-dots record and, as a bonus, come up with a clever twist to answer this “Atari programmer” jab :)

The Goal

As a nod to Hannibal’s remark, I immediately had the idea of playing Atari music on the Amiga during my future dot record attempt. To do that, I needed to emulate the YM2149 sound chip. I had already written an Atari music emulator for the Amiga back in time in my AmigAtari demo.

However, accurately reproducing modern Atari music effects, such as SID voices, Sync Buzzer, and Digidrums, requires emulating not only the YM2149 but also the Atari’s hardware timers. This kind of emulation is extremely CPU intensive, consuming around 50% of the frame time in my 2020 AmigAtari demo. That makes it impossible to break a sin-dots record at the same time.

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