Brion Vibber @brion@bikeshed.vibber.net

I'm kind of inclined to target the parallel bus just for that audio channel -- that allows full-ass mono audio with decent precision, at zero cost in CPU cycles.

The POKEY sound precision is only 4-bit (urrrgghhh) and you have to run a tight timing loop on the CPU to bitbang samples because there's no DMA buffer.

However -- the 400/800 also have a big metal RF interference shield around the cartridge (with a lid and everything), so to be compatible you have to either be able to run off the internal power provided by the Atari without an external power source, or you have to prop the lid open and ignore 1979-era FCC regulations about radio interference ;)

The parallel bus on the XL and XE models (which inconveniently have a different physical connector) extends the cartridge with more address lines, some fancier controls that should allow taking over more memory space and other cool tricks, and an analog audio output channel, which would be nicer than pushing 4-bit PCM audio samples every scan line.

But, no 800/400 support.

The cartridge interface is most universal, working across the entire 8-bit Atari line. But it allows you only 8 KiB of main address space and 256 bytes of control registers!

This *is* however enough for a full 192-scan line graphics screen, or a little smaller with palettes and PCM audio.

This is currently way beyond my ability and I'd have to brush up my electronics skillz first, but I really like the idea of making a thingy that plugs into the Atari cartridge or parallel bus that provides a modern RISC co-processor (even if quite modest) with a reasonably large memory blob that's shared with the Atari host.

Hooking into the memory bus is key because it should allow full-motion video by pointing the ANTIC's display list at the shared RAM.