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Current status (2022-08-28; project is currently in hibernation as I work on PicoGUS):

Rough plan of experimentation/implementation:

✅ = done, 📝 = work in progress, 💭 = still to be done

✅ Design prototype PCB, with level shifters and lines for IRQ and DMA
✅ Source parts & build prototype PCB
✅ Run RPiISA code on Linux on prototype PCB, validating level shifters, changing design/bodging as needed
✅ Port RPiISA code to Circle, using PWM audio out
✅ Extract GUS emulation code from DOSBox
✅ Implement port-only emulation
✅ Implement IRQs
📝 Implement sample upload via DMA
💭 Investigate moving GPIOs to free up I2S pins for better audio output quality via external DAC
💭 Design release candidate PCB

Known issues

• DOSBox's GUS emulation faithfully reproduces sample rate drop when going above 14 channels. I've implemented a resampler using Speex's resampling code, but it's not working well
• The GPIO header does not give enough clearance for the Raspberry Pi when plugged directly in via straight headers. A ribbon cable must be used instead, or a pair of right-angle headers
• There might not be enough GPIO pins to handle proper DMA
• The I2S GPIO pins are in the middle of the address lines, preventing the use of a high quality external DAC and preventing use of CM4 or Zero 2 modules.