Today I worked on a PCB for proof-of-concept, essentially it will be an adapter to a Raspberry Pi header.
One module will have a Raspberry Pi, one module will have a Pi DAC module.
Once this is working, I will be able to use it to check other custom modules.
So far I have discovered one problem, that using a PCIe x1 connector probably isn't going to allow me to use more than one signal source. Pretty much all the pins are used up with 8 channels of audio, all using the same clocks. I suspect it will be impractical to sync multiple sources with the same clock (although I will look into it), so I will probably have to go to a bigger connector to allow multiple sets of 8 audio channels. Maybe I'll go for a few sets of stereo and a couple of 8 channel sets (or is stereo falling out of favour?)
Schematics are on github, unfinished, not laid-out yet.
Next: Finish schematic, layout, send for manufacture.