I read this most fascinating article about how misunderstood the concept of “ground is” as well as watching a few yt videos from some people who know what is what.
tl;dr the fool-proof way to avoid noise getting from your digital to your analog circuits is NOT separate ground fills (that creates an antenna), and it is NOT to use star grounding (which you really can’t do in any real circuit).
The solution is to use differential inputs (and outputs). That is, each audio connection has its own + and -, and you run them next to each other, so any interference occurs to both and is cancelled out. Just like humbuckers on a guitar.
That’s a very simple summary, and of course you should be laying your digital stuff out away from your analog stuff, and other sensible things too.
There’s two major problems I see with this.
- The DaisySeed only does single ended in/out (i.e. it has a common ground); and
- Guitar leads (patch cables) and infact guitars themselves don’t have differential audio
I’ve looked at a bunch of fairly random PCBs that use ADCs, from the old Variax circuit to the Line6 HX Stomp and the Kemper Player, and they all seem to use differential ADCs.
There are a number of such ADCs or CODECs that would be compatible with the DaisySeed as external components (some giving 4 channels). I quite like TIs TLV320ADCx140 series which allow you to daisy chain (ikr) 4 units together to get 16 channels (or 32 PDM channels), and there are some big brother to the PCM3060 that have differential options though the ones I looked at were a bit intensive on external components.
I’m just throwing all that out there as “food for though”. In terms of working with what we have in the DaisySeed, I think what makes the most sense is to use op-amps in differential mode with the negative side connected directly to the AGND pin.
I have no idea about what to do about the ground of guitar cables.
(Side note: I got interested in this topic because I wanted to add enough inputs to have two for every pickup in my guitar – either side of the humbucking pairs, though you can substitute that for having one input for each string via piezo under-saddle mics like Variax used to).
(Other side note: if you do end up adding a bunch of TLV320ADCx140s you can do really fun “far field microphone” experiments with multiiple digital microphones.)