Scrapheap Challenge modular synths

Before I embark on this post, you should probably know that I’m far less educated in electronics and hardware synth design than I am with soft-synths (this is one of my babies:), so I might just end up getting carried away, suggesting impossible things and sounding stupid here…

I saw that one of the design inspirations for Mutable Instruments was the 2CV. Being from the UK I don’t know a lot about that, but I do have an affiliation with machines like the Land Rover or the JCB - machines that were built on the simplest technology, with the widest possible range of serviceable parts and upgrades. In the UK we have a TV show called Scrapheap Challenge (I believe there’s a US version called Junkyard Wars, although I haven’t seen it) where teams have to build some wacky machine out of scrap. Invariably they find an old Land Rover engine/chassis to build their machine on top of because they’re so simple and easy to extend and bend in certain ways. I’m wondering if the next iterations of the Shrut(h)i might be like that.

What I’m imagining is a “bare bones” modular synth framework which you can customise with whatever parts you might happen to have to hand. Something that comes with enough default parts to build a perfectly decent synth, but if you happen to have a SID chip or two, and some parts scavenged from a Minimoog filter, that you can put them together pretty easily, tweak the firmware to cope with that hardware, and build something totally unique. Something that comes as monophonic “out of the box”, but if you can source the parts is designed to be extended to provide polyphony. Or, to put it another way, something that’s a bit like building your own PC - so long as you choose one of the motherboards out of a given manufacturer’s range, you can put in whatever processor, graphics card, soundcard etc that you like. How open and customisable is it possible to make a synth platform?

I’ve not played with MIDIbox stuff, so I guess that could already be the sort of thing I’m thinking of, but from what I’ve read I’m wondering if there’s something that could be built that’s a bit smaller, cheaper, tighter in focus than that. Something that is pretty standardised in terms of the power supply, MIDI and audio input/output, and with a reasonably fixed and simple interface (like the Shruti-1 has), something that’s very definitely just a synth (rather than something that could also be a controller or a sampler or whatever), but where the components of that synth are relatively easily interchangeable.

Is it possible? Is it sensible? Is there already stuff out there that does it as cheaply, or with as tight a focus? Is it the sort of thing pichenettes is considering?

This is a very ambitious mission, and it boils down to one problem: interfaces. What would be the right interface between the modules? In the modular synths world, CV makes sense… But this doesn’t cover all the use cases you describe: for example addressing a SID requires some rather specific digital messages that no other chip talks. One has to be careful in defining an interface that wouldn’t require too much translation glue between the modules! What if you want to swap SID-generated oscillators with an analog VCO?

A “good” way of breaking things down would be:
1/ control module (MIDI in, switches/pots and main navigation UI, and CV generation for the VCA, VCF…)
2/ oscillator module: takes a set of note messages on a digital bus/line (could be MIDI, simply!), converts them into audio signals
3/ signal shaping/processing module: filters/processes the outputs of the oscillator module, using control signals from the control module

At this stage, you can get creative with the oscillators (a SID, a simple AVR chip running the same kind of synthesis algorithms as in the Shruti-1, a micro and DACs for MIDI->CV conversion and a VCO, an AVR generating square waves and an analog waveshaper), and with the filters/effects.

I was considering doing this, what made me a bit reluctant was the whole issue of the firmware update… As soon as the stuff that needs to be upgraded lives on several chips, this makes things a bit more complicated! So for the Shruthi-1, 1 and 2/ are tied together, indeed they happen in the same chip with no communication overhead. It’s not like you’re stuck with the same oscillator forever, because the oscillators happen in software so it’s hackable… And there’s still the option of replacing the analog signal processing board by whatever you want to. Indeed I plan to post Eagle file “templates” to develop compatible analog filter/signal processing boards for the Shruthi family… The “top” board being dedicated to UI/MIDI/control signals generation/oscillators only.