"Beyond Obnoxious"

Zoxnoxious is a hardware analog music synthesizer with a software interface. The intent is to provide a platform for card/module development ("card" may be a better term than "module", as the latter is a well established term in the modular synth world). Cards interface via a Raspberry Pi to a host controller over USB, and the host controller runs VCV Rack which provides control signals.

Z-Cards are the voice cards that plug into the Zoxnoxious backplane. Each card receives digital signals such as I2C and SPI along with analog signals from other Z-Cards. Then each Z-Card drives a defined set of pins with its own analog output.

The Z-Cards are agnostic to the microcontroller; the interface being I2C and SPI most any microcontroller can be used. While the overall Zoxnoxious project focuses on a Raspberry Pi implementation a different project can pickup the voice cards and do something different. These cards are ripe for re-use in developing an analog synth with a microcontroller if you want to do your own thing and not use a Raspberry Pi like I have done in this project.

All Z-Cards are in the kicad directory.

Cards developed to date include:

• AS3340 VCO
• AS3372 Signal Processor (VCF, VCA)
• SSI2130/AS3394 Dual VCO + VCF Synth Voice
• Pole Dancer multimode filter with morphing/crossfade

The backplane design allows for up to 8 cards to be addressed with itself counting as one. One control address goes unused as the backplane board design only has 6 physical slots.

A Raspberry Pi Zero 2 attaches to the Zoxnoxious backplane. To a host computer, the Pi appears as USB Audio and USB MIDI devices. Incoming USB Audio and MIDI are dispatched to a card's driver, which may choose to send SPI or I2C messages to the card.

The application the Pi runs is contained entirely in the raspberry_pi directory.

The user interacts with the synth through a VCV Rack plugin. Zoxnoxious VCV Rack plugins map the Rack modules to audio channels and MIDI events that are sent via USB to the synthesizer.

These are all contained in the vcv_rack directory.

Deep dive 1: Project background and user interface

Deep dive 2: Raspberry Pi and Gadget mode

Deep dive 3: Hardware detail and autotune