An inexpensive Eurorack audio generator

This project is an extension of previous "Cheep" Eurorack modules which have been built as demonstrations for simple, low-cost audio generators using low-end MCUs and DACs. Cheep3a targets the recently introduced CH32V003 RISC-V based MCU from WCH Inc. in order to explore the feasibility of audio generation with a sub-$1.00 microcontroller.

Cheep3a provides the following features:

• Three Control Voltage inputs with 10-bit 10Vpp instantaneous range over a +/-11V adjustable range using offset potentiometers.
• One digital input for trigger/gate/sync.
• Two +/-5V audio outputs with 16-bit resolution and 48kHz sample rate.
• One mode select button
• One RGB LED for mode indication.
• 4-pin programming interface, including 1-line WCH standard SWDIO, UART TX and Reset signal.
• JST SH-4 connector with optional I2C signals compatible with StemmaQT/Qwiic connections for off-board expansion.
• Standard 16-pin Eurorack power with shrouded header and +/-12V supply.

Firmware included in this project supports seven operating modes with various audio signal outputs in each mode:

1. [Red] Two independent sine oscillators with amplitude control - 10Hz - 10kHz frequency range.
2. [Green] Same as 1 but with 0.3Hz - 300Hz frequency range.
3. [Yellow] One sine oscillator with two outputs, phase offset and amplitude control - 10Hz - 10kHz frequency range.
4. [Blue] Pulse wave oscillator with two outputs and independent PWM controls - 10Hz - 10kHz frequency range.
5. [Magenta] 2-operator FM with index and ratio controls. Mod and Carrier outputs.
6. [Cyan] Two independent clocked noise outputs.
7. [White] White and Pink noise outputs.

Mode selection is stored in the MCU's on-chip flash memory and will persist across power cycles.

The Cheep3a firmware is based on cnlohr's "ch32v003fun" project and expects that the github directory for that project is located in an adjacent directory in order for the make-based build system to operate correctly. A recent RISC-V GCC toolchain is also required and once you have that installed (and in your search path) the following steps should work:

git clone [email protected]:emeb/cheep3a.git
git clone [email protected]:cnlohr/ch32v003fun.git
cd cheep3a/firmware
make

Version 0.1 of the hardware was fabbed and assembled at JLCPCB at modest cost and all unsupported components (pots, jacks, connectors, LED and button) were added by hand. Aside from a few minor layout errors which were easily corrected, the system performed as expected.

The hardware can be recreated with the following steps:

1. Open the PCB project in Kicad and generate Gerber and Drill files. Zip them up together and submit to the PCB fab of choice.
2. Generate component placement information and format as required by your PCB assembly house.
3. The JLCPCB formatted BOM is included in the hardware directory and maps to components that were available at the time I designed this board. If you don't use JLCPCB the BOM may need to be reformatted and substitutions of parts will likely be necessary.
4. Follow the submission process for your PCB house, including correction of placement and approval of components.

The CH32V003 MCU performs well for simple audio generation tasks but falls somewhat short for more complex DSP. Hardware support for good digital audio is impaired by the overall low CPU clock speed and limited memory capcity, as well as the need to "work around" the lack of true I2S serial audio interfacing. The biggest shortcoming however is the subset RV32E RISC-V CPU which doesn't provide hardware multiply and thus significantly increases the cost of even simple DSP operations needed for good-quality audio generation.