USB Audio & MIDI Interface

Turns the Workshop Computer into a class-compliant multichannel USB audio interface and USB MIDI device with integrated CV/Gate support.

• 6-Channel USB Audio: Route up to 6 audio channels into and out of your DAW.
• USB MIDI to CV/Gate: Control modular gear with MIDI notes, clock, and CCs, or send CV into your DAW as MIDI.
• Web MIDI Editor: Configure routing, channel counts, and sample rates directly in your browser.

Operating Modes

Set Switch Z on the Workshop Computer panel to select the active mode:

Alt Mode (Switch Up) Jack Mapping

When Switch Z is set UP, hardware jacks provide MIDI to CV/Gate integration:

Inputs

• Audio In 1 & 2: Stereo passthrough routed to USB audio channels 5 & 6.
• CV In 1: 1V/Oct pitch CV input (converts incoming CV to MIDI notes).
• CV In 2: CC modulation source (converts CV to CC values).
• Pulse In 1: Gate input associated with CV 1 pitch tracking.
• Pulse In 2: External clock or run gate input.

Outputs

• Audio Out 1 & 2: Stereo audio playback from USB channels 5 & 6.
• CV Out 1: 1V/Oct pitch CV output (converts incoming MIDI notes to 1V/Oct).
• CV Out 2: CC modulation CV output.
• Pulse Out 1: Gate / trigger output (fires on MIDI note events).
• Pulse Out 2: Clock / run gate output.

Controls & MIDI CC

When MIDI functionality is active (Middle or Up mode), physical knobs send MIDI CC messages:

• Main Knob: Transmits MIDI CC 1 (Modulation).
• Knob X: Transmits MIDI CC 2 (Breath / Expressive).
• Knob Y: Transmits MIDI CC 3 (Undefined / Custom).

Web Configuration & Settings

Configure sample rates, channel masks, and custom CC/Gate routing using the USB Audio Web Editor. Settings save directly to the device's onboard flash memory.

Default Factory Settings

• Sample Rate: 44.1 kHz
• Active Channels: 4 Channels (Audio 1/2 + CV 1/2) enabled for both Input and Output.
• Input Mapping: Audio 1/2 (Audio Stream), CV 1 (Pitch Ch 1), CV 2 (CC 4 Ch 1), Pulse 1 (Gate).
• Output Mapping: Audio 1/2 (Audio Stream), CV 1 (Pitch Ch 1), CV 2 (CC 4 Ch 1), Pulse 1 (Gate), Pulse 2 (Clock).

[!NOTE] OS & USB Bandwidth Guidance

The RP2040 USB interface operates at Full Speed (USB 1.1). Bandwidth varies by host OS:

• macOS: Up to 6 Channels @ 48 kHz.
• Windows: Recommended 4 Channels @ 44.1 kHz or 6 Channels @ 24 kHz for optimal stability.
• Linux: 2 Channels up to 48 kHz; 4 Channels up to 24 kHz.

Firmware Flashing

1. Hold the BOOTSEL button on your Pico / Workshop Computer board while plugging in the USB cable.
2. Drag and drop usb_audio_midi_interface.uf2 onto the mounted RPI-RP2 drive.

Building from Source (Developer Info)

[!IMPORTANT] TinyUSB Library Update Required

When building from source using the Pico SDK, ensure the embedded tinyusb library is updated to avoid USB audio class stability bugs:

cd $PICO_SDK_PATH/lib/tinyusb
git checkout master
git pull

Build Steps

mkdir build
cd build
cmake ..
make

Created for the Music Thing Modular Workshop System by Vincent Maurer with assistance from Google Gemini. Special thanks to Chris Johnson for the initial USB-Audio base and the ComputerCard library.