Lanmower's Looper

A bare-metal audio looper running on Raspberry Pi 4, controlled by an Akai APC Key 25. No operating system, no drivers, no latency — just hardware and code.

What it does

• 5-track loop recording with overdub, quantized to Ableton Link tempo
• Live pitch shifting via keyboard keys (signalsmith-stretch DSP)
• Formant-preserving pitch control
• Tape-style delay with smooth time scrubbing
• Reverb with adjustable decay
• 2-pole SVF lowpass filter with resonance
• 2-pole SVF highpass filter (Butterworth)
• USB audio I/O via Behringer UCA222 (48kHz)
• USB-C OTG audio gadget for direct computer connection
• WiFi Ableton Link sync (joins or creates "ticker" network)
• VU metering on APC pad grid

Hardware

• Raspberry Pi 4 (runs bare-metal, no Linux)
• Akai APC Key 25 (pads + keyboard + knobs)
• Behringer UCA222 (USB audio interface)
• SD card with firmware

MIDI CC Map (APC Key 25 Knobs)

CC	Function
48	Reverb amount
49	Delay amount
50	Time (reverb decay + delay length, 10ms-1000ms)
51	Highpass cutoff
52	Pitch shift (continuous, +/-6 semitones)
53	Pitch shift formant preservation
54	Lowpass resonance
55	Lowpass cutoff

Keyboard

APC keyboard keys (MIDI channel 1) set live pitch shift based on distance from middle C (note 60). Press C4 for no shift, E4 for +4 semitones, C3 for -12, etc. Pitch locks on key press.

Pad Grid

• Column 0: Record/play/stop per track (5 rows = 5 tracks)
• Column 1: Tap to mute, hold to erase track
• Columns 2-7: VU meters per track

Building

CI builds automatically via GitHub Actions. The build:

1. Clones circle (bare-metal framework) and circle-prh (audio extensions)
2. Applies patches from patches/ directory
3. Compiles for RASPPI=4, AARCH=32
4. Produces kernel7l.img

Output goes on the SD card alongside the Pi bootloader files.

Dev Server

node dev-server.js runs TFTP (port 69), DHCP (port 67), and syslog (port 514) for network boot and log capture. Requires admin/root for privileged ports.

Architecture

USB Audio In (UCA222) --> AudioInputUSB --> loopMachine --> AudioOutputUSB --> USB Audio Out
                                              |
                                    pitch shift (signalsmith-stretch)
                                    effects (SVF filters, tape delay, reverb)
                                    loop record/playback (5 tracks, crossfade)
                                              |
                              OTG Audio Gadget (USB-C, side-channel)

All audio processing runs in interrupt context on core 0. No OS scheduler, no context switches, no jitter.

License

Based on circle by Rene Stange and circle-prh by Patrick Horton.