BRUME

Overview

Brume is a standalone multi-timbral instrument built on a Raspberry Pi Compute Module 5. It runs four synthesis engines across four independent parts, each with six voices of polyphony (24 total), an SVF filter per voice, dedicated modulation routing, and a shared effects chain. The entire interface runs on a 10.1-inch touchscreen through a GEM-style windowed desktop, with no laptop or external computer required for operation.

The four engines correspond to four distinct approaches to electronic sound design: a dual-oscillator complex architecture with FM, AM, and wavefolding; an additive harmonic engine with spectral scanning; a triangle-core timbral engine with wave-multiplier shaping; and a granular engine that generates pitched clouds of micro-oscillator grains. Each engine occupies one of Brume's three parts, and any part can be switched between engines at runtime. Incoming MIDI can be routed to any combination of parts through a configurable channel map.

Interface

The top menu bar provides access to all pages: the four engine mode tabs (COMPLEX, HARMONIC, TIMBRAL, GRANULAR) on the left, and navigation pages (MOD, MIX, MIDI, SYS, LIBRARY) on the right. Tapping a mode tab selects that engine for editing on the synth page, while tapping a navigation tab switches to the corresponding full-screen view.

Each engine's synth page is divided into tabbed sections. Complex, for instance, has six tabs: OSC, MOD, FOLD, SHAPE, FILTER, and AMP. Parameter sliders respond to horizontal drag, and their values persist across tab switches. A touch keyboard at the bottom of the synth page provides direct note input with glissando, hold, retrigger, and chord modes.

Signal Path

Audio flows through the following chain for each voice, regardless of which engine is active:

All four parts render independently and are summed at the master bus, where a safety limiter with a -1 dB threshold prevents clipping before the signal reaches the DAC.

Synthesis Engines

Complex Oscillator

The Complex engine pairs a carrier and modulator oscillator, both capable of continuous waveshape morphing from sine through triangle, sawtooth, and square. The modulator feeds into the carrier through several modulation paths — phase-modulation FM, amplitude modulation, ring modulation, through-zero FM, and hard sync — while a cross-FM feedback path returns the carrier's output to the modulator for self-referential timbral complexity. Independent wavefolders on each oscillator and a phase-distortion stage round out the architecture.

OSC tab — Oscillator waveforms and frequency relationship

MOD tab — Modulation paths between the two oscillators

FOLD tab — Per-oscillator wavefolding

SHAPE tab — Phase distortion

Harmonic Oscillator

The Harmonic engine generates sound through additive synthesis, combining eight individually controllable harmonics with spectral shaping tools. A Gaussian scanning window sweeps across the harmonic series, isolating or blending partials as its center and width change. Each harmonic can be morphed from sine to square, and an internal FM oscillator modulates the fundamental frequency, producing coherent spectral shifts across all eight partials simultaneously.

HARMONICS tab — Individual harmonic amplitudes

SCAN tab — Spectral scanning and waveform morph

SPECTRUM tab — Spectral shaping

FM tab — Internal frequency modulation

Timbral Oscillator

The Timbral engine starts from a triangle-wave core and shapes it through a wave-multiplier circuit that transitions from soft saturation to cascaded wavefolding as the timbre control increases. An expanded symmetry parameter tilts the triangle core before it enters the shaper, feeding different harmonic content into the multiplier depending on the tilt direction. The engine also provides linear FM, a sub-oscillator at half the fundamental frequency, and a self-modulation feedback path where the output feeds back into the oscillator's own frequency.

SHAPE tab — Wave multiplier and feedback

FM tab — Linear frequency modulation

SUB tab — Sub-oscillator

Granular Oscillator

The Granular engine generates pitched clouds of micro-oscillator grains — short bursts of waveform output (1–500 ms) that overlap to produce evolving textures, shimmering pads, and atmospheric tones. Unlike sample-based granular systems that slice recorded audio, Brume's granular engine synthesizes each grain from scratch using the same morphable oscillator architecture as the Complex engine. A pool of 32 concurrent grains spawns continuously while a note is held, with each grain receiving its own randomized pitch, pan position, and waveform shape based on the scatter and spread parameters.

CLOUD tab — Grain generation and spatial distribution

SHAPE tab — Grain waveform and envelope

FM tab — Per-grain frequency modulation

Filter & Envelope

SVF Filter

Each voice includes a state-variable filter operating in lowpass mode, with a dedicated ADSR envelope that modulates the cutoff frequency. The filter and envelope parameters are identical across all four engines and are configured through the FILTER tab on each engine's synth page.

Amplitude Envelope

The AMP tab controls a standard ADSR envelope that gates the voice's output amplitude after all synthesis and filtering stages.

Modulation

Each of Brume's parts has an independent modulation router with two LFOs and two step sequencers. These four sources can be assigned to any synthesis parameter through the MOD page, where each assignment specifies a source, destination, depth, and response shape. Multiple assignments can target the same parameter, and their offsets are summed before being applied.

LFOs

Both LFOs offer identical capabilities: a selectable waveform shape (drawn from a library of 24 transition curves), adjustable rate, and a mode switch between free-running loop and one-shot triggered playback. In loop mode the LFO cycles continuously regardless of note activity; in trigger mode it plays once from the beginning each time a note-on is received, then holds at its final value until the next trigger.

Step Sequencers

Each step sequencer holds between one and eight steps, where each step stores a normalized value from 0 to 1. The sequencer advances one step per trigger event, wrapping to the first step after completing the sequence. Four trigger modes determine when advancement occurs:

Routing Assignments

A modulation assignment connects one of the four sources (LFO1, LFO2, SEQ1, SEQ2) to any parameter in the current part. Each assignment has a bipolar depth control ranging from -1 to +1, where negative values invert the modulation source. A response shape — drawn from the same 24-shape library as the LFOs — can be applied to curve the source signal before depth scaling.

Shape Library

Brume's 24 transition shapes serve as both LFO waveforms and modulation response curves. Each shape maps a phase value (0 to 1) to an output value (0 to 1), with different mathematical curves producing different modulation characters.

Mixer

The MIX page provides a four-channel mixer with horizontal faders for each part (Complex, Harmonic, Timbral, Granular), along with mute and solo controls. Each part's level defaults to 0.8 and can be adjusted from 0 to 1. Each part also has independent delay and reverb send levels, which control how much of the part's signal reaches the shared delay and reverb buses. A master volume control governs the final output level before the limiter. All level changes are smoothed to prevent clicks.

Effects

The EFFECTS panel on the MIX page provides four processing stages organized as tabbed controls: Saturator, Chorus, Delay, and Reverb. Saturator and Chorus operate as inserts on the dry master bus, processing all parts equally. Delay and Reverb operate as send effects, where each part's independent send level controls how much signal reaches the effect. This architecture follows the model used in hardware instruments like the Elektron Digitone, where a dry bass line can coexist with a heavily reverbed pad without one treatment bleeding into the other.

Saturator

Chorus

Delay

The stereo delay supports both free-running millisecond timing and tempo-synced musical divisions. When a sync division is selected, the delay time auto-calculates from the transport BPM. The delay uses cross-channel ping-pong routing with a damping filter in the feedback path to darken successive repeats.

Reverb

Four reverb algorithms are available, each built from networks of allpass diffusers and parallel comb filters with damped feedback. All algorithms share the same parameter set; the internal topology and delay lengths differ to produce distinct spatial characters.

MIDI

Brume receives MIDI over USB from any class-compliant device — control surfaces, keyboards, sequencers — and from the Meridian USB-C bridge to a host DAW (see below). All sources feed the same engine in parallel, so a controller plugged into a USB-A port and the DAW’s MIDI output over Meridian both reach Brume simultaneously without any routing setup. DIN 5-pin gear connects through any class-compliant USB-to-DIN bridge (e.g. iConnectMIDI4+).

The MIDI page has three sections: channel routing on the left (a part-centric list where each part shows its assigned MIDI channel; tap the channel box to cycle through CH 1–16 or NONE), CC mapping in the middle (the LEARN workflow plus the list of persisted CC→parameter bindings), and the clock controls on the right. Multiple parts can share the same channel for layered playback, and a single channel can be reassigned at any time without stopping playback.

Note-off messages use a promiscuous release strategy: when a note-off arrives on any part, Brume also releases that note on all other parts. This prevents stuck notes when channel assignments change during a performance. MIDI CC messages 120 (All Sound Off) and 123 (All Notes Off) are recognized and release all active voices on the targeted part.

CC Mapping — LEARN workflow

The CC MAPPING panel binds incoming MIDI CCs to Brume’s synthesis parameters. Brume ships with sensible defaults — CC 1 → Filter Cutoff, CC 2 → Filter Resonance, CC 7 → Master Volume, CC 11 → Filter Envelope Depth, plus a few part-specific bindings — and the LEARN workflow lets you add or override any binding directly from the touchscreen. No keyboard required.

Transport & Clock

The CLOCK window on the MIDI page controls Brume's internal transport, which drives tempo-synced delay divisions and step sequencer timing. Three clock modes determine how the transport derives its tempo:

The BPM slider ranges from 20 to 300 and affects all tempo-dependent features: delay sync divisions, step sequencer beat triggers, and future arpeggiator patterns. When the clock is in Sync mode, the BPM display shows the estimated tempo from the incoming MIDI clock source.

Meridian — USB-C to your DAW

A line you cross. Meridian is Brume’s integrated audio-and-MIDI bridge to a host computer — comparable in role to Elektron’s Overbridge for the Elektron range, but built on open USB class-compliant standards so it works on Mac, Windows, and Linux without proprietary drivers. A single USB-C cable carries 8-channel audio out, bidirectional MIDI, and MIDI clock simultaneously.

In a DAW, arm one or more audio tracks with Brume’s capture as the input, route MIDI out from a track to Brume’s MIDI port, and Brume behaves as a first-class external instrument just like a Prophet or Moog patched into a multi-channel audio interface — with the additional convenience that it’s all on a single cable.

Incoming notes, CC, and MIDI clock over Meridian are parsed identically to other MIDI sources — they flow through the channel map, respect CC bindings, and drive the Sync clock mode. Audio output is structured for multi-track DAW workflows by default.

Per-part stems (8 channels)

Each of Brume’s four parts writes its own dry mono signal to a dedicated stereo channel pair on the USB capture endpoint:

Stems are dry — no master saturation, chorus, delay, or reverb. The intent is that DAW users already have better plugin FX than anything Brume ships internally, and recording dry preserves total flexibility. Brume’s master FX chain still computes for the local monitoring path (HDMI / DAC) so your touchscreen monitoring still sounds finished, but the USB stems are clean. The SYS page includes a compact channel-mapping reference (`Ch 1-2 • Complex / 3-4 • Harmonic / 5-6 • Timbral / 7-8 • Granular`) so you can cross-check your DAW’s input picker without leaving the device.

Meridian expands in capability over firmware releases. Already shipped: 8-channel per-part stems, bidirectional MIDI with clock, the channel-mapping SYS reference. Active follow-ons: a USB-NCM virtual network interface for the brumectl companion CLI, optional stripping of the UAC2 Feature Unit so the macOS volume slider disappears for bit-perfect passthrough.

Library

The LIBRARY page provides preset storage and recall for each engine. Presets capture all oscillator, filter, and envelope parameters for a single part and are stored as JSON files on the device's filesystem, organized by engine type. Mode tabs at the top of the library page switch between the Complex, Harmonic, Timbral, and Granular collections.

Tapping SAVE captures the current state of the active engine and writes it to disk using an atomic write-then-rename operation that survives unexpected power loss. Tapping a preset name in the list loads its parameters into the corresponding engine and switches the display to that engine's synth page. Presets can be deleted with the × button beside each entry.

The preset format is versioned and uses optional fields for future expansion. Current presets store oscillator and filter parameters; future versions may include modulation scenes, effects chain state, and multi-part combinations without breaking compatibility with existing files.

System

The SYS page is Brume’s diagnostic and configuration surface. Every value on the page is read live from the running system — no hardcoded numbers. Plug in a different display, swap controllers, change sample rate, switch audio outputs, and SYS reflects it immediately. Sections are accent-colored: green = audio, cyan = MIDI, orange = system, magenta = display.

AUDIO OUTPUT

The audio-output section picks where Brume’s master mix lands. Brume enumerates every ALSA card on the device — the USB Meridian gadget, the HDMI output through the touchscreen speakers, any DAC hat, plus any USB audio interface plugged into a USB-A port — and renders one row per detected output. The active device shows in the section accent color with a filled dot and an "ACTIVE" label; alternate devices render below as tappable "OPTION" rows. Tapping an option tears down the current audio stream and reopens on the chosen device with a brief (~100–200 ms) silence gap. The choice persists to ~/.brume/settings.json and is restored at startup.

Below the device list, the CHANNELS block is the on-device reference for how Brume’s eight Meridian stems map to the spatial-position labels Apple’s class-compliant UAC2 driver displays in the DAW:

Use this in the DAW to figure out which incoming spatial-labeled channel is which Brume part, or as a reference when you do a one-time DAW-side rename of the channels to "Complex", "Harmonic", etc. (Bitwig, Logic, Ableton, Reaper all support persistent channel renaming).

SAMPLE RATE shows the actual rate the audio backend opened the device at — 48 000 Hz on Meridian, HDMI, and any modern audio interface. FORMAT shows the engine’s internal sample format.

MIDI

DEVICE shows the connected MIDI input by its OS-reported name (e.g. nanoKONTROL2). PORTS lists every MIDI port currently feeding the engine, joined with " + " when multiple sources are attached — a USB control surface and Meridian’s f_midi endpoint commonly run side by side. With no MIDI device connected, both rows show “—”.

SYSTEM

Hardware diagnostics for the CM5 reference platform. TEMPERATURE reads from the CM5 thermal sysfs and updates once per second. MEMORY shows the in-use resident memory for the Brume process. UPTIME is system uptime since boot. ENGINE reports the configured polyphony layout (4 parts × 6 voices = 24 total).

DISPLAY

RESOLUTION is detected from the connected monitor at startup via GDK and used to compute the auto-fit zoom level — Brume is canvas-designed against a 1024×600 logical resolution and auto-scales to whatever panel you plug in (the reference 1920×1200 touchscreen runs at scale 1.875). FPS shows the animation frame-rate cap. TOUCH reads the connected touch input device’s OS-reported name — useful when troubleshooting input issues or confirming the touchscreen probed correctly at boot.

Extending Brume with Lua

Brume embeds a Lua 5.4 scripting engine that enables users to write custom control scripts, generative sequencers, and audio effects. Scripts are loaded from the ~/brume/scripts/ directory and managed through the SCRIPT page, which provides a browser for loading and unloading scripts, a parameter panel for Lua-defined audio effects, and an interactive console for direct Lua evaluation.

The scripting system operates at three levels. Control scripts respond to musical events and manipulate Brume's parameters in real time — generating arpeggios, cycling through chord progressions, or slowly evolving a drone. Sequencer scripts emit note and parameter events on the transport clock, enabling generative patterns and algorithmic composition. Audio effect scripts define custom signal processing using DSP primitives exposed from the Rust engine, running as insertable slots in the effects chain.

API Reference

All scripts have access to the brume table, which provides functions for controlling the instrument:

Music helpers:

Callbacks

Scripts define global functions that Brume calls at appropriate moments. All callbacks are optional — define only those you need:

DSP Primitives

Scripts have access to the dsp table, which provides factory functions for creating real-time audio processing objects backed by the same Rust DSP library that powers Brume's built-in engines:

Audio Effects

Lua scripts can define audio effects that run in the FX chain alongside the built-in Saturator, Chorus, Delay, and Reverb. Effect scripts are placed in ~/brume/scripts/fx/ and loaded through the AUDIO EFFECTS section of the SCRIPT page. Each effect defines a global fx table with a name, parameter definitions, an init function, and a process function that receives and returns stereo audio buffers:

When loaded, the effect appears in the LUA FX panel with sliders for each defined parameter. Parameters are controllable from the touchscreen and can be CC-mapped to hardware MIDI controllers. The effect's mix value controls the dry/wet blend — at 0 the effect is bypassed, at 1 the output is fully wet.

Screen Drawing

The screen module provides a canvas drawing API for custom visualizations, pattern editors, and interactive displays. Drawing commands are batched and rendered at 20fps in the LUA FX panel. The API covers lines, rectangles, circles, text, paths, color, and line width — enough to build anything from simple meters to complex generative graphics.

Coroutine Clock

The clock module provides coroutine-based musical timing inspired by the monome norns clock system. Instead of responding to callbacks, musicians write sequential code that reads top-to-bottom, with clock.sync() pausing execution for a musical duration:

Multiple coroutines can run simultaneously — a common pattern is one for bass, one for melody, and one for chords. Each runs independently on the transport clock and pauses when the DAW sends MIDI Stop.

Script Parameters

Any script can expose tuneable controls in the UI by calling brume.add_param() during init(). Parameters appear as sliders in the LUA FX panel on the SCRIPT page, and their values can be read at any time with brume.get_param_value():

Script parameters are cleared when the script is unloaded. They can also be CC-mapped to hardware MIDI controllers through the FX tab in the CC MAPPING window.

Studies

Brume ships with progressive tutorial scripts in the scripts/studies/ directory. Each study makes sound immediately on load and teaches one concept through commented, modifiable code: