Thorns

Fractal MIDI sequencer for norns + grid

Requirements

• norns (any version)
• Grid (16x8 minimum)
• MIDI output device

Features

• Monophonic step sequencer (2-16 steps)
• Binary transformation tree with 7 levels of depth
• Five transformation types:
  • Pitch modification (nonlinear power function)
  • Velocity modification (nonlinear power function)
  • Mutate (±1 semitone shifts)
  • Gate chaos (mute/unmute/create notes)
  • Time shift (rotate pattern forward/backward)
• Real-time path navigation through transformation space
• Scale quantization
• Grid-based editing with octave windows
• Separate gate/pitch and velocity editing screens
• Audio engine (PolyPerc) or MIDI output

Controls

Encoders

• E1: Branches (tree depth: 0-7)
• E2: Path (navigate variations: 0.0-0.99)
• E3: Tempo (20-300 BPM)

Keys

• K2: Play/Stop
• K3: Toggle gate/pitch ↔ velocity screen (edit mode only)

Key Combinations

• K1+E1: Shift octave window (edit mode)
• K1+E2: Change pattern length (2-16 steps)

Grid

Edit Mode (Stopped):

• Screen 1 (Gate/Pitch): Click cells to set/clear notes
  • Columns = steps (1-16)
  • Rows = pitches (8 visible at a time from 3-octave range)
  • Scale degrees shown dimly
• Screen 2 (Velocity): Click column to set velocity height
  • Vertical bar height = velocity (8 levels)

Play Mode:

• Displays currently playing transformed sequence
• Current step highlighted
• Shows notes in current octave window

Parameters Menu

• Output Mode: Audio / MIDI / Both (default: Audio)
• Base Pitch: Root note of the 3-octave range (C0-C4)
• Scale: Quantization scale (Major, Minor, Dorian, etc.)
• Transformations:
  • Pitch Mod Prob: Probability of applying pitch modification (0.0-1.0, default 0.5)
  • Velocity Mod Prob: Probability of applying velocity modification (0.0-1.0, default 0.5)
• Gate Chaos Probability: Chance per step for gate chaos (0.0-1.0, default 0.05)
• Mutation Probability: Chance per step for mutation (0.0-1.0, default 0.5)
• Shift Freedom: Controls time shift distribution (0.0-1.0, default 0.5)
  • 0.0: no shifting (always stays in place)
  • 1.0: all shifts equally likely (uniform distribution)
  • 0.0-1.0: favors smaller shifts
  • Note: Time shift is always applied, but shift_freedom controls its behavior
• Audio Engine:
  • Release: Note release time (0.1-5.0s, default 0.5)
  • Cutoff: Filter cutoff frequency (50-5000Hz, default 1000)
  • Gain: Output gain (0.0-4.0, default 1.0)
• MIDI Output Channel: 1-16
• MIDI Device: Virtual or hardware device
• Clock Source: Internal (only)

How It Works

Pitch Range

• 24 semitones total: 8 below base pitch to 15 above
• Displayed in 3 windows of 8 semitones each
• Input is chromatic (all 24 semitones available)
• Output is quantized to selected scale

Transformation Tree

When you hit play with a modified trunk:

1. Generates a complete 7-level binary tree
2. At each branch level:
   • Pitch modification and velocity modification are checked against their probabilities
   • Time shift, gate chaos, and mutation are always applied (but have their own control parameters)
3. Multiple transformations can be applied to the same branch (they stack in sequence)
4. Pre-generates all random values (so playback is deterministic)
5. Stores full sequences at every node

Control parameters:

• Pitch/Velocity Mod Prob: 0.0 = never applies, 1.0 = always applies
• Shift Freedom: 0.0 = never shifts, 1.0 = all shift amounts equally likely
• Gate Chaos/Mutation Prob: per-step probabilities (even when transformation “always applies”)

Playback

• Path parameter (0.0-0.99) determines which branch to follow at each level
• Path value is decomposed into binary choices (left/right at each level)
• Plays trunk → level 1 → level 2 → … → level N (where N = Branches)
• Then loops back to trunk

Example: If Branches = 2 and Path = 0.3:

• Path 0.3 falls in range [0.25, 0.375) = binary “01”
• Playback sequence: trunk → left branch → right branch → loop

Transformations

1. Pitch Modification

• Nonlinear pitch transformation using power function
• For each note: r = pitch relative to base (-8 to 15), p = (r - 3.5) / 12
• Random exponent g between 1 and 2
• If p > 0: transformed_p = p^g
• If p < 0: transformed_p = -(-p)^g
• Transform back: r = 3.5 + 12 × transformed_p
• Branch A: uses exponent g (expands high/low ranges)
• Branch B: uses exponent 1/g (compresses toward center)

2. Velocity Modification

• Nonlinear velocity transformation using power function
• For each note: v = velocity (0-127), p = (v - 63.5) / 64
• Random exponent g between 1 and 10
• If p > 0: transformed_p = p^g
• If p < 0: transformed_p = -(-p)^g
• Transform back: v = 63.5 + 64 × transformed_p
• Branch A: uses exponent g (expands dynamic range)
• Branch B: uses exponent 1/g (compresses toward middle velocity)

3. Mutate

• Randomly shifts both pitch and velocity for each step
• Pitch: shifts by -1, 0, or +1 semitone with probabilities: -1 (p/2), 0 (1-p), +1 (p/2) where p = mutation probability
• Pitch bounces off range limits (-8 to +15 semitones relative to base pitch)
• Velocity: shifts by -8, 0, or +8 units with same probability distribution
• Velocity wraps around range (1-127)
• Branch A: Apply shifts as generated
• Branch B: Apply negated shifts

4. Gate Chaos

• Each step has probability p (gate chaos probability) to:
  • If gate on: turn off
  • If gate off but was originally on: turn on
  • If gate off and was never on: create new random note
• Both branches apply same logic to same steps

5. Time Shift

• Shifts pattern by m steps where m is chosen from 0 to pattern_length using weighted distribution
• Distribution: P(m) = x^m · (x-1)/(x^(n+1)-1) where x = shift_freedom parameter
• Special case: if m = pattern_length, reverse the sequence instead of rotating
• Branch A: Rotate forward by m steps (or reverse if m = n)
• Branch B: Rotate backward by m steps (or reverse if m = n - reverse is its own inverse)
• Shift freedom controls distribution:
  • x = 0: always m = 0 (no shift)
  • x = 1: uniform distribution over all m
  • 0 < x < 1: favors smaller shifts
• Preserves all note data, changes timing/position/order

All random values are pre-generated when tree is built, so the same path always produces the same result.

Installation

1. Copy the thorns folder to ~/dust/code/
2. Restart norns or run SYSTEM > RESTART from the norns menu
3. Select THORNS from the norns script selection

Tips

• Start with Branches = 0 to just hear your trunk pattern with scale quantization
• Increase Branches gradually to explore deeper transformations
• Sweep Path slowly to hear smooth transitions through transformation space
• Use short patterns (4-8 steps) for rhythmic variation
• Use longer patterns (12-16 steps) for melodic development
• Try different scales - quantization happens at output, so transformations work in chromatic space
• Adjust transformation probabilities to control how often each type applies
• Set a transformation probability to 0.0 to completely disable it
• Set a transformation probability to 1.0 to always apply it
• Try combinations - for example, high pitch mod + low velocity mod for melodic variation with consistent dynamics
• Lower Shift Freedom (< 0.3) for subtle rhythmic variations
• Higher Shift Freedom (> 0.7) for more dramatic pattern rearrangement
• Transformations stack - multiple can apply to the same branch, creating complex combinations

Credits

Inspired by Qu-Bit Electronix Bloom hardware sequencer. This is an independent implementation with different features and approach. Not affiliated with or endorsed by Qu-Bit Electronix.

Built for norns.

License

Copyright (c) 2025 Marc Pelath

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.