Introduction

**This manual is a work in progress so there may be some missing features or rough wording but it should improve over time**

Accidental Synthesizer (AccSyn) is a standalone four‑oscillator Monosynth desktop app for macOS.

It is designed to provide enough standard synth features to be usable for standard synth duties while leaving room open for me to experiment and let it get weird as it evolves.

Hopefully it helps you get weird too.

Getting Started

Requirements

AccSyn is a macOS application. A MIDI keyboard or controller is recommended but not required — you can trigger notes via MIDI from a DAW or other software.

You can download the release binaries for Apple Silicon Mac's. They are unsigned so you will have to do the dance with Apple security setting to get it running.

To build AccSyn yourself you will need to setup a rust development chain as per: Rustup or Rust's Getting started Page

Installation

Build from source using Cargo:

cargo build --release

The compiled binary is at target/release/accidental-synth. You can run it directly or copy it somewhere in your shell path.

First Launch

On first launch AccSyn creates its user patch directory at:

~/Library/Application Support/AccidentalSynthesizer/patches/

Any patches you save in the UI will live here. You can modify or even create your own patches from scratch in this folder. This directory is scanned when the application starts and your patches will appear at the end of the patches dropdown list at the top center of the application.

Audio Setup

Open the Settings panel and select the Audio Settings section. Choose your output device, left/right channels, sample rate, and buffer size. Changes take effect immediately.

A lower buffer size gives less latency at the cost of higher CPU load. If you hear clicks or dropouts, increase the buffer size.

MIDI Setup

AccSyn creates a virtual MIDI input port named AccSyn MIDI Input that appears in CoreMIDI automatically. Any DAW or MIDI software on your Mac can send MIDI to it.

You can also choose a hardware or software MIDI device to listen to for MIDI messages. In Settings → MIDI Settings, select your input port and optionally restrict AccSyn to a specific MIDI channel (it responds to all channels by default).

You can actually use both midi mechanisms at the same time. You might for example choose a device to listen to for MIDI clock but point a midi controller at the Virtual Port for MIDI notes and CC. This could cause weird issues if you get crazy with it but it has been useful for me. Both methods share the same channel number selected in the settings menu.

Playing AccSyn

Once audio and MIDI are configured, you can use it like any monophonic soft synth. It responds to MIDI note on & off, CC, Mod Wheel, monophonic aftertouch/channel pressure, etc. See the MIDI Implementation section for all the details.

Loading Patches

The patch dropdown in the header selects the active patch. Factory patches are listed first (prefixed with *), followed by your saved user patches in alphabetical order. You can send MIDI Program Control messages to change patches. There is only one Bank and it ignores it anyway so don't worry about it. The number next to the patch name in the Patches dropdown is the Program Control number to send for that patch.

Interface Overview

The interface has two main tabs and a settings panel:

• Synth tab — oscillators, filter, envelopes, LFOs, mixer, and performance options
• Effects tab — all effects slots processed in series after the synth signal chain
• Settings — audio device, MIDI device, synth options, and patch management

See the Controls section for a full description of every control, and Settings Menu for the settings panel.

Controls

There are 3 sections to the UI. The Synth and Effects Tabs and the Settings Menu.

The main Synth tab contains all controls for the synth voices, the mixer, and the performance controls like Mod Wheel and Pitch Bend parameters.

The Effects tab contains all the effects modules and sit in the over all signal chain between everything in the Synth panel, with the exception of the output mixer, and the final audio output. The effects are laid out in order of signal flow starting from the top left and proceeding effect to effect. The last effect in a row then goes to the first (left most) effect in the next row down.

The Settings Menu has is own page here

Most controls should reset to the defaults for the patch you selected, or the init patch if you haven't, when you double click them.

Synth Tab

Oscillators

AccSyn has four oscillators: Sub Oscillator, Oscillator 1, Oscillator 2, and Oscillator 3. They are identical except that the sub oscillator is pitched 1 octave below the played note.

Waveforms

Filter

A resonant lowpass filter placed after the oscillator mix.

Filter Key Tracking Note - The value is bipolar. The key tracking centers around E4/MIDI note 64 which will always get the filter value you set with cutoff and then notes above or below are altered as you change the key tracking. In the middle there is no tracking, To the right you get normal key tracking where the filter opens up as you play higher notes. To the left you get negative key tracking where the higher notes get darker and the lower notes get brighter.

Filter Envelope

An ADSR envelope dedicated to filter cutoff modulation. An ADSR envelope to modulate the cutoff of the filter over time with the played note. The inverted mode flips the envelope so that the filter starts at maximum cutoff frequency (full brightness) and then the envelope lowers the cutoff and brings it back to full based on how you set it.

Due to the interaction of the filter envelope sustain and the need for there to be somewhere for the cutoff to go I suggest starting with the cutoff either all the way off, or maybe in the center and tune by ear from there.

Filter LFO

A dedicated LFO for filter cutoff modulation

The LFO modulates the cutoff around (above and below) the current frequency set by cutoff slider. It also follows the cut off value produced by the Filter Envelope value so using both can produce some interesting effects.

Amp Envelope

An ADSR envelope to modulate the amplitude of the played notes over time. The inverted mode flips the envelope so that the note starts at maximum amplitude and then the envelope lowers the amplitude and brings it back to full based on how you set it.

Pitch Envelope

An ADSR envelope that modulates oscillator pitch. Each oscillator has a Pitch Envelope Amount control that determines how much it is affected.

Mixer

Controls the level, stereo position, and mute state for each oscillator and for the master output.

Per-Oscillator (Sub, Osc 1, Osc 2, Osc 3)

Output

Oscillator Options

Global and per-oscillator performance settings.

Modulation (Mod) Wheel

The midi mod wheel parameter CC#1 controls the range parameter of a dedicated LFO which controls the depth/amount of the effect on the oscillators.

With some exceptions noted in this table that LFO is applied to the pitch of the oscillator producing a Vibrator effect. Here is the effect it has on each oscillator.

The Mod Wheel control section has the following controls for the underlying Mod Wheel LFO

Effects Tab

All effects are applied in series after the synthesizer signal chain. Each has an Enable toggle. Disabled effects pass audio through unchanged.

1 — Saturation

Adds harmonic distortion.

2 — Colour Compressor

Dynamics compression with a colored character.

3 — Wave Folder

Folds the waveform back on itself, adding upper harmonics.

4 — Bit Shifter

Reduces bit depth for lo-fi aliasing and quantization noise.

5 — Clipper

Hard clips the signal, adding aggressive saturation at high levels.

6 — Gate Clipping

7 — Wave Rectifier

Flips or removes negative samples, changing the waveform symmetry and adding harmonics.

8 — Chorus

2 voice chorus effect

9 — Flanger

Flanger effect

10 — Auto Pan

Automatically pans the signal between left and right.

11 — Tremolo

Modulates the output amplitude.

12 — Delay

A simple stereo delay.

Signal Flow

How a note travels through AccSyn, from MIDI in to audio out. AccSyn is monophonic, so there is a single signal path — no voice allocation. Note that the oscillator mixer comes before the amplifier and filter.

Click the diagram to open it full size.

The effects are laid out in the same three rows as the Effects tab, and they process in that order — left to right, top to bottom. For what each control does, see Controls.

Settings Menu

Audio Settings

To record AccSyn in your DAW either send it through audio loopback software or do what I do and assign it to a stereo pair of output channels on your audio interface and patch them back to a stereo pair of input channels. Then you can treat AccSyn like a hardware synth and route it through outboard effects, a mixer, or what ever else you do with your synths.

MIDI Settings

To prevent needing to use MIDI loop back software to connect to AccSyn from you DAW you can simply point it at the AccSyn virtual port. This port is available no matter what you have chosen for the Input port. The two can also be used simultaneously if you wanted to say send notes to one and CC or clock to the other.

Synth Options

Polarity

This option swaps the polarity of the waveform the synth outputs. It is a simple polarity inversion at the very end of the signal chain. The change will not be audible unless you are getting some kind of interference with another signal's phase/polarity.

If you find that you are getting some kind of cancelation or destructive interference, say with a bass patch and your kick, you can try swapping the polarity. It shouldn't be needed most of the time but if you aren't using a DAW where you can do the polarity swap this might come in handy in certain circumstances.

User Patches

In this section you are able to save the current state of the Synth And Effects controls as a user patch. You can also delete previously saved patches. See the Patch Format section for full details on patches.

Save a Patch

This allows you to save the current state of the Synth and Effects tabs. The settings menu state is not saved in patches. Files will be saved using the name provided. Patch names can be a maximum of 24 characters long. The patch name is used for the file name so some character restrictions apply. Names can include upper and lower case letters (including some accented characters), spaces, numbers, as well as most special characters.

Delete a Patch

Choose the patch to be deleted and press delete. WARNING!: This CAN NOT be undone.

MIDI Implementation Chart v2.0

Manufacturer: Dave White Model: Accidental Synthesizer (AccSyn) Version: v2026.07.13.520 Date: 2026-07-15

Basic Information, MIDI Timing & Synchronization, and Extensions Compatibility

Control Number Information

Patch Format

Overview

Patch files are JSON documents that capture the complete state of a synthesizer patch, including oscillator settings, filter parameters, envelope configurations, LFO modulation, mixer levels, keyboard response, and effects processing. They allow you to save, share, and recall synthesizer configurations.

It does not store the settings from the settings panel like midi channel or device as these are something that is likely to change from session to session.

Patches are stored in your user level library directory here: ~/Library/Application\ Support/AccidentalSynthesizer/patches/

User patches are stored in JSON format text files so that they are human readable. You can manually create, edit, or delete files. Changes will be loaded when you restart the application.

While manually creating and editing patches is in no way required to use the Accidental Synthesizer, it does give you a little more control and provides some creative opportunities that are not available in the UI. For example, while the UI Portamento controls control all the oscillators at once, the patch files provide portamento controls per oscillator allowing you the freedom to set them all to unique values.

Get creative with it but be careful about the levels when combining effects as things could get very loud. Also some combinations will create very quiet or even no output. If that happens you might have to play with the settings in the UI until you get it where you needed it and then save that state as a new patch.

Root Structure

A patch file has the following top-level keys:

{
  "clock": {...},
  "effects": [...],
  "envelopes": [...],
  "filter": {...},
  "keyboard": {...},
  "lfos": [...],
  "mixer": {...},
  "oscillators": [...]
}

All keys are required. The following sections define the structure and valid ranges for each.

Clock

Single clock object that stores the last known MIDI clock tempo. Used by the engine when no external MIDI clock is present. Changing this wont have any real impact on the synth though as it needs midi clock signal to work and that will just over write it.

Example Clock

{
  "bpm": 120
}

Oscillators

Array of 4 oscillator objects, in order: Sub, Osc 1, Osc 2, Osc 3.

Example Oscillator

{
  "wave_shape_index": 3,
  "course_tune": 0,
  "fine_tune": 0,
  "clipper_boost": 0,
  "shape_parameter1": 0,
  "shape_parameter2": 0,
  "pitch_bend": 0,
  "portamento_enabled": false,
  "portamento_time": 7,
  "hard_sync_enabled": false,
  "key_sync_enabled": false,
  "gate_flag": false,
  "pitch_envelope_amount": 0
}

Filter

Single filter object controlling the resonant lowpass filter.

Example Filter

{
  "cutoff_frequency": 16800,
  "filter_poles": 4,
  "resonance": 0,
  "key_tracking_amount": 0.5,
  "current_note_number": 0
}

Envelopes

Array of 3 envelope objects: [Amplitude Envelope, Filter Envelope, Pitch Envelope].

Each envelope is an ADSR (Attack, Decay, Sustain, Release) generator.

The Pitch Envelope (index 2) modulates oscillator pitch. Each oscillator's pitch_envelope_amount controls how much it is affected. The amount, is_inverted, sustain_pedal, and gate_flag fields on the Pitch Envelope are not used; set them to their defaults.

Example Envelope

{
  "amount": 1,
  "attack_ms": 200,
  "decay_ms": 200,
  "sustain_level": 0.8,
  "release_ms": 200,
  "is_inverted": false,
  "sustain_pedal": false,
  "gate_flag": 0
}

LFOs

Array of 2 LFO objects: [LFO 1, LFO 2].

Low-frequency oscillators provide modulation sources for other parameters.

Example LFO

{
  "wave_shape": 0,
  "frequency": 10,
  "synced_frequency": 0.5,
  "sync_triggered": false,
  "thirty_second_notes": 32,
  "clock_synced": false,
  "key_synced": false,
  "center_value": 1,
  "range": 0,
  "phase": 0,
  "reset": false,
  "gate_flag": false
}

Mixer

Single mixer object controlling oscillator levels and master output.

Each quad_mixer_inputs object has:

Example Mixer

{
  "level": 0.8,
  "balance": 0,
  "is_muted": false,
  "quad_mixer_inputs": [
    { "level": 0, "balance": 0, "mute": false },
    { "level": 1, "balance": 0, "mute": false },
    { "level": 1, "balance": 0, "mute": false },
    { "level": 1, "balance": 0, "mute": false }
  ]
}

Keyboard

Single keyboard object controlling MIDI response and velocity sensitivity.

Example Keyboard

{
  "pitch_bend_range": 12,
  "velocity_curve": 0.5,
  "aftertouch_amount": 0,
  "mod_wheel_amount": 0,
  "polarity_flipped": false
}

Effects

Array of 12 effect objects in fixed order. Each effect has:

Effects are processed in this order:

All parameters use the range 0.0 to 1.0, including "unused" fields which should be set to 0. Toggle parameters are 0.0 for Off and > 0.0 values for on but 1.0 is the preferred On value.

Example Effect

{
  "name": "Saturation",
  "is_enabled": false,
  "parameters": [0.5, 0.5, 0.5, 0]
}

Waveforms

Both oscillators and LFOs use the same waveform index table:

Notes for Manual Patch Editing

Performance State Fields

Some fields reflect live performance state — they are updated in real time by MIDI input or the synth engine during playback. While you can set these in a patch file, they will be overwritten as soon as the corresponding MIDI event occurs. In most cases they should be left at their default values.

• oscillators[*].pitch_bend — Leave at 0 (overwritten by MIDI pitch bend wheel)
• oscillators[*].gate_flag — Leave as false (overwritten by note on/off)
• filter.current_note_number — Leave at 0 (overwritten by note on)
• envelopes[*].sustain_pedal — Leave as false (overwritten by MIDI sustain pedal)
• envelopes[*].gate_flag — Leave at 0 (overwritten by note on/off)
• lfos[*].synced_frequency — Leave at 0.5 (computed at runtime from MIDI clock and thirty_second_notes)
• lfos[*].sync_triggered — Leave as false (set by the engine at each 32nd-note clock boundary)
• lfos[*].gate_flag — Leave as false (overwritten by note on/off)

Key Tracking

key_tracking_amount is stored as a normalized 0.0–1.0 value but represents a bipolar range internally:

The conversion is: bipolar = (key_tracking_amount - 0.5) * 2.0. The reference note is MIDI note 64 (E4) — notes above shift the cutoff up or down relative to that center depending on the tracking direction.

Pitch Bend

pitch_bend is stored in cents (100 cents = 1 semitone), not raw MIDI values. Its effective range is determined by the keyboard.pitch_bend_range setting:

• Maximum positive bend = pitch_bend_range × 100 cents
• Maximum negative bend = -(pitch_bend_range × 100) cents

For example, with pitch_bend_range: 12 (12 semitones = 1 octave), the range is -1200 to +1200 cents. With pitch_bend_range: 2, the range is -200 to +200 cents.

A value of 0 means no pitch bend (center position). This is a performance state field — it will be overwritten by MIDI pitch bend input during playback.

Velocity Curve

velocity_curve controls how MIDI velocity maps to amplitude. The value 0.5 is the midpoint:

Parameter Validation

JSON files are validated at runtime. Invalid values will be clamped to their ranges. However, it is best practice to respect the documented ranges when editing patches manually.

File Format

• Use standard JSON formatting (valid JSON is required)
• All numeric values must be valid JSON numbers (not strings)
• Boolean values use JSON true and false (not strings)
• Trailing commas are not valid JSON

Creating New Patches

Start from init.json as a template and modify parameters as desired. The initial patch is a good reference for structure and default values.

Example: Creating a Saw Lead

Here is a simplified example of a sawtooth lead patch with portamento:

{
  "oscillators": [
    {
      "wave_shape_index": 3,
      "course_tune": -12,
      "fine_tune": 0,
      "clipper_boost": 0,
      "shape_parameter1": 0,
      "shape_parameter2": 0,
      "pitch_bend": 0,
      "portamento_enabled": true,
      "portamento_time": 27,
      "hard_sync_enabled": false,
      "key_sync_enabled": false,
      "gate_flag": false,
      "pitch_envelope_amount": 0
    },
    {
      "wave_shape_index": 3,
      "course_tune": 0,
      "fine_tune": 0,
      "clipper_boost": 0,
      "shape_parameter1": 0,
      "shape_parameter2": 0,
      "pitch_bend": 0,
      "portamento_enabled": true,
      "portamento_time": 27,
      "hard_sync_enabled": false,
      "key_sync_enabled": false,
      "gate_flag": false,
      "pitch_envelope_amount": 0
    },
    {
      "wave_shape_index": 3,
      "course_tune": 12,
      "fine_tune": 0,
      "clipper_boost": 0,
      "shape_parameter1": 0,
      "shape_parameter2": 0,
      "pitch_bend": 0,
      "portamento_enabled": true,
      "portamento_time": 27,
      "hard_sync_enabled": false,
      "key_sync_enabled": false,
      "gate_flag": false,
      "pitch_envelope_amount": 0
    },
    {
      "wave_shape_index": 0,
      "course_tune": 0,
      "fine_tune": 0,
      "clipper_boost": 0,
      "shape_parameter1": 0,
      "shape_parameter2": 0,
      "pitch_bend": 0,
      "portamento_enabled": false,
      "portamento_time": 7,
      "hard_sync_enabled": false,
      "key_sync_enabled": false,
      "gate_flag": false,
      "pitch_envelope_amount": 0
    }
  ],
  "filter": {
    "cutoff_frequency": 8000,
    "filter_poles": 4,
    "resonance": 0.5,
    "key_tracking_amount": 0.8,
    "current_note_number": 0
  },
  "envelopes": [
    {
      "amount": 1,
      "attack_ms": 50,
      "decay_ms": 300,
      "sustain_level": 0.7,
      "release_ms": 500,
      "is_inverted": false,
      "sustain_pedal": false,
      "gate_flag": 0
    },
    {
      "amount": 1,
      "attack_ms": 10,
      "decay_ms": 200,
      "sustain_level": 0.5,
      "release_ms": 300,
      "is_inverted": false,
      "sustain_pedal": false,
      "gate_flag": 0
    },
    {
      "amount": 0,
      "attack_ms": 1,
      "decay_ms": 200,
      "sustain_level": 0,
      "release_ms": 10,
      "is_inverted": false,
      "sustain_pedal": false,
      "gate_flag": 0
    }
  ],
  "lfos": [
    {
      "wave_shape": 0,
      "frequency": 5,
      "synced_frequency": 0.5,
      "sync_triggered": false,
      "thirty_second_notes": 32,
      "clock_synced": false,
      "key_synced": false,
      "center_value": 1,
      "range": 0,
      "phase": 0,
      "reset": false,
      "gate_flag": false
    },
    {
      "wave_shape": 0,
      "frequency": 0.1,
      "synced_frequency": 0.5,
      "sync_triggered": false,
      "thirty_second_notes": 32,
      "clock_synced": false,
      "key_synced": false,
      "center_value": 0,
      "range": 0,
      "phase": 0,
      "reset": false,
      "gate_flag": false
    }
  ],
  "mixer": {
    "level": 0.8,
    "balance": 0,
    "is_muted": false,
    "quad_mixer_inputs": [
      { "level": 0.3, "balance": -0.3, "mute": false },
      { "level": 1, "balance": 0, "mute": false },
      { "level": 1, "balance": 0.3, "mute": false },
      { "level": 0, "balance": 0, "mute": true }
    ]
  },
  "keyboard": {
    "pitch_bend_range": 12,
    "velocity_curve": 0.5,
    "aftertouch_amount": 0,
    "mod_wheel_amount": 0,
    "polarity_flipped": false
  },
  "effects": [
    { "name": "Saturation", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Colour Compressor", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Wave Folder", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Bit Crusher", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Clipper", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Gate Clipping", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Wave Rectifier", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Chorus", "is_enabled": true, "parameters": [1.0, 0.2, 0.0, 0.3] },
    { "name": "Flanger", "is_enabled": false, "parameters": [0.5,0.0633,0.5,0.5] },
    { "name": "Auto-Pan", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Tremolo", "is_enabled": false, "parameters": [0, 0, 0, 0] },
    { "name": "Delay", "is_enabled": false, "parameters": [0, 0, 0, 0] }
  ],
  "clock": {
    "bpm": 1
  }
}