A 16-band parametric and dynamic equalizer for macOS, by Catastrophic Audio
Download · User guide · Features · Under the hood · Building · Development
Spectral Fault is an open-source equalizer plugin with analog-matched filters, dynamic bands with side-chain, per-band stereo processing, a linear-phase mode and tools that turn a spectrum, a drawn curve or a reference track into bands. It is a learning project written from published papers, with every filter checked numerically against its analog prototype.
- 16 bands, each free, active or bypassed, with ten shapes: Bell, Low/High Shelf, Low/High Cut, Notch, Band Pass, Tilt Shelf, Flat Tilt and All Pass.
- Cuts from 6 to 96 dB/oct in 6 dB steps, plus a 192 dB/oct Brickwall.
- Analog-matched response: bells and shelves stay close to their analog shape right up to Nyquist, without the cramping of textbook digital designs.
- Click-free editing: parameters glide, and type, slope or bypass changes crossfade.
- Dynamic bands (Bell and shelves): threshold, range or ratio law, attack, release, Peak or RMS detection.
- Frequency-aware detection: each band listens to its own region of the spectrum.
- External side-chain: let another track drive a band, for ducking or de-masking.
- Spectral dynamics: switch a dynamic band to Spectral and it acts per frequency slice, pulling down only the resonances or harsh spots that cross the threshold instead of the whole band.
- Per-band channel mode: Stereo, Left, Right, Mid or Side, freely mixed in one chain.
- Split curve display: L/R or M/S sums shown when bands act on different channels.
- Zero latency (minimum phase) or Linear phase with three filter lengths (about 96 / 181 / 352 ms of latency at 48 kHz), reported to the host for automatic delay compensation.
- Real-time analyzer: input and output spectra, with adjustable resolution, speed, range and freeze, and a stereo output meter.
- Peak pick: rings mark the most prominent peaks; drag one to create a band tuned to that peak.
- EQ Sketch: Option-drag a curve over the display and it becomes bells, shelves and cuts.
- EQ Match: learn a reference track (side-chain or capture) and your own, preview the difference and fit it into bands.
- Factory and user presets, A/B comparison (both slots saved with the project), undo/redo of your edits.
- Auto Gain: loudness compensation computed from the curve, K-weighted.
- Interaction: drag nodes, wheel or pinch for Q, multi-select, right-click menus, double-click to add a band.
| Area | What it does | Source |
|---|---|---|
| Filter design | Matched second-order bells, band passes and cuts; matched two-pole shelves; matched one-pole tilt | M. Vicanek, Matched Second Order Digital Filters (2016); Matched Two-Pole Digital Shelving Filters (2024/25); Matched One-Pole Digital Shelving Filters (2019) |
| Real-time safety | The audio thread never allocates, locks or does file I/O; UI data flows through lock-free SPSC FIFOs; heavy work runs on background threads | Verified by an allocation-counting test |
| Spectral dynamics | Short-time FFT (2048-point frames, 75 % overlap, square-root Hann analysis and synthesis for exact reconstruction), a follower and gain law per frequency slice, weighted by the band's region | |
| Linear phase | Frequency-sampled zero-phase 2×2 stereo matrix, 4-term Blackman-Harris window, own uniformly partitioned overlap-save convolution that keeps its input history across filter changes | F. Harris, On the Use of Windows… (1978); F. Wefers, Partitioned Convolution Algorithms for Real-Time Auralization (2015) |
| Stereo model | Every band as a 2×2 transfer matrix, so L/R and M/S bands combine exactly (display, Auto Gain, linear phase) | |
| Auto Gain | Pink-noise power ratio of the curve, K-weighted | ITU-R BS.1770-5 |
| Curve fitting | EQ Sketch and EQ Match fit bells, shelves and cuts to a target with Levenberg-Marquardt least squares on the plugin's own designs | |
| Testing | ~290 Catch2 tests: responses against analog prototypes with stated bounds, stability sweeps, latency, no-allocation checks; pluginval (strictness 5) and auval |
Built with JUCE 9.0.2 (AGPLv3 option) and Catch2, C++20, CMake.
Download the latest SpectralFault-<version>.pkg from Releases
(builds of every change are also kept as artifacts of the build workflow).
It needs a Mac with Apple silicon and macOS 12 or later, and lets you choose Audio Unit, VST3 and the Standalone
app. The .dmg holds just the Standalone app.
The builds are not signed with an Apple Developer ID, so macOS asks once: if your host skips the plug-in, open System Settings → Privacy & Security and click Open Anyway next to Spectral Fault, then restart the host. In Logic Pro the plug-in appears as Catastrophic Audio: Spectral Fault.
New to it? The user guide has an annotated tour of the interface and walkthroughs for common jobs: cleaning up a vocal, taming resonances, dynamic de-essing, side-chain ducking, mastering in linear phase, matching a reference and more.
Requirements: a Mac with Apple silicon, Xcode command-line tools, CMake 3.22+ and Ninja.
git clone --recurse-submodules https://github.com/CatastrophicCoder/eq.git
cd eq
cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
cmake --build buildThe plugins are in build/SpectralFault_artefacts/Release/ and are also installed into ~/Library/Audio/Plug-Ins/.
./packaging/package.sh builds the installer and disk image.
Running the tests, validating the plugin, the architecture and the project's conventions are described in
docs/DEVELOPMENT.md.
All planned milestones (0–9) are done, including the deferred features: A/B, undo/redo, peak pick, EQ Sketch, EQ Match and spectral dynamics. The plan and a detailed log are in docs/PLAN.md and docs/PROGRESS.md. The primary test host is Logic Pro (AU); the VST3 is checked with pluginval.
GNU Affero General Public License v3.0. JUCE is used under its AGPLv3 option.