calf: a Linux LV2 and JACK plug-in pack in maintenance mode

End-of-life, and it says so in the first paragraph

The first section of the README is titled Important, and it is the most important thing in the document.

The project is considered end-of-life because its plug-in user interface relies on the already phased-out GTK2 framework. It will be kept alive for other projects that depend on its code or the plugins, but no further development other than maintenance will happen here. And the README points to a new VST3 project, calfNXT, for anyone looking for a successor to the plugins.

That is a clean statement of where things stand, and it splits the audience in two. For a distribution, a plugin manager or a project that embeds Calf's code, a maintained-in-place repository is genuinely useful, because the alternative is forking a dead codebase yourself. For a musician or producer choosing a plug-in pack to install, it is a dead end with a warning label, and the successor project is the thing to evaluate.

The code is C++, the licence is LGPL-2.1-only, and the project site is calf-studio-gear.org, which the README points to for further information, screenshots and installation instructions. The last push to the repository was on 2026-10-13, and there are no GitHub releases at all, so the version you get is whatever your distribution packages.

Instruments, effects and tools, for a professional audience

Calf Studio Gear is described as an audio plug-in pack for LV2 and JACK environments under Linux operating systems, and it aims for a professional audience.

The catalogue is broad in the way a studio rack is broad. The instruments and tone generators are an Organ, a Monosynth, a Wavetable and Fluidsynth. The modulation effects include Multi Chorus, Phaser, Flanger, Rotary, Pulsator and a Ring Modulator. The delay section runs from Reverb and Vintage Delay through a Compensation Delay Line to a Reverse Delay.

The dynamic processors are the densest group, with a Compressor, a Sidechain Compressor, a Multiband Compressor, a Mono Compressor, a Deesser, a Gate, a Sidechain Gate, a Multiband Gate, a Limiter, a Multiband Limiter, a Sidechain Limiter and a Transient Designer. The filters and equalizers include Filter, Filterclavier, an Envelope Filter, and equalizers in 5, 8, 12 and 30 band, plus a Vocoder and an Emphasis stage. The distortion group covers Saturator, Exciter, Bass Enhancer, Tape Simulator, Vinyl and Crusher, and the tools are Mono Input, Stereo Tools, a Haas Stereo Enhancer, Multi Spread, an Analyzer and three crossover variants.

What is missing from that list is as informative as what is in it. There is no drum machine, no sampler beyond the SF2 player, and no modular environment. The pack is a set of single-purpose processors, which is exactly what a mixing chain wants and what a producer building an instrument rack does not.

LV2 and JACK, and that is the whole platform story

The format support is narrow, and it determines who can use Calf at all.

The pack ships for LV2 and JACK environments on Linux. LV2 is a plugin format for Linux audio hosts, and JACK is the audio routing system underneath a large part of the Linux audio world. A studio running that stack gets the whole catalogue. A studio running a commercial DAW on macOS or Windows gets nothing, because the plug-ins are not built for VST3, AU or AAX, and the successor project, calfNXT, is the VST3 route.

There is a related detail in the tree. The top level contains a vcpkg.json file, and the build is driven by CMake with an Automake path that is explicitly marked deprecated. The presence of a Windows-oriented dependency manifest alongside an Automake configuration marked deprecated is a good illustration of a codebase that has been carried across a decade of different build practices.

This is where the project is strongest as a dependency and weakest as a product. The audio code is portable, well understood and under a weak copyleft licence that still allows embedding, which is exactly why the README says it will be kept alive for other projects that depend on it. The plug-in binaries are Linux and GTK2 specific, which is why it is end-of-life.

The GTK2 dependency is the whole story

It is worth being precise about why the project is over, because the reason is narrow and instructive.

The plugins are not over because the DSP is dated. The catalogue includes a 30 band equalizer, a transient designer, a sidechain compressor and a wavetable synth, and none of that is obsolete. The project is over because the plug-in user interface relies on GTK2, which has been phased out. A user interface that no longer has a supported toolkit underneath cannot be maintained, and the plugins that carry that user interface cannot be either, because the UI is not separable from the plug-in in this architecture.

That single dependency is also the reason the repository is not simply closed. The README says it will be kept alive for other projects that depend on its code or the plugins, and the successor project exists to solve the same problem with a modern format and, implicitly, a modern UI toolkit.

So there is a narrow path for anyone who wants the old binaries: a distribution package, a static build, or a container. What there is not is a path to contributing a new feature, because the README rules that out, and it would be misleading to pretend otherwise in order to look more positive about the project.

The licence stack is deeper than the headline

The project is licensed under LGPL-2.1-only, and then the README lists what it uses: GTK and FluidSynth under LGPL-2.1-or-later, Expat under MIT, and LV2 under ISC. The pointer for the full detail is licenses/README_LICENSES.txt in the source tree.

That structure matters for two reasons. First, the LGPL is a weak copyleft licence, and the way it bites depends on whether you distribute the plug-ins as a library or as a separate shared object loaded at runtime, which is the usual arrangement for plug-ins. Second, the tree contains three separate COPYING files, COPYING, COPYING.ASSETS and COPYING.GPL, which suggests that not everything in the tree is under the same terms, and assets such as the bundled soundfonts and icons are exactly the kind of thing that carries its own licence.

The sf2 directory in the tree and the presence of COPYING.ASSETS point in that direction, and the point to take away is procedural rather than legal: the headline licence covers the project, and the file that tells you what covers the assets and the bundled libraries is the one inside licenses. Whether any of that constrains your use is a question for your own counsel, and this is not legal advice.

For a project in maintenance, the licensing files are the part most likely to have gone stale, which is another reason to read them rather than assume.

Building with CMake, because Automake is deprecated

The installation section is short, and it is honest about the mess.

The README says that in all cases, looking at the build workflow file in the GitHub Actions directory gives a quick hint of how the build is done on the systems that are tested. From there it points to two documentation files, INSTALL for CMake and INSTALL-AUTOMAKE for Automake, and the Automake one carries a deprecated marker in the same line.

The repository reflects that split. There is a CMakeLists.txt, a cmake directory, a Makefile.am, an autogen.sh, and a file named configure.ac.deprecated. The dot-deprecated suffix is unusual and tells you plainly which path is legacy. There is also a build workflow badge, so the tested systems are the ones to look at first, and the ChangeLog, NEWS and TODO files sit alongside the AUTHORS file as the project's own record of its history.

Usage documentation, for once, is a solved problem. There are manuals available once the pack is installed: man calf for general instructions, man calfjackhost for hints on the calf rack application, an HTML manual under the share doc directory of the installation path, and the plug-in manuals reachable from the GUI menus. The calf rack is the standalone host that the JACK environment implies, and it has its own man page, which is a hint at how the pack is meant to be used day to day.

When to reach for Calf and when not to

The case for using Calf today is narrow and specific.

It fits a Linux audio workstation that runs an LV2 or JACK host and needs one particular instrument or effect, where installing a distribution package is faster and safer than porting that plugin to a modern format yourself. It fits a project that embeds Calf's code, since the LGPL terms and the maintenance guarantee both make that a reasonable dependency. And it fits a user who wants to experiment with a plugin whose UI is dated but whose DSP is not, on a machine where GTK2 still exists.

It is the wrong choice for a new production setup, and the reason is not the age of the effects but the age of the interface and the age of the format support. A producer buying a plugin pack today wants VST3 or AU, wants an interface that matches the rest of their studio, and wants a vendor that will fix a bug. Calf offers none of those, and its own README says where to look instead.

The practical decision is therefore: if the plugin you want is in the catalogue and your host can load LV2, install the distribution package and use it, because there is no upstream work left to wait for. If you are starting from nothing, look at calfNXT, and if you need the VST2 route, look for a maintained alternative rather than the deprecated Automake path in this tree.

And if you are considering a fork, the honest starting point is that you would be maintaining GTK2 in a world that has already moved past it, which is a decision about your own time rather than about the software.

Editorial conclusion

calf still suits a Linux audio workstation that needs a specific instrument or effect for an LV2 or JACK host and would rather not rebuild it, and it is the wrong choice for a new setup, because the project is end-of-life and the README points to calfNXT as the successor. Verify first that the plugin you actually need is present in the catalogue and that your host loads LV2 or JACK, and read the licence files rather than assuming the LGPL covers the whole tree, because the assets and the bundled dependencies carry their own terms listed in licenses/README_LICENSES.txt.

Frequently asked questions

Official sources

1. calf-studio-gear/calf on GitHub
2. Issues
3. License: LGPL-2.1
4. Project website
5. README