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OXI Instruments Coral

Eurorack Module By Robin Vincent
Published July 2023

OXI Instruments Coral

The Coral from OXI Instruments is a polyphonic synthesizer in a compact Eurorack module. It takes algorithms from the Mutable Instruments Plaits firmware, expands them out across eight voices and builds in a filter and VCA to take it from oscillator to synthesizer. We’ve seen Plaits sound engines expressed in modules before and polyphonically in synthesizers like the Arturia MiniFreak. However, what the Coral does differently is that it opens it up to eight parts of multitimbrality.

So What Is Coral?

Coral is an eight‑voice, eight‑part, 10‑sound‑engine synthesizer in a 14HP module. The sound engines include virtual analogue, waveshaping, FM, wavetable, supersaw, and string. Then there are three percussive synths covering hi‑hat, snare and kick drum. And finally, in a departure from the Plaits, we have a wave player sample engine. All the sound engines get three parameters to play with for shaping, detuning, folding, pulse widthing, indexing and feedback depending on the chosen algorithm.

The middle section of the module houses a resonant low‑pass filter for tonal shaping. Beneath that is a slightly odd pair of envelopes. One side handles the VCA while the other plays with the filter. They have a shared Sustain control, and the Decay knob is also the Release knob. It makes for simple enveloping but sometimes feels like it is not always working with you. Beneath that, you have a good range of CV inputs for modulation and CV control. Unusually for a Eurorack module, the Coral is designed primarily for use over MIDI, the assumption being that you’ll plug in a MIDI controller.

The Coral uses a ring of LEDs around the central purple/pink encoder to navigate the sound engines. To reach deeper levels or secondary parameters, you must hold in the encoder while rotating it or moving other knobs. As a system, it works well enough and avoids the need for a display or menu buttons. However, your hand is often trapped there holding in the encoder and can get in the way of the LEDs. It’s at its most clumsy when allocating parts to MIDI channels, with lots of holding, turning and trying to see around your hand, but it’s an annoyance rather than a problem.

You can share the eight voices between anything up to eight parts, by giving them a different MIDI channel. So you could allocate the kick, snare and hi‑hat engines to three channels, use the virtual analogue engine for a monophonic bass line on another channel, and you are left with four voices for a multi‑oscillator supersaw pad on channel 5. You have level and pan control over the individual parts and a Space Reverb and Chorus effect. Coupled with the right sequencer, that’s a complete electronic music studio right there.

While MIDI offers the fullest Coral experience, OXI Instruments haven’t ignored control voltage. As a compact monophonic synth voice via CV/gate, the Coral still has a lot going for it and the CV modulation options would make it at home in any modular system. You can also make use of the multitimbrality via the Part CV input, using voltage to switch between which part the CV/gate input is playing. Plug in an LFO and that makes for some very interesting melody lines.

At the time of the review I was using firmware 0.4.7, and it’s not perfect. There were some odd glitches, and sometimes it got into the sort of muddle that only switching it off and on again would cure. The sample engine could be considered a work in progress. It was troubled by clicks and artefacts and felt a bit unstable. The potential is enormous, though, if it could be rendered a bit more solid. OXI wanted me to stress that they are working vigorously on the firmware and improvements, and many more features will be forthcoming.


Overall, the Coral is an amazing source of sound in a small space. It works well as a MIDI synthesizer, giving you a lot of choices, and a workable synth architecture while having some well‑thought‑out CV patchability. You could argue that complete synth voices, MIDI and polyphony are not really what modular is about, but the beauty of an open format like Eurorack is that we don’t have to reach a consensus and can use whatever fits our particular needs. Coral can bring a lot to a small system or offer those occasional big synth sounds to a larger modular environment, and for performing, you’re not having to lug a big keyboard around.