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User Perspectives on the OXI Coral

A Full Multi-Timbral, Multi-Part Synth in a Eurorack Module

Robin Vincent · 01/08/25

The Coral from OXI Instruments is an 8-voice, multi-part, multi-engine synthesizer and sampler voice built into a 14HP Eurorack module with a filter, modulation and effects. What on earth will we do with such a beast when Eurorack is more commonly a playground of monophonic beeps and boops? It's a good question.

First of all, let's have a look at the specs. Coral is a digital synthesizer, onto which OXI Instruments has managed to squeeze ten different sound engines. Eight of the engine algorithms are taken from the open-source Mutable Instruments Plaits. OXI has enhanced them with a filter and envelopes, expanded them into 8-voice polyphony, and added effects and MIDI control. Two additional engines offer custom wavetables and sample playback. You can share the eight voices across multiple engines for multi-timbral playback. It responds to MIDI and CV in different ways, has some alternative firmware, and looks unlike any other module in your rack.

The user interface is a little bit dense and multi-layered, but you quickly get the hang of things - the use of colour and lighting is extremely helpful. The big pink knob in the middle is the main character. It's a continuous, pushable, detented encoder that manipulates the ring of LEDs. The primary function is to select the sound engine. With a push, it can also select voices and parts or act as a shift to access secondary parameters on other knobs and toggle between different utility pages.

The four surrounding knobs have slightly different functions depending on the sound engine but on the whole, mirror the MI Plaits to offer control over timbre, harmonics, morph and frequency. Secondary functions offer the ability to mix in some noise, add chorus, fine-tune the engine and select MIDI channel.

In the middle we have the yellow section containing a lowpass filter. The centre knob handles the cutoff frequency, while the other two handle resonance and envelope depth. Secondary functions let you control the overall level, panning and turn on the rather nice Space Reverb.

In the next blue and purple row, we have the amp and filter envelopes. You have independent attack and release and then a shared sustain knob in the middle. These knobs can also act as attenuators for the CV inputs. There’s also a CV input for the VCA if you’d like to use a different envelope.

Finally at the bottom there is a patch bay for CV control over the main parameters, a MIDI In port and a stereo output. Coral is ready to give you some fantastic sounds.

The Sound Engines

The available sound engines change depending on which firmware you've got loaded into the Coral. The original firmware had six melodic and three percussive algorithms taken from the Plaits, plus the sample playback. Since version 2.0, they've mixed it up a bit.

There's a "No Drums" version that replaces the percussion synths with an Additive Distortion, ACID synth and a 3-Osc virtual analog engine. The "Drums" version is closest to the original but keeps the Additive Distortion in place of the String engine. The "ACID + 3VCO Moog" version squeezes in the ACID and 3-Osc VA engines alongside a combined Snare/Hi-hat and bass drum synth. The latest update gives us a Dual Wavetable OSC, which is available through the "No Drums Dual Wavetable" version. It's pretty comprehensive, if a little confusing and not very well documented. At present, the full list is as follows:

  • Virtual Analog: Two detuned analog-style oscillators that can morph between triangle, saw and pulse waves with variable width.
  • Waveshaper: A triangle wave with a waveshaper and wavefolder.
  • FM: Two sine wave oscillators modulating the heck out of one another.
  • Wavetable: The harm, timbre, and morph knobs let you navigate a wavetable in three dimensions. You can only load one wavetable, but you can import your own using the OXI Wave software.
  • MDO: Multiple Detuned Oscillator that contains eight detuned oscillators doing their thing. You can morph between saw and square waves, modulate the pulse width and then push them all apart in a swarm of detuned chaos.
  • String: Your classic Karplus Strong resonating struck string with controls over excitations, decay and inharmonicity.
  • Additive Distortion: Harmonic synthesis with wavefolding and diode distortion.
  • 3VCO: A massive sounding 3-oscillator virtual analog sound designed to give you a bit of a Minimoog vibe. You can detune both the second and third oscillators and morph between triangle, saw and pulse wave with pulse width modulation.
  • ACID: This is a 303-style analog synth with morphing between saw, square and pulse waves. Not a lot else happens until you bring in the filter, where you get some really gritty distortion. You can control accents via CV or MIDI and add glide.
  • Dual Wavetable: Two wavetable oscillators with detuning and separate position control for each.
  • Hi-hat: A blend between two hi-hat sounds with a highpass filter and balance between filtered and metallic noise.
  • Snare: Similar to the hi-hat but with two snare sounds and balance between drum, noise and body.
  • Hi-hat/Snare: A single sound from each camp that you can morph between and then balances the harmonic and noise elements.
  • Kick: Same idea again, this time with a kick drum and control over sharpness and overdrive. You can use the Morph CV control to swap between the two sounds on all the percussion engines.
  • WAV Player: A good old-fashioned sample player that can handle mono or stereo samples in 16, 24 or 32 bit at 44.1kHz or 48kHz. There are ten folders that can hold 32 samples each on the SD card, and you can fill them up with your own.

CV or MIDI?

Within Eurorack you will naturally lean towards using CV to play the sound engines. That's completely fine. You have a 1v/Oct input and a trigger input to fire the envelopes, as well as a lot of sound variation and modulation control. Using CV is resolutely monophonic; there is no overlap, chord mode or sustaining through notes, polyphony is only available over MIDI. So, you can use your choice of sound engine as a completely CV-controlled monophonic voice in your modular, but there are some key aspects that you'll be missing out on.

I should add that using it as a massive CV-driven monophonic sound source is enormous fun and shouldn't be underestimated.

Polyphony and Parts

The Coral supports a full eight voices across every single sound engine, including the sample playback. Each voice has a filter, amplifier and envelopes to support that. You can plug in a MIDI keyboard and play Coral as a polysynth or sequence it from your DAW. Polyphony is not unheard of in Eurorack but what OXI do next most certainly is. Coral is multi-timbral, which, in the land of synthesizers, means that you can use different MIDI channels to play different sounds at the same time. This is not something I’ve ever seen before in Eurorack modular.

OXI has packed a heck of a lot into 14HP, but it's not limitless. Coral can only produce a maximum of 8 voices at any one time and so when you start using multiple sound engines, the voices have to be shared between them. The way OXI has dealt with this is to let you create "Parts" that are allocated a sound engine with a specified number of available voices. So, Part 1 could be the ACID engine playing monophonically, Part 2 could be 4-voices of the Wavetable engine, Part 3 could be a single voice kick drum, Part 4 a single voice snare, leaving us with Part 5 as a monophonic lead using the MDO engine. With each part given a different MIDI channel, you have an entire song ready to be arranged from your DAW or hardware sequencer in a single module.

To get all this to work within the context of the Coral requires quite a bit of finger and encoder kung-fu. The process requires you to hold down the encoder while being directed by the colours of the ring of LEDs. One colour lets you select individual voices, which you allocate to MIDI channels via another colour, which then become parts you can choose with another colour. It can be a bit challenging because your fingers are always in the way of the LEDs. But with patience and resilience, it all comes together.

Playing and Sequencing

So, you've got your multi-layered, multi-part, multi-voice Coral all fired up; how are you going to use it?

I've already talked about using CV, which is awesome, will give you some juicy sounds and your rack will probably already have sequencers ready to fuel it. You can't select sound engines via CV, but you can select parts via the "Part" CV input. This, in effect, lets you swap between sound engines on the fly. You could patch it to a voltage generator and use that to select different parts. You could plug in a source of randomness to have your sequence leap between different sounds. It's a nice extra level of versatility for people working in a MIDI-free environment.

Otherwise, we need to be all about the MIDI. You could grab a MIDI controller keyboard and play Coral directly, but it's more interesting to use a sequencer. Many Eurorack sequencers have MIDI outputs but not many of them do it polyphonically. Some interesting options would include the Cycle Instruments Tetrachords and the Squarp Hermod+, but they are all that immediately come to mind. And so, you'll probably have to look for an external sequencer to take full advantage of the polyphony. One module worth mentioning is the enigmatic Chord Pilot from Knobula. It can generate all sorts of chords in response to triggers and modulation and then output them as MIDI. It could be a useful companion to the Coral if you want to stay within the modular paradigm.

In terms of polyphonic desktop MIDI sequencers there are many. If we are integrating a sequencer with our modular then, ideally, it's going to need to have CV and MIDI sequencing. A good place to start is the Arturia Keystep Pro. It has four channels of sequencing to play with which is probably about right for the Coral, a built-in keyboard and lots of control. It handles both modular and MIDI really well and gives you a very familiar musical front end. The Korg SQ-64 offers three channels of sequencing plus drum triggering and while it doesn't have a keyboard, it has lots of interesting performance and manipulation options. For something a bit different, check out the Torso Electronics T-1. The T-1 is designed as an algorithmic generator of rhythms and harmonic structures. It has 16 channels of polyphonic sequencing that explores voicing and chord progressions through randomizing melodic sequences.

However, we, of course, shouldn't forget the ONE from OXI Instruments themselves. A fabulous button-matrix of 4-channel polyphonic sequencing that matches up perfectly with the Coral.

If you're more at home on a computer, then using a software DAW would be a great solution. You could treat the Coral just like any other MIDI synthesizer. However, unless you have some specific hardware it's more difficult to interface a computer with other CV/Gate-driven modular. You might need a CV-to-MIDI converter module to help you connect up to other parts of your Eurorack. It really depends on how you want to work.

Modulation

The patch bay at the bottom of Coral gives plenty of opportunity to incorporate some modular modulation. Just because you're playing it via MIDI doesn't mean you can't patch in a bunch of LFOs and other modulators to mess about with the sounds.

MIDI also opens up a whole load more parameters that you can access via CC numbers. Along with the main sound controls, level, panning and filter, you can also control the envelopes, noise level, chorus depth, reverb and delay parameters. Perhaps more importantly, you can scan through the sound engines using CC #30 for instant patch changes. However, you can also do a version of this using a Program Change message. Instead of changing the sound engine, a Program Change selects one of the 10 preset slots. A preset can hold all the sound and voice configuration information, letting you load up a whole new arrangement of Parts, sounds and settings.

Lastly, over MIDI, Coral supports MPE control if you have a controller that supports it. This will give you individual pitch bend control over notes along with aftertouch and pressure, which can be mapped to any of the CC parameters.

To Summarize

Coral packs an extraordinary amount of power into a 14HP module. With CV you can use it like a super-enhanced Plaits with filter and effects, capable of pulling off a wide variety of sounds. Plug in a bit of MIDI and you open it up to 8-voices of virtual analog synthesis, polyphonic Plaits algorithms and old-school sample playback. If you allocate those voices to different parts, you can run a whole arrangement with multiple sounds in one module. There's nothing else like Coral in Eurorack.