Qu-Bit Aurora: A Stereo Spectral Reverb Module

The Long-Awaited Eurorack Reverb is Finally Coming

Jacob Johnson · 05/10/22

Qu-Bit Electronix have always held onto a unique niche in the world of Eurorack modules, exploring aspects of synthesis largely untouched by other companies. Their first module, the original version of Nebulae, was one of the earlier takes on hardware granular synthesis that later took the modular world by storm, while others like Scanned and Data Bender were inspired by Qu-Bit's love for computer music and circuit-bending, respectively. Now, they're finally introducing their long awaited effect module: Aurora, a one-of-a kind spectral reverb.

We briefly got a peek of Aurora way back at NAMM 2020, and we've been eagerly awaiting its arrival since then. That day is finally upon us, and we've got some fresh insight into what makes Aurora different from just about any other reverb effect out there—and we're not just talking about the beautiful LED lightshow on the panel.

What is a Phase Vocoder?

Aurora isn't Qu-Bit's first foray with phase vocoders—it's also the basis of Nebulae's granular engine. But what exactly is a phase vocoder? Without getting too deep in the technical side of things, phase vocoding is essentially a process that makes use of Fast Fourier Transforms (FFTs) to decouple the time and frequency components of an audio signal.

If you think about traditional audio playback methods, like those used in most loopers or samplers, changes to pitch and time always affect each other. For example, if you record a loop of yourself playing an instrument and then drop down to half-speed playback, the loop becomes twice as long but the pitch of what was played also moves down an octave—halving the pitch as well as the speed. Thus, pitch and time have a direct correlation to each other.

Phase vocoding is able to overcome this relationship through the FFT process, as the spectral content of the audio input source is analysed and then resynthesized at an arbitrary rate at the output. As such, phase vocoding a great way to achieve time-stretching and time-compression effects without repitching the audio, or conversely repitching audio without changing its playback speed. This is why Qu-Bit previously made use of a phase vocoder engine for the Nebulae—granulating audio with independent control over its pitch and playback speed proved to be very fun! But this time around, phase vocoding appears on Aurora as the core of its reverb effect, which has its own interesting implications.

Why Spectral Reverb?

Because reverb is a time-based effect, it's a go-to choice for not only manipulating the perceived space that a sound exists in, but also for smearing even the most brief of musical events into lush drones. Aurora does certainly do this well, but adds its phase vocoding engine to the front-end of the reverb, providing the means to transform the spectral and timing identity of the wet reverb signal separately from the dry signal. And to put it simply, this is really cool.

On Aurora's front panel, you'll find full stereo inputs and outputs, along with a host of other features and parameters to sculpt and shape time and spectra. Because of Aurora's unique architecture, the parameters all interact with each in ways you might not expect, leading to both familiar and totally foreign reverb territories.

To start, the Reverse and Freeze buttons respectively pipe backwards audio into the reverb or take a snapshot of the incoming signal's frequency spectrum to be processed. From there, Time, Blur, and Reflect have a substantial impact on how Aurora's reverb is shaped. Time dials in smeary effects by obscuring transients, whereas Blur will stretch and manipulate the harmonic spectra—easily capable of creating the lovably bizarre sounds that FFT processes are known for. Finally, Reflect coordinates the delay networks in the reverb algorithm, covering everything from early reflections to multi-tap-like delays. Combining all three of these together make it possible to manipulate space in ways that aren't achievable with other reverbs, making otherwise expansive drones feel claustrophobic and imparting infinite tails onto even the most minute of transients.

Aurora doesn't end there though, as there are more ways to shape and sculpt your reverbs. Warp offers frequency domain pitch shifting, for a slightly different flavor of shimmers and ominous suboctave drones. And rather than implementing a traditional filter or tilt EQ to shape the reverb tails, the Atmosphere parameter taps into the resynthesis side of the phase vocoder, not only able to remove harmonic content but also add spectral information. This presents Aurora as more than just any old reverb effect, but also as a highly effective sound design tool as well.

Aurora is built on Electrosmith's Daisy DSP platform, like Noise Engineering's Versio line. And like the Versios, Qu-Bit seems to hint at the ability to seriously customize Aurora's behavior, to the point of supporting alternative firmwares in the future. The USB port on the front panel is a clear giveaway, and the included thumb drive allows some of Aurora's features to be tailored to your needs. For example, you can decide whether you'd like the phase vocoder to precede the delay, or have the delay feed into the phase vocoder, among other options like Latency Compensation and only outputting the wet signal when freeze is engaged.

Not Your Average Reverb

We love to see fresh, new takes on common effects and synthesis methods, and in an increasingly crowded space for reverb modules, Aurora is the new sonic spice we've been looking for in our musical culinary craft. Phase vocoders and FFTs generally have a unique, ethereal sound to them, and cleverly parameterizing them as a part of a reverb is a perfect effect pairing. And after over two years of looking forward to this module, we're so glad that Qu-Bit took their time make sure Aurora was done right—and we're excited to hear the sounds we can get out of it.

We're also glad to note that Qu-Bit is donating a portion of all Aurora proceeds to Polar Bears International—helping to protect our polar ice caps and the furry friends that inhabit them.