Mod Squad: Rossum Electro-Music Morpheus

A classic filter design that's more powerful than ever
Publish date:
Social count:
A classic filter design that's more powerful than ever
Image placeholder title

Among the many innovations Dave Rossum is known for is z-plane filtering. As part of the E-mu Morpheus synthesizer of the early ’90s, the feature allowed you to interpolate (or morph) between a set of filter states. It was revolutionary technology at the time that yielded a unique sound, even if only one of the planes could be controlled in real time.

Nearly 30 years later, Rossum has made major improvements to the original design in order to create a Eurorack module named Morpheus ($499), with stereo I/O and real-time control over all three axes of the z-plane filter.

The OLED gives you a handy visualization of levels and changes in frequency response as you morph through a cube’s 3-dimentional space.

Image placeholder title

Morpheus utilizes a 14-pole filter to shape frequency responses that are musically interesting. Sets of eight frequency responses are assembled into a group, which is referred to as a cube, as if each response is positioned in a corner of a virtual three-dimensional space. This underlying structure makes it possible to create complex timbral gestures by morphing between the eight frequency responses. Morpheus comes with 289 user-editable cubes.

The included cubes are organized into categories based on effects types. The Harmonic Shifter, Diphthong, and Vocal Formant categories are sonically rich and kept me busy for hours. Other categories include Instrument Formant, Vari-pole, Distortions, Parametric Tracking, and Flanger.

As you explore the categories, you’ll quickly notice how wide the module’s dynamic range is: A number of cubes have resonance components that are capable of generating extreme output levels.

Positions along the cube’s three axes—Frequency, Distortion/Transform, and Morph—can be set manually and with CVs/attenuverters; the two levels are summed for each axis. An additional CV input, Full Level, is unattenuated and summed with the other Frequency-related levels. Further voltage offsets can be programmed from the Edit menu.


Another thing that makes this module a keeper is the ability to program repeating sequences of cubes and control the step-to-step movement manually or using a trigger. The Increment, Decrement and Reset inputs, together with the programmed behavioral commands (Halt, Pause, Bumper), can alter sequence playback in surprisingly musical ways. Morpheus stores 199 sequences, each with up to 200 steps.

The Select/Data encoder is used to load cubes and sequences, and the buttons access the module’s editing and programming features. To help you keep track of what the module is doing, the colorful OLED crams as much useful information into its screen as possible: It shows sequencer step and command status; input level, Distortion/Transform-axis levels, and output level; the Frequency, Morph and Distortion/Transform values; and the current position within the virtual 3D space using an animated dot moving inside a cube.

At any given moment, there can be a lot going on. Yet, like other Rossum modules, Morpheus is simple to use despite its deep feature set. Once you’re ready, it’s easy to navigate the module’s advanced functionality with the help of the online manual.


Morpheus not only offers a wide range of filter types—from subtle to aggressive—but also it gives you the tools to morph between them while modulating their parameters.

And although the factory cubes and sequences are expertly designed and fun to use, they merely hint at this module’s capabilities. It’s only when you edit the cubes that you begin to see the inexhaustible potential within Morpheus.