study: Rossum Morpheus

The Rossum Electro-Music Morpheus is based on the unique Z-plane filter of the Emu Morpheus synth from 1993. Years before the first wave of virtual analog synths like the Yamaha AN1X and Nord Lead, the design goal was less focused on modeling the exact behavior of analog filter circuits, and more on delivering a wide variety of filtering options, including the resonances of acoustic instruments. The module updates the original with the much more hands-on Eurorack format, enables simultaneous modulation of all three dimensions instead of one, and adds about a hundred more filter cubes.

Each program (or “cube”) in the Morpheus is a set of 8 different 14-pole filter configurations, arranged as the vertices of a cube. Three parameters control the interpolation between these configurations. So you might have one dimension controlling cutoff frequency, another morphing from 2-pole to 4-pole, and a third affecting the resonance. Or you could have a comb filter cube where one dimension shifts all the frequencies, one changes the spacing between notches, and a third dimension silences the low and high bands around the comb — it’s the same code, but with different data. So it’s extremely versatile.

There are also some programs (with a “.4” in the name) which have just 4 vertices; the third parameter controls distortion by default (though you can set it to interpolate to a “null filter” instead).

All of this means that, while it can sound approximately like typical analog filters, it also does a whole lot of other stuff. There are 289 different filter cubes, some of which are exotic. It’s a lot to take in.

There’s an excellent manual on the Morpheus product page with descriptions of nearly all of the cubes (only a few are missing). In some cases these reveal the cube’s design intent, and I think it’s worth checking out. But I found some things in the descriptions didn’t agree with what I experienced using the cube, and the format doesn’t lend itself to a sort of quick lookup when you’re trying to decide what cube to use.

I went through all of them, making a spreadsheet that’s more concise and (to me) a better quick guide than the descriptions in the manual, but I can’t help but feel this is a bit less of a deep dive and more of a quick border survey. I could park Morpheus on “Chiffin.4” and spend weeks making a huge variety of music with that, getting a feel for all of its subtleties.


  • Above all else, this study made me realize that the difference between a poor-sounding filter and an incredible one is the audio that you feed it. Maybe it’s great for bass and crap for higher notes or vice versa. Maybe it’s lovely for a triangle wave and awful on a sawtooth. Dynamics matter. How you modulate the filter has a huge impact too; fast vs. slow, smooth motion vs. sudden discontinuities; audio-rate.
  • Another way to say that is, sometimes when I judge a cube in Morpheus to be awful, I’m just not feeding it its preferred diet. Some of these cubes have an extremely specialized purpose… although most of them can be abused to do something different. When I started this study, I expected to find maybe a dozen favorite cubes that I would return to regularly in the future, and at least a hundred of them I would never want to touch. But I really don’t think that’s the case.
  • Let your ears guide you. The names can be misleading, the official descriptions in the manual can be misleading (or factually incorrect in a few cases), and my own brief classification was subjective and hasty. Even the display of the frequency response might not tell you the same story that your ears do. If it says “flanger” but sounds like an LPF, have fun with that LPF. If it says “piano” and it works on drum loops, great. If I said it was an HP filter but it’s a low shelf, sorry, I probably did that several times.
  • There’s nothing wrong with running a second filter or an EQ before or after Morpheus. Some cubes do cool things to the midrange but need a bit of the high end rolled off. For resonators, often you want to use filtered white noise at the input.
  • Sometimes mixing the dry signal, or a second filter in parallel with Morpheus, is just the ticket. Some of the flanger/comb or highpass-ish filters seem to do well with this. You could also try mid/side processing with different filters (just be careful of phase correlation/mono compatibility).


  • Distortion on Morpheus is not the same as distortion before or after the module, nor the same as input gain. It emphasizes the filter’s action, often making peaks peakier and notches notchier.
  • Sometimes a bit of distortion tends to round out and warm up the sound. And sometimes, if a low to moderate amount sounds harsh, an extreme amount will actually be smooth and delicious (it’s like brewing coffee).
  • Increasing distortion rarely has a linear effect on the output. There are all kinds of thresholds along the way, sometimes dramatic.
  • Be aware that the filters can freak out utterly. They can go into barely-contained resonance that rings out for 30 seconds or more… and sometimes they might do that at below audio rates. They can pin the signal to a constant +12V or -12V DC for some time. (They can certainly have a DC offset while sounding normal, too.) Or everything sounds great until you can cross some unexpected threshold when a certain frequency aligns just so and have sudden, loud, nasty clipping. You might have some odd noises and squeaks and bonks. Distortion makes all of this far more likely. Sometimes you can use this creatively… just protect your speakers and your ears (use a DC-blocking filter and a limiter if you need to).
  • The way audio-rate modulation interacts with resonances can also act like distortion, including in all the ways it can go wrong. But it’s focused on particular frequency bands. You can take advantage of this, but you may need to turn down the distortion if so.
  • For “.4” programs: the Transform knob controls distortion by default. You can disable “Xform Controls Dist’n” in the menu if you want it to interpolate to a “null filter” instead — this kind of averages the filter out to be flatter, which in a few cases gives you a more controlled and pleasant sound. You can still set distortion in the menu.
  • For the true cubes: you can adjust distortion from the menu, or you can reassign Transform to control distortion directly. (In most cases you’ll want to keep it on Transform.)
  • The default setting for distortion in the true cubes is 0dB. This is not the minimum! I’m sorry to say that with many cubes you’ll get better results if you lower distortion to -30dB and resave it. (Maybe tweak the input gain too while you’re in there.)


  • Probably most people have gotten into the habit of modulating the cutoff of a filter with an envelope or a gestural controller and maybe tracking pitch with it; maybe occasionally venturing a little beyond that. With Morpheus, it pays to try a lot of different modulation sources into each of the CV inputs. Envelopes, LFOs, pitch tracking, sequences, gates/patterns, noise, audio rate modulation from an oscillator, mult the filter’s input or output… experiment. Different kinds of cubes will respond differently.
  • Sometimes “Frequency” isn’t the main frequency control — it could be Morph or Transform — or a cube combines a lowpass and comb filter with separate frequency controls. Sometimes all three parameters affect the frequency but with subtle differences.
  • Turn up the CV Gain in the menu if you’re not getting the full range of modulation you need. At 1.0x, my Make Noise Function can’t bring Frequency from its minimum to its maximum.
  • That said, sometimes subtle modulation is best.
  • In general, peakier filters and resonators don’t benefit from envelope modulation unless you like cheezy laser pitch sweeps. I have a column in my spreadsheet for that.
  • Audio rate modulation can be especially lovely, widening the stereo field if you have a slight phase offset between your left and right inputs. If using an oscillator tracking the same pitch as your input, try different ratios.
  • Sometimes a fast envelope attack, pitch CV or other modulation with sudden changes can excite resonances. This can lead to add little transient squeaks, thumps and clanks. Softening up the attack time or slewing the signal may help. Of course sometimes you want this for effect.


There are a few things I wish were different about the module:

  • My top wish is a dedicated knob for distortion, instead of switching between Transform and Distortion. It doesn’t even need a CV input (though that would be nice). It’s something that often needs to be tweaked, or turned down quickly in a panic.
    (Failing that, the default should have been -30dB on all presets. And a single parameter instead of split L/R for both gain and distortion would have been less awkward. And maybe a panic button to kill distortion and reset the filter state when things go wrong.)
  • Several of the cubes have frequencies that don’t go as low as I’d like. LP and HP filters should typically have minimum cutoff somewhere under 20Hz. Combs and resonators and even vocal filters should often be able to go lower than they do.
  • There are a few cubes where Morph is a better frequency control than Frequency. Consistency would have been nice.
  • A dedicated knob for input gain would be a nice-to-have.
  • A mix knob maybe, but that only allows the dry signal in parallel with the filter when it might be better to patch another filter in parallel instead, so I’ll let that one go.
  • Since this isn’t a filter that will get you to sub-audio cutoff frequencies for useful processing of CV, DC blocking out on the output would probably have been a good idea, whether as part of the DSP or just an extra capacitor after the DAC.
  • The filter sequencer features don’t appeal to me at all, personally, and that’s panel space that could have gone to more useful controls. (After writing this page, I went back to check them out to see if it could be used as a sort of categorization system. The answer is yes, but grouping by filter type doesn’t seem to help navigation as much as I thought it might. There are 57 cubes I’ve marked as “LP” and 80 more that are LP and something else! I think rather than putting more effort into categorization I’ll just stick with this chart.)


I made this with LibreOffice Calc. I’m providing the .ODS file for download (Excel can read it too) as well as an HTML page.

For testing I used a Bitwig Grid patch with:

a saw/square mix droning at 65Hz, with a slight delay on one side for a phase offset;
a triangle droning at 65Hz, same channel delay;
white noise through a lowpass filter;
a sawtooth LFO for pings

  • Number / name: self explanatory.
  • Type: my subjective categorization of the type(s) of filters in the cube. “Complex” means there are multiple parametric bands and it doesn’t really fit a conventional description.
  • Distortion: “YES” if I found it useful to turn up the distortion. “REQUIRED” if I didn’t like the cube without it. “maybe” if it’s situational. Again, this is subjective opinion!
  • Xform: for .4 cubes, a YES means I found it worthwhile to turn off “Xform Controls Dist’n” in the menu. This doesn’t mean it’s not useful to keep having the knob control distortion though, it’s just a choice.
  • Envelope: YES if patching an envelope to Frequency works well just like modulating cutoff on a conventional filter. “maybe” if it depends on settings or your tolerance for sweeping resonant peaks. No entry means I didn’t much like it, but YMMV.
  • AR: YES if patching an audio-rate oscillator (probably at a frequency consonant with the input, or slightly detuned from that, but not necessarily) resulted in especially nice things. “maybe” if it was more situational. No entry here doesn’t mean it was bad, just ineffective or not really more exciting than doing the same for an analog lowpass filter (which, hey, that can be pretty nice at times).
  • Reso: YES if this cube “sings” nicely with filtered white noise or pinging the input, DISTO if it does it only after turning up the distortion. I didn’t count resonant BP style filters here, only groups of peaks that stood out enough from the background noise level to sound nice. Subjectively of course.
  • Notes: suggestions, opinions, complaints or confusion. The most subjective column of them all.

If an entry only lists the number, name and type, it’s not that I didn’t test it, it’s that no modulation, distortion, etc. seemed useful.

Here are those links:


Please let me know if you have any suggestions (that don’t involve adding another column!) or corrections or cool patch ideas for the Morpheus.