U-HE Zebra 2.1 (Mac/Win)

A new kind of wireless modular synth.
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Web Clips: Listen to an audio example from the U-he Zebra software synthesizer

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FIG. 1: The signal-flow grid in the center of Zebra''s Synthesis tab is flanked by a sound-module rack (left) and a modulator-module rack (right). The Global/FX tab in the bottom panel reveals Zebra''s modular effects processor.

Zebra, the brainchild of independent software developer Urs Heckmann, started out as a great-sounding plug-in synth for the Mac; it was especially noted for the quality and flexibility of its oscillators. Version 1.5, which was reviewed in the July 2004 issue of EM (available online at www.emusician.com), added new effects and modulators along with the signal-patching grid. But Zebra 2.1 is a whole different animal, much more modular and loaded with new sounds.

The big changes in version 2.1 are complete modularity, morphing multiwaveform oscillators, a pattern-based arpeggiator, and the new Cross Modulation Filter (XMF). Zebra comes with two bonus plug-ins: Zebralette is a simpler, nonmodular version of Zebra intended to be a hands-on tutorial for the oscillators, and ZRev is an enigmatic digital reverb — no instructions but lots of fun. Zebra has also gone cross-platform and multiformat, with VSTi and AU versions for the Mac and a VSTi version for the PC.


Zebra's user interface is divided horizontally into two tabbed panels. The top panel's tabs — Perform, Synthesis, and Presets — respectively house four x-y controllers, the synthesis engine, and the preset browser. The bottom panel's tabs reveal the fine structure of Zebra's global effects, oscillators, multistage envelopes, controller assignments, and arpeggiator. The grid at the center of the Synthesis tab is at the heart of Zebra programming, so I'll start there (see Fig. 1).

The grid contains 48 cells arranged in 4 columns. Audio modules are inserted in the cells by right-clicking and selecting from a pull-down menu, and the signals flow down each column to be mixed at the bottom. The modules are either sound generators (oscillators and noise generators) or sound processors (filters, waveshapers, mixers, and ring modulators). All modules have one or two inputs. Sound generators usually mix whatever sound they are generating with their input (there are some exceptions), whereas effects modify their input before sending it on. Input mixing makes the grid much more flexible. For example, you can build a Minimoog-style synth (oscillators and noise through a filter) in a single column as well as in the more obvious 4-column configuration (see Fig. 2).

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FIG. 2: A simple Minimoog-style synth can be wired in several ways. Input mixing allows the linear configuration on the left. The more obvious approach on the right uses more modules to produce the same result.

Zebra's signal path ends in an effects chain with a 3-column grid of its own. Here the columns are fed by three separate buses: Master, Send1, and Send2. Each of the main grid columns can feed any of those buses, and modules in the effects grid can also take input from other columns in the effects grid. You can, therefore, use the effects section as one complex effects block or as three separate, linear effects chains. Effects include a chorus-phaser multi-effect, delay, reverb, EQ, filtering, compression, shaping, and ring modulation.

Although you can move modules around freely in either grid, you get a fixed number of modules of each type. For sound generators, you get four waveset oscillators (a waveset is a table of 16 waveforms), four FM oscillators, and two noise generators. The noise generators offer four flavors of noise, lowpass and highpass filters in series, and their own pan and stereo-spread controls.

The FM Oscillator (FMO) gives you five ways to modulate its built-in sine-wave oscillator. You can also use the sine-wave oscillator as the modulation source, or you can use the module's input for that. Using the oscillator (called self-modulation) produces a kind of feedback modulation effect that becomes increasingly chaotic as the modulation amount increases. A second self-modulation mode, called Self2 (+), results in a signal with only odd harmonics (think square wave). The self-modulation modes mix the input with the output as usual. When the input is used as the modulation source, you can choose between ring modulation, attenuated FM, and filtered FM. For filtered FM, the modulation-amount control sets the cutoff of a lowpass filter applied to the input. You can accomplish standard FM synthesis by using one FMO to modulate another.

The Wave's the Rave

Zebra's morphing waveset oscillators are so powerful that you could build a whole synth around one of them, and that's exactly what Heckmann has done with Zebralette. Playing with Zebralette is a great way to get a handle on these oscillators, and you can load Zebralette presets into Zebra for further expansion.

The Zebralette oscillator is based on a waveset. Currently, you get neither waveset presets nor any way to copy individual waveforms between wavesets, although you can copy entire wavesets.

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FIG. 3: Waveform morphing (left) and blending (right) are different ways of moving through oscillator wavesets.

Waveforms can be created geometrically (Geo) or additively (Spectro), and in either case you can move through the table by crossfading (Blend) or morphing (Morph). That results in four oscillator modes: GeoMorph, GeoBlend, SpectroMorph, and SpectroBlend. In the Morph modes, you create breakpoints, and moving through the table converts one shape to another by moving the breakpoints (see Fig. 3). In the Geo modes, the graph represents the waveshape, whereas in the Spectro modes, it represents the levels of the sine-wave components of an additive waveform. It can take a while to bend your brain around the terminology, but the tools for creating waveforms are both simple and powerful.

Once you have a waveset, the oscillator gives you several ways to mess with it. You can modulate the oscillator's Wave knob to automate morphing or blending through the waveset. Envelopes and LFOs are good sources for that. You can layer two or four copies of the oscillator and spread them across the stereo field. You can add a phase-shifted copy of the waveform to itself either inverted or not, and you can hard-sync a pitch-shifted copy with itself. The oscillator also has an effects section consisting of two waveshaping processors.

Filters, Filters, Filters

Zebra gives you three types of filter module: four VCF, two Comb, and two XMF. VCF is a versatile multimode filter. In addition to quite a few variations on the usual suspects (lowpass, bandpass, highpass, and notch), you get shelving, peaking, and phasing-allpass filters. One of the most interesting lowpass variations, LP formants, follows a 2-pole lowpass filter with a formant stage with morphing between the five vowel formants A, E, I, O, and U (see Web Clip 1).

The Comb filter is actually a feedback-delay line with very short delay. An impulse fed to a delay of a few milliseconds with a lot of feedback produces a self-sustaining tone much like an oscillator, and the Comb modules have a built-in noise source for that purpose. Thus, they can act as oscillators as well as process an incoming signal. As oscillators, they're ideal for plucked, bowed, and metallic sounds reminiscent of physical modeling. As signal processors, they produce resonator, phasing, and flanging effects.

Like the Comb filter, the Cross Modulation Filter (XMF) will self-oscillate, and it doesn't even require an impulse — high resonance is sufficient — although a click impulse is provided for pingy sounds. The XMF is a stereo filter, and you can offset the tuning (cutoff) of the two sides. You can also modulate the cutoff by the filter's sidechain input for filter-FM effects. A bank of presets called XMF Files contains good examples of the XMF's capabilities.

Around and About

You can apply modulation to most Zebra parameters, and you get several ways to do so. Many knobs have adjacent, smaller knobs for applying modulation. In those cases, clicking on the smaller knob reveals a pull-down menu for selecting a modulator from among several MIDI sources or one of Zebra's modulation generators. The latter includes LFOs, ADSR and multisegment envelope generators (MSEGs), modulation mixers (allowing several sources to modulate a single target), and two controller lanes in Zebra's pattern-based arpeggiator. The arpeggiator and MSEGs are programmed from tabs in the lower panel. All other modulators are created on demand and housed in a rack to the right of the grid. (Audio modules are housed in a similar rack to the left of the grid.)

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FIG. 4: The MSEG1 pattern shown in the foreground is modulating waveset selection. MSEG2 holds the pitch-modulating step sequence shown in the background.

You can apply modulation from the ModMatrix, which is an alternate tab of the mixer at the bottom of the grid. The ModMatrix houses four modulation routings, and it gives you pull-down-menu access to every parameter of every module in the grid. Each routing has Amount and Via controls, the latter of which routes MIDI or another modulator to control the amount.

Finally, you use the XY Assign tab in the bottom panel to route either dimension of any of Zebra's four x-y controllers to the same parameters accessible from the ModMatrix. The x-y controllers are, of course, MIDI controllable.


Zebra excels at pattern-based modulation. First there's the arpeggiator, which uses its pattern sequencer to select the note played (from those currently held), the step length, the gate, and the transposition. The pattern can have up to 16 steps and play forward, backward, or alternating. You can use the arpeggiator's two modulation sequencers to control module parameters such as the waveform selector of a waveset oscillator.

Next up are the four MSEGs — 32-step breakpoint envelopes with independent control of each segment's slope — linear, concave, convex, or S-shaped (see Fig. 4). You can activate looping between any two breakpoints, and you can scale the rate of the preloop (attack), loop, and postloop (release) stages independently.

Zebra's LFOs also have a pattern sequencer as one of their waveform options. You get up to 32 steps, transitions can be linear or stepped, and you can force the two global LFOs to restart after a specified number of measures.

Sounds Abound

The manual is an online work in progress, with PDF versions released periodically. Given all that's going on in the program, the manual may leave you feeling a little adrift. Because Zebra is the work of a one-man development team, less manual means more Zebra. On the other hand, you get a huge, well-categorized collection of presets. With a bit of exploring and tinkering, you can gain a basic knowledge fairly quickly. Probably only Heckmann knows all the intricacies lurking beneath the stripes, and that might be a good thing.

The vastness of its sound palette, the programming flexibility, and the bonus plug-ins Zebralette and ZRev make Zebra 2.1 a real bargain. This synth covers a lot of bases and gives you plenty of room to grow — you won't get tired of it anytime soon.

Len Sasso is an associate editor of EM. For an earful, visit his Web site atwww.swiftkick.com.


Zebra 2.1

software synthesizer



PROS: Huge sound palette. Flexible modular architecture. Bonus plug-ins Zebralette and ZRev. Great sound. Hundreds of factory presets.

CONS: Manual is a work in progress. CPU intensive.



Web Clips: Listen to an audio example from the U-he Zebra software synthesizer