Z-Plane Synthesis is E-mu’s proprietary technology for providing dynamic timbral control of sample-based waveforms. Conventional synthesizer filters consist of a single section that simply allows you attenuate a waveform’s harmonic content above a single frequency with (in some cases) an optional resonant peak at that frequency. In contrast, a Z-Plane filter consists of seven sections, each (very much like a band of parametric EQ) allowing independent control of frequency, bandwidth and degree of peak or notch. As a result, Z-Plane Filters can model virtually any resonant characteristic; whether that of an acoustic instrument body, the human vocal tract or even something that does not exist in nature.
This modeling capability alone makes Z-Plane Filters extremely powerful filters, but their real power comes from their ability to smoothly interpolate (or “morph”) between resonant models. Whether in response to velocity, pressure, or a variety of real-time controls, Z-Plane filters let you dynamically transform sounds.
Z-plane synthesis was first implemented in the Morpheus (the name has nothing to do with the figure from Greek mythology but refers to ‘morphing’, a term which means to change from one thing to another), and its use of interpolation between two filter shapes is very reminiscent of how the Fairlight ‘merged’ from one waveform to another. Extremely complex filter shapes are created through the use of up to eight filter components, each of which is comparable to the traditional low-pass, band-pass or high-pass filters or parametric equalizer bands. The resulting sculpting of the sound is far more precise and subtle than in any previous type of synthesis. In addition to the basic function of the filter, starting by removing the high and/or low end, peaks and notches can be placed at will anywhere across the entire audible frequency range.
Along with being able to tailor the most precise filter responses ever, Z-plane synthesis is then able to interpolate smoothly between two of them. This not only allows the user access to a myriad of additional filter responses, if the filter is held static in any of the transition positions, but as the interpolation can be carried out in real time, radical changes in the filter response can be made in the course of a sound being played back, with the ‘Morph’ parameter enabling the user to change backwards and forwards at will between the starting and ending filter shapes. With Emu’s long-established modulation matrix providing a host of possible controllers for this Morph parameter, these timbral changes can be controlled by anything from velocity, envelopes or wheels, through to custom Function Generators. Whilst this is all similar in concept to controlling the cutoff frequency of a conventional filter using an envelope or LFO, the actual results produced are far more striking to the ear.
Surprising as it may seem, we still haven’t scratched the surface of Z-plane synthesis. In fact, the basic Morph parameter on its own might be thought of as X-axis synthesis. Another parameter, Frequency Tracking, introduces the equivalent of a Y-axis into the equation. This is the closest parameter to the conventional filter cutoff, in that it moves the complex Morph filter up and down the frequency range.
In combination with the Morph parameter, Frequency Tracking gives two-dimensional control over the filter shape. Unlike a conventional filter cutoff, though, the Frequency Tracking parameter cannot be moved in real time, but must be set at Note On (presumably because there has to be some limit on the processing power required). This makes it suitable for hooking to parameters like keyboard tracking and velocity, but unavailable for controlling from aftertouch or envelopes. However, the real-time Morph parameter allows much more radical effects than filter cutoff movement, and thus more than makes up for the fact that you have to fix the Frequency Tracking at Note On.
We still haven’t mentioned the ‘Z’ axis that completes Z-plane synthesis: a third parameter, Transform 2. The function of this varies from Z-plane filter to Z-plane filter, but one example of what it can do is increase the size of the peaks and notches in the filter contour (similar to the individual peak which is increased in a conventional filter by the resonance control). Now that we’ve introduced the Z-plane into the equation, we can begin to visualize the three-dimensional variations possible in the resulting filter contour.
With Z-plane synthesis, we’ve started to touch on the technology used in physical modeling, which is currently where all the big strides in synthesis are being made.