A digital synthesizer design originally developed by E-Mu Systems. The outstanding feature of the Morpheus was a concept called the Z-Plane filter. This provided the ability to interpolate between a variety of complex filter response curves, in real time. Selecting between sets of response curves allowed the performer to not only create simple subtractive synthesis, but also an approximation of resynthesis.
The E-mu Morpheus Synth
The original E-mu Morpheus, released in 1993, was a 1U rackmount synth, packaged in a manner resembling the Proteus synth line, but containing unique hardware. (No keyboard version was ever released.)
The Morpheus' basic voice architecture consisted of a digital oscillator-filter-amplifier chain, fairly standard on its surface. Each voice contained two layers, which fully duplicated each other. The outputs of the two layers went to a mixer section, which allowed each layer to be panned across any of three stereo pair outputs, and also routed to the effects section.
The digital oscillator was of the wavetable type, allowing the user to choose from a large set of single-cycle waveforms im ROM. The oscillator for each layer fed into a "tone" circuit (a simple, non-resonant lowpass filter), and then into the Z-Plane filter. The output of this went to the digital amplifier and then to the mixer section.
The Z-Plane Filter
The Z-Plane filter was the outstanding feature of the Morpheus, from which the synth took its name. The Z-Plane filter interpolated between eight different complex response curves, which the manual described as being, conceptually, at the eight corners of a cube. Three parameters, known as Frequency, Morph, and Transform 2, selected a point within the volume of the cube. The filter interpolates between all eight filter curves depending on how "close" the interpolation point is to each corner of the cube; the closer the point is to a corner, the more influence that the response curve in that corner has on the result.
On the original Proteus, due to CPU limitations, only the Morph parameter could be varied in real time. The Frequency and Transform 2 parameters were set at note-on and remained fixed as long as the note sounded. The Frequency parameter was somewhat misleadingly named; it was called such because its value was usually assigned to the MIDI note number, and the filter cube was usually arranged so that the Frequency parameter usually interpolated between two version of the same response curve, with the higher-value one "slid to the right" so that all features of the curve were at higher frequencies, to somewhat emulate the behavior of a conventional filter having its cutoff frequency modulated by note value. However, nothing about the filter architecture required that the cube be set up in this way, and in fact, some of them weren't.
The Transform 2 dimension was set up in some cubes so that it morphed between two versions of the same shape, but with one having its peaks and notches more emphasized, and Transform 2 was conventionally assigned to velocity. Again, the filter architecture did not require this, and some cubes were not set up this way. Some "cubes" were actually just "planes", with four corners and four response curves; on these, the Transform 2 parameter had no effect.
The Effects Processor
E-mu introduced the original Morpheus in 1993. It was not a big seller; the Z-Plane filter mystified many users, and that, combined with the awkwardness of patch editing typical of the Proteus-derived modules, probably were what held down sales. The following year, E-mu introduced the Ultraproteus, which used the same basic architecture but had a simplified and stripped-down filter cube set.
The Rossum Morpheus Module
Dave Rossum, one of the co-founders of E-mu, revived the Z-Pane concept in 2016, in the form of a Eurorack module. It was the first product introduced by Rossum Electro-Music. With the advances in processor technology, the restrictions on he Frequency and Transform 2 axes are eliminated; all three axes can be animated in real time, by control voltage inputs. The module also contains a useful color display which can be used to display the individual curves in a cube, and where in the cube the interpolation point is at any given moment.
The Rossom module also contains "sequences", or chains, of filter cube setups. Trigger signals input to the module can cause the sequence to advance from one filter to the next. Transitions are done by interpolating, and the interpolation time can be set in the sequence. This is in addition to the interpolation taking place within the cubes. The module has non-volatile patch memory for the sequences, with 64 user-writeable locations.