We must confess to having totally forgotten about VirSyn's weird and wonderful "spectral modelling synth", Poseidon, which scored a solid review a few years back.
With the recent release of the iPad version, some eight years later, however, we're glad to have it brought back into our line of sight, as it's one of VirSyn's more esoteric instrumental offerings.
An additively generated waveform called a "spectral sound model" serves as the source signal for the oscillator. This is essentially a 512-partial additive construction, shown in the main Wave page of the GUI as a 3D spectral waveform, with colour representing amplitude.
Unlike the desktop version, you can't load in your own samples for resynthesis, but that's far from the end of the world, as a well-stocked library of them is built in, comprising all manner of sampled synth waveforms, instruments, vocals, environmental sounds and more.
Like a wavetable synth, the movement of the playhead through the spectral model can be controlled and modulated by various sources (LFOs, envelopes, keytrack, etc), and is fully - and very smoothly - animated in the Wave display, which can also be zoomed and rotated via multitouch.
As well as its main tonal qualities, the spectral model also has a noise component, and both can be manipulated using a range of controls, including mixing between the two, reducing the number of partials, modulating the brightness of the sound, and "blurring" the frequency and level transitions between partials.
Beyond that, it's fairly standard synth fare, with two LFOs, two envelopes, the same high-powered 32-step arpeggiator seen in other VirSyn synths, a wicked multimode filter (featuring the morphable Z-Plane-style PoleZero mode), seven very capable effects modules, and randomisation buttons scattered throughout.
The colour-coded tabs of the Synth and FX pages work really well, keeping the two most important controls for each element (LFO, filter, Spectrum, etc) constantly visible, while the strip at the bottom hosts the rest of the parameters for the currently selected element.
Poseidon Synth's primary areas of proficiency are pads, ambiences, textures and otherworldly effects of all kinds, from pure and melodious to jagged and inharmonic; but it's also pretty good at basses, leads and other more 'mono' tones.