ZynAddSubFX, shown in KXStudio 12.04 above (main page, ADsynth page, SUBsynth page, and PADsynth page), was created by Paul Octavian Nasca in 2002. It is a realtime, polyphonic/monophonic, multitimbral, microtonal synthesizer capable of rescaling to up to 128 microtones per octave, as well as key mapping. Although it incorporates the traditional additive and subtractive methods of sound synthesis, as well as a PAD synth capability, it really is in a category all its own, because of how it expands, enriches and compresses harmonic spectrums, thereby obtaining extraordinarily rich sound timbres. It can be run standalone and also as an LV2 plugin. ZynAddSubFX has also been forked into a version called Yoshimi which is being developed and maintained at Github (link provided below). Recent developments include some useful additions such as Midi bank control, and an increased number of parts. A nice demo of Yoshimi below as well.
The "ADsynth" section functions by adding voices together, each one provided with "traditional" features such as filters and LFOs, but also including morphing and modulation capability. Each voice has a waveform generator capable of reproducing up to 128 wave types. These waves can be reshaped, and Fourier synthesis is also provided.
The "SUBsynth" subtractive section subtracts harmonics from white noise. The envelope stages for amp, filter and pitch can be switched to "freemode," where multiple steps and values can be set, similar to the Casio CZ series synths of the 80s.
The "PADsynth" section produces pad sounds which can be exported.
Several effects are included in ZynAddSubFX: Reverb, Echo, Chorus/Flange, Phaser, AlienWah, Distortion, EQ and Dynamic Filter. These can be applied to parts and globally. Several types of filters are provided, including formant and state variable filters. Envelopes can also be adjusted, beside the traditional ADSR structure.
Below are various soundbanks and patches (in .xiz format), which should all work just fine in either ZynAddSubFX or Yoshimi.
ZynAddSubFX at KVRaudio
Folderol Banks 1
Folderol Banks 2
My own little bank: Vanilla
01 PADsynth strings
02 Simple Electric Guitar and strings
03 Space Choir
04 Example Instrument and strings
05 Complex Strings video demo
06 PADsynth different profiles and bandwidths
07 PADsynth sweep pads
08 PADsynth different ways to get choir sounds
09 Arpeggio using many effects
10 Analog synth
Yoshimi demo, full midi control
ZynAddSubFX at Sourceforge
Yoshimi at Github