I just released a couple of Csound User Defined Opcodes (UDOs), including a
controllable formant filter optimised for use with MIDI, as well as three
voice processing modules, a vocoder, a pitch correction plugin and an
auto-tune like opcode that sets a new pitch based on a played MIDI note.
Additionally there is a frequency inverter, which mirrors frequencies around a
supplied centre frequency and the latest addition the Risset filter. This is a
multi-bandpass filter, whose frequencies are controlled by a Risset glissando.
You can specify the centre frequency, q-value or bandwidth, number of
octaves, direction of the glissando and the modulation rate or tempo.