From NonLinear Circuits website:
This 4HP module is a thru 0 VCO using the SSI2130 chip. It is designed as an FM operator as per the concepts delivered by John Chowning, so for best results 2 or more are needed (get 6!), along with suitable EGs and mixers. The NLC Valmorification (VC EG) and 4HP Mix are intended as support modules to develop an all analogue FM system. It contains the oscillator and VCA, the EG must be patched in. The thru-0 oscillator has a good variety of controls – control voltage, frequency modulation, phase modulation and hard sync.
The module only has a sine output as per a standard FM operator, although it generally spits out some very complex waveforms. I rarely do any further processing to the signal, such as VCFs or wave folding, there is enough going on.