A literal time-domain translation of each analog functional module into digital form is surprisingly challenging. Creating band-limited sawtooth and square waves over a wide variety of pitches is computationally expensive. Some filter topologies used in VCFs are not realizable digitally. To avoid these problems we have devised a new method based on operations on spectral descriptions of analog modules and additive synthesis techniques. This strategy is also used to simulate analog noise generating modules.
Audio output is qualitatively close to that of analog synthesizers.
The implementation runs in real time and responds to external controllers via MIDI or Open Sound Control. In order to prevent aliasing, the source signals must be band-limited, or contain only partials between the fundamental and the Nyquist frequency. The additive synthesis strategy can also be used to simulate analog noise generation. The VCO noise signal is represented as a set of noise bands, each with a center frequency and an amplitude. White noise is generated by using a constant amplitude over all frequency bands, and pink noise is generated by setting the amplitude of each band to be inversely proportional to its center frequency.