A digital synthesizer that mimics the circuitry found in an analog synthesizer. A Virtual Analog synth emulates analog characteristics by implementing mathematical models of analog circuitry. Analog modeling is a type of physical modeling, which imitates the electronic properties of circuit components rather than the mechanical or acoustical qualities of some device.
It’s important to understand that digital synthesizers as they are currently implemented don’t exactly model the changes in voltage an analog synth uses to operate. Analog’s voltage fluctuations are smooth, continuous and infinitely variable, and the interactions that take place between all the components under different conditions are highly variable and dynamic. Digital synths, on the other hand, represent signal changes as numbers. Digital signals and parameter values are quantized into a finite number of discrete steps. How these steps ultimately become manifested as an analog signal (or control how that signal is generated), and ultimately the quality of that signal, will depend upon the implementation of the software and the D/A converter at the end. Even where this implementation may be “perfect,” and can produce a perfect replica of an analog signal at a moment in time under one set of conditions, it may fall short in the next moment under a slightly different set of conditions just due to the enormous complexity of all the possible variables of an analog system as the components interact with each other and the environment. In theory every signal aspect of a device’s operation can be modeled, but in practice they are not and this is one place inaccuracies in the final result can creep in. There are many ongoing advances in this type of modeling so expect to see better and more realistic implementations in the future.