Modulation is a central concept of music synthesis. Ever since the 1950s when radio engineers first hooked one oscillator up to another, controlling fundamental signal parameters (amplitude, frequency, phase) from other signals has been the cornerstone of electronic sound design.
Modulation expresses itself in our physical electronic instruments: A Eurorack filter module, for instance, will include at least one modulation input that accepts any signal to offset the base value set by the ‘Cutoff’ knob. A polyphonic synth will include at least one low frequency oscillator (LFO) you can route to e.g. filter cutoff or the pitch of an oscillator for cyclic effects. Modern synthesizers, especially software, feature extensive modulation routing matrices, allowing the user to control the sound in a myriad of subtle or dramatic ways during a musical performance.
In the world of music production, however, much of the modulation capability in the plug-in is hidden from the host application. Although popular plug-in formats allow the host to automate parameter values - the equivalent of turning knobs in a Eurorack system - the host cannot modulate them: There are no virtual modulation inputs.
CLAP transcends this restriction: Each parameter not only has a base value, but can also respond to modulation offsets sent from the host. This means that, as well as simple automation, CLAP-capable hosts can provide non-destructive modulation of all plug-in parameters that support this feature.
But that’s not all: CLAP also extends this model into full polyphony: Modulation can not only target a parameter in the synth engine, it can even target individual voices within that engine. For polyphonic virtual instruments collaborating with a host, this allows a degree of expressivity that is far more powerful than the modulation methods specified by other popular standards.