Musically, modulating a sound means introducing some variations of pitch or intensity to make the sound more salient, or "lively". There are 2 sorts of modulations: amplitude (or loudness) modulation and frequency (or pitch) modulation, also called, respectively, tremolo and vibrato.
When the modulating signal reaches audible frequencies (above 20Hz), the modulation effect is not heard anymore as a rapid pulsation of the modulated sounds, but as a change in the tonality or color (waveform) of the original sound. The effect of rapid variations of amplitude or frequencies is to add or remove harmonics to the sound, effectively producing a change of the waveform of the sound.
These 2 classes of modulations used to modify the timbre of an instrument are commonly called Ring modulation (for variations of amplitude), and FM synthesis (for frequency modulation).
These 4 classes of modulations (tremolo, vibrato, FM synthesis and ring modulation) are implemented in HighC in a particularly simple and general unifying scheme: HighC makes it possible to designate modulated and modulating signals, and link them to one another to create a modulation effect. If the modulating signal is at a low frequency, this will produce a tremolo or vibrato effect ; if they are in audible range, they will produce a modulation of the timbre/waveform.
Grasping the effect of modulation requires some experience and sometimes understanding a bit of maths helps a lot. Because modulation effects can be added or composed with one another, the possibilities are quite endless, so that spending a bit of time exploring the possibilities of modulation in HighC is rather rewarding.
This chapter proposes a gentle introduction in 5 sections: