The last of the basic synthesis control topics pertaining to sample playback is LFO modulation.
Modulation literally means “a change.” In traditional musical terms, a modulation is the change from one tonal center to another tonal center (say from C major to G major). In audio synthesis terms, modulation refers to the changing of an audio parameter over time, typically through the use of a Low Frequency Oscillator.
An oscillator generates a signal with a periodic fluctuation in amplitude (digitally, it’s all numbers, but the numbers represent the same thing as described). In synthesis terms, these fluctuations have names based on the shape of the output waveform, or the mathematical function used to generate the waveform. The common waveforms are sine, triangle, square, sawtooth (up or down), and random (noise).
A low frequency oscillator (LFO) generates a waveform at sub-audio rates – below 20 Hz (although many LFO generators go slightly above the 20 Hz range). At sub-audio rates you don’t hear a tone from the vibrations, but rather the individual vibrations themselves, if you hear anything at all.
LFO modulation can be applied to any synthesis parameter, but most commercial samplers/synths (physical and virtual) limit the desitinations. LFO modulation can be used
- to vary the amplitude of a signal (amplitude modulation), which creates tremelo. It is the sampe principle as the vibrato used in flute performance, where changes in the intensity of the air stream result in a loudness fluctuation.
- to vary the pitch of a signal (frequency modulation), which creates a pitch vibrato like that used in violin performance.
- to vary the cutoff or center frequency of a filter.
- to vary the resonance of a filter.
- to vary the panning of a signal.
The NN19 sampler in Reason only allows for pitch modulation, filter frequency modulation, and panning modulation.