Digital Sound & Music: Concepts, Applications, & Science, Chapter 6, last updated 6/25/2013
Figure 6.23 Master volume controller for the signal amplifier in a synthesizer
Modulation is the process of changing a shape of a waveform over time. This is done by
continuously changing one of the parameters that defines the waveform by multiplying it by
some coefficient. All the major parameters that define a waveform can be modulated, including
its frequency, amplitude, and phase. A graph of the coefficients by which the waveform is
modified shows us the shape of the modulation over time. This graph is sometimes referred to as
an envelope that is imposed over the chosen parameter, giving it a continuously changing shape.
The graph might correspond to a continuous function, like a sine, triangle, square, or sawtooth.
Alternative, the graph might represent a more complex function, like the ADSR envelope
illustrated in Figure 6.25 illustrates a particular type of envelope, called ADSR.
We'll see look at mathematics of amplitude, phase, and frequency modulation in Section
3. For now, we'll focus on LFOs and ADSR envelopes, commonly-used tools in synthesizers.
LFO stands for low frequency oscillator. An LFO is simply an oscillator just like the ones
found in the sound generator section of the synthesizer. The difference here is that the LFO is not
part of the audio path of the synthesizer. In other words, you can’t hear the frequency generated
by the LFO. Even if the LFO was put into the audio path, it oscillates at frequencies well below
the range of human hearing so it isnt' heard anyway. A LFO oscillates anywhere from 10 Hz
down to a fraction of a Hertz.
LFO’s are used like envelopes to modulate parameters of the synthesizer over time.
Typically you can choose from several different waveforms. For example, you can use an LFO
with a sinusoidal shape to change the pitch of the oscillator over time, creating a vibrato effect.
As the wave moves up and down, the pitch of the oscillator follows. You can also use an LFO to
control the sound amplitude over time to create a pulsing effect.
Figure 6.25 shows the LFO controls on a synthesizer. The Waveform button toggles the
LFO between one of six different waveforms. The Dest button toggles through a list of
destination parameters for the LFO. Currently, the LFO is set to create a triangle wave and apply
it to the pitch of Oscillators 1 and 2. The Rate knob defines the frequency of the LFO and the
Amount knob defines the amplitude of the wave or the amount of modulation that is applied. A
higher amount creates a more dramatic change to the destination parameter. When the Sync
button is engaged, the LFO frequency is synchronized to the incoming tempo for your song
based on a division defined by the Rate knob such as a quarter note or a half note.