Low Frequency Oscillators
Low Frequency Oscillators (LFOs)
Like the Envelope Generators, the LFO section on a synthesizer is a ‘modulator’. That is to
say, instead of forming a part of the sound
generating
process, it is used instead to modify
(modulate) other synthesizer sections. For example, altering the oscillator pitch or filter cutoff
frequency.
Most musical instruments produce sounds that vary not just in volume but also in pitch and
timbre. Sometimes this variation can be quite subtle, but still contributes greatly towards
shaping the final sound.
Where an Envelope is used to control a one-off modulation event which occurs during the
lifetime of a single note, LFOs modulate by using a cyclic repeating wave pattern. As discussed
earlier, oscillators produce a constant waveform which can take the shape of a repeating sine
wave, triangle wave etc. LFOs produce waveforms in a similar way, but at a frequency normally
too low to produce an audible pitched vibration that the human ear can perceive.
The waveforms generated by the LFOs may be fed to other parts of the synthesizer to create
the desired movements in the sound. A typical LFO will generate a variety of different control
waveforms. These ‘look’ exactly the same as those we have seen with the audio oscillators
except that, because their function is to modulate or control pitch, tone and amplitude, the
effect they have is very different. The usual waveforms are:
WAVEFORM
WAVESHAPE
Triangle wave
The triangle wave will cause pitch to gradually rise and fall in accordance with its shape. The
triangle wave is commonly used at a fairly fast speed (around 7Hz or a value of 75 on the
V-Stations LFO SPEED control) to create vibrato effects.
When applied to the filter cutoff frequency, it will cause the filter to gradually open (get brighter)
and close (get softer). When the LFO speed is set quite fast, it can be used to emulate the
‘tremolo’ effect of instruments such as flute. With extreme settings, it can create distinctive
‘bubbly’ effects.
Square wave
The square wave will cause the oscillators to jump abruptly from one pitch to another. This can
be useful for creating ‘trills’ and other musical effects. When applied to the filter, it will cause the
cutoff frequency to jump from one value to another, alternating bright - soft - bright - soft and so
on.
Sawtooth wave
The sawtooth wave will cause pitch to rise gradually and then drop suddenly. When applied to
the filter, the filter will open gradually then close suddenly. It is often possible to ‘invert’ this
waveshape (i.e. turn it upside down) so that pitch falls gradually then rises abruptly and the filter
closes gradually and opens abruptly. Generally speaking, the sawtooth wave (and its inverted
variation) is used for special effects.
18
Synthesis Tutorial
