The LFO gives a control voltage in the shape of a waveform. It is basically an oscillator which generates a cyclic waveform at low frequencies below the audio range.
The LFO can produce different wave-shapes used to modulate various other modules. Common uses of LFO modulation are to obtain vibrator (by modulating the pitch of a VCO), wah-wah (by modulating the filter cut-off frequency), tremolo (by modulating the gain of a VCA).
Some LFOs can have there frequency voltage controlled like with a VCO. Some have a reset input. When a gate or trigger signal is received at the reset input, the LFO waveform will be reset to zero, re-starting its wave cycle. Rarely found on LFO modules is a Delay time setting in conjunction with a Delay trigger input. The Delay adjusts the rate it takes the LFO signal to go from zero to full intensity, initiated by a gate or trigger signal at the Delay trigger input.
Modulation is the most vital consideration when using the synthesiser as an electronic musical instrument. Why? Sound is not music, merely a carrier of information. It is the change in sound that our ears perceive as music; our brains, via our ears, are good at detecting and assessing these changes and thus sound is merely the carrier of musical concepts.
Modulation is the business of making changes to the constituent elements of sound, so really if we are to accept the above paragraph music and modulation sources go hand in hand. Below we will go into detail of the major modulation source in an electronic music synthesiser – the low frequency oscillator.
Low frequency oscillations are defined as periodic vibrations at frequencies below the audio threshold. You will all be aware of the fundamental ideas behind oscillators and voltage control. The LFO cannot act as a sound source, it is a modifying element that is used to process and control other elements of the sound. The ‘low’ in LFO defines the oscillator as operating at frequencies below the audio threshold, which for most of us is around 30Hz.
If we examine the LFO in its simplest block diagram format, below, it can be seen that it is a source signal, i.e. the LFO is not reliant on any other input such as CVs or triggers; it produces a periodically varying output CV that is purely a function of the front panel LFO rate control. This isn’t always the case, though, as there are some more advanced LFOs that can be controlled themselves.
But to continue with our simple LFO, it is generally the case that it will provide triangle and square wave output signals at rates between 0.01 up to about 30Hz. In some cases you will find that the LFO produces sine instead of triangle waveforms – this is in fact preferable, but it was the case that manufacturers found it more expensive to incorporate sine wave generators into their instruments and consequently we often have to make do with the sharp edges of the triangle wave. This isn’t too horrific a proposition,as one has to listen quite closely to the modulated signal to detect its nature, however of course, in the audio spectrum the difference brought about by the higher harmonic content is considerably more noticeable.
In the more elaborate LFOs a wide range of additional waveform outputs and features are provided – ramp and pulse in particular.
The Role of the LFO
The most common use for the LFO is pitch or frequency modulation. Sine or triangle wave modulation of the VCOs is known as vibrato, square wave modulation as trill. Generally frequency modulation applied via the modulation performance controls (wheels, ribbons, joysticks or whatever), however it is possible to permanently route the LFO to the VCO if it is so desired, though the effect is generally rather monotonous for musical applications.
Vibrato is generally applied so that the pitch deviation is less than plus or minus a semitone, applied around the initial frequency of the oscillator, the overall modulated sound output retains its original tuning.
However trill is the oscillation between two steady frequencies. The square wave oscillates between a zero volt level (which has no effect on the pitch of the VCO) and a positive value, which determines the upper note to which the trill is set.
The other use of the LFO in conjunction with the VCO is to provide a control signal with which to modulate the pulse width.
Sine or triangle modulation of the filter goes by various names, the most common and unlikely being that of ‘growl’, though sometimes the terms ‘wow’ or ‘wow-wow’ appear. LFO modulation of the filter is less commonly utilised, as musically it is not as important as vibrato or trill; however, it is often useful when routing the LFO to the oscillators, to parallel the signal to the filters in order to enhance the effect.
LFO modulation of the VCA usually employs either sine or triangle wave for tremolo effects or the ramp down waveform which can be used for simulated echo effects.