Guide to Electronic Music Techniques (Part 4)
The Low Frequency Oscillator
Modulation is the most vital consideration when using the synthesiser as an electronic musical instrument. Why? Well, without getting too much into esoterics — 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 the 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 go hand in hand. This month we are looking at one of the major modulation sources 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, VCOs (or DCOs) — the generators of the audio tones which are the fundamental signal elements in the chain of voice production. The low frequency oscillator 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; we'll deal with modulating oscillators that operate above this frequency at a later date.
If we examine the LFO in its simplest block diagram format — Figure 1, it can be seen that it is a source signal, i.e. the LFO is not reliant on any other input such as control voltages or triggers; it produces a periodically varying output control voltage that is purely a function of the front panel LFO Rate (or Frequency) control. This isn't always the case, and in fact as we shall discover, there are many different types of LFO, which can be fed control data.
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 0.01Hz up to 25-30Hz. In some cases you will find that the LFO produces sine instead of triangle waveforms — this is in fact preferable, but manufacturers find 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 — take for example the LFO section from a Yamaha CS-70M, this offers considerably more than the illustrated LFO in Figure 1.
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 is 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 application. Figure 2 illustrates the effect of the LFO on the frequency. In 2a we have a triangle wave modulating the oscillator, and you will notice that the frequency of the VCO has been shifted so that it oscillates around the steady state pitch. Consider an acoustic instrument such as a violin. What is happening when the violinist applies vibrato? He is rocking his fingers on the strings to effectively lengthen and shorten the string by a small amount. Vibrato is generally applied so that the pitch deviation is less than plus or minus a semitone, and so as the modulation is, as with the violinist, applied around the initial frequency of the oscillator (string), the overall modulated sound output retains its original tuning.
However, as you can see it is a different matter when we consider trill. Trill is the oscillation between two steady frequencies. A trumpet player will introduce a trill to the passage by jumping between the root note and one a semitone or two higher. So, if you were to apply a square wave oscillation so that the frequencies moved either side of the initial steady state pitch, the tuning would go right out of the window. Thus a state of affairs as shown in Figure 2b is necessary, with the square wave oscillating between a zero volt level (which will have no effect on the pitch of the VCO) and a positive voltage, 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. I don't propose to discuss the theory of pulse width modulation here and now, as the subject has already been dealt with in some detail.
Sine or triangle wave modulation of the voltage controlled 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 voltage controlled amplifier usually employs either the sawtooth (sine) wave for tremolo effects or the ramp down waveform which can be used for simulated echo effects.
The rate, speed or frequency of the LFO is very important in determining the role the device will have. At oscillations of up to 0.5Hz the LFO is acting so as to provide a rhythm, and this generally rubs off on the rhythmic structure of the music — a 0.5Hz oscillation is, of course, equivalent to 120 beats per minute.
With a greater emphasis these days being put on electronic rhythm units and sequencers/arpeggiators, the LFO has taken on a whole new role — that of the master clock. At frequencies around 120 bpm the LFO is the ideal element to act as the reference frequency for syncing all these facilities. In effect, so that eighth and sixteenth notes can be accommodated the clock will run at a multiple of the 120 figure, nevertheless the LFO has taken on a whole new field of operation with such devices.
A similar concept that utilises the LFO as a rhythmic source is that of the sample and hold circuit, which essentially relies on clock pulses derived from the LFO being used to sample a random waveform (usually the noise generator). The result is still a random signal, but quantised so that each element lasts for an equal 'time packet' (see Figure 3).
It is at frequencies between 4 and 15Hz that the LFO is used for most of the 'expressive' musical modulations (FM in particular). These frequencies are best suited to transmitting the 'feel' to the music, whilst above this frequency, LFO oscillator modulation tends to set up audible sidebands — a subject which we shall discuss in more detail next month, as well as taking a look at more complex breeds of LFO.
Feature by Dave Crombie
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