Making sounds with analogue electronics - Part 4: Voltage-controlled amplifiers

LFOs
LFOs are used to produce low-frequency CVs. They are in two forms: VCOs and special-purpose oscillators. VCO-based LFOs can have their frequency controlled with an external CV, whilst special-purpose oscillators cannot.

Unlike audio frequency VCOs, LFOs need to produce waveforms where the shape is normally more important than the harmonic content. So, in addition to the sine, square, pulse and sawtooth waveforms, additional shapes such as an inverted sawtooth are also provided. These might be used when the LFO is connected to a source such as a VCO and is controlling the pitch of the VCO. The basic sawtooth, or ramp-up waveform, would then produce a pitch that rose slowly and dropped quickly. The inverted shape, although still called a sawtooth, would now be a ramp-down waveform and would give a pitch that rose quickly and dropped slowly.

Two specialized LFO waveform outputs are often found on LFOs: (i) sample and hold and (ii) arbitrary.

Sample and hold is the name given to a random or repetitive sequence of CVs, that are produced by using the LFO to repeatedly take the value of another voltage source, and then keeping that value until the next time that it measures the value again (Figure 3.3.32). This process is called 'sampling' the value, and that value is then 'held' until the next sample is taken. The technique is thus called sample and hold.

FIGURE 3.3.32 Sample and hold circuits take regular 'samples' of a noise (or other waveform) and then maintain that level until the next sample is taken. The rate of the samples is normally controlled by an LFO. The output consists of a series of steady voltages with rapid transitions, but whose level is not predictable. If the noise source is replaced with a repetitive waveform, then the output levels depend on the timing relationships between the sample LFO and the waveform being sampled.

If the voltage source that is sampled is noise, then the sample values will be random in level. This produces a series of values which do not repeat and are not regular or predictable. The regular timing from the periodic sampling is the only known quantity. If another LFO or VCO is sampled, then one of two results is possible.

If the second LFO or VCO is not synchronized to the sampling LFO, then the output of the sample and hold will be a series of values which are partly random and partly repetitive – the exact pattern depends on the relative frequencies and the LFO/VCO waveform. If the sampling LFO and the second LFO/VCO are synchronized so that they are locked together with the LFO/VCO being a multiple or fraction of the sampling LFO, then the output pattern will repeat.

Sample and hold is often used to control the cut-off frequency of a resonant low-pass filter. This is an effective way of providing 'interest' and 'movement' in a sound when it is in the sustain segment of an ADSR envelope.

Unfortunately, the rhythmic random changing timbres, that this type of filter modulation produces, have become an overused cliché. But by reducing the amount of variation of cut-off frequency and using a slow LFO, or preferably a slow LFO triggered by key gates, it can be used as a way of making the timbre of successive notes slightly different.

Arbitrary waveforms are the ones, which are constructed from a series of simpler waveform segments (Figure 3.3.33). There are many variations possible:

• two or more levels (rather like a simple sequencer)
• two or more straight-line slopes (much like an envelope)
• two or more curves (exponential, linear, sine, power law, etc.).

Arbitrary waveform generators are also called function generators. They can be used to replace EGs, control panning and effects settings and even act as simple sequencers to produce a series of pitched notes.

FIGURE 3.3.33 Arbitrary waveshape generators extend the concept of the multi-segment EG by providing additional shapes for the transition from one break-point to the next.

LFO output waveforms are frequently available simultaneously, so that a sine wave can be used at the same time as a square waveform (Figure 3.3.34). The common outputs are as follows:

• sine
• triangle
• square
• sawtooth/ramp-up
• inverted sawtooth/ramp-down
• pulse
• inverted pulse (100% pulse width)
• sample and hold
• arbitrary.

FIGURE 3.3.34 LFO outputs are normally provided in a variety of shapes to give additional control possibilities; although in practice, the sine wave is almost always used for vibrato or tremolo, and the square wave is almost exclusively used for trills. The other shapes are often presented in normal and inverted forms, and are often used for special effects sounds.