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Quick Guide To Modulation

Modulation is a core element of almost every sound from vocals to guitar but it comes in a variety of guises. We examine the warps and whoops of musical life...

Modulation is the process of changing an aspect of a sound such as pitch or volume. The changes can be small and subtle, wide-ranging and extreme, regular or irregular, fast or slow, and under the modulation umbrella lurk a vast range of processes and effects. The modulation effect we're most familiar with is vibrato which is a gentle varying of the pitch but modulation can be applied to many sound parameters to create many effects.

In a synthesiser, modulation is typically applied via a LFO and the sine wave is the most common choice of waveform. This creates a regular, undulating modulation which, at low levels, is very pleasing.

The most musical vibrato, for example, occurs between 6-8Hz and the change in pitch should only be small, typically less than a quarter of a semitone. You can't stray too far away from these settings without the sounding becoming distinctly unmusical. If you increase the pitch range, the gentle oscillation becomes a siren. If you increase the modulation rate, the vibrato becomes a buzz.

Shake it a baby...

Tech terms LFO Low Frequency Oscillator, an oscillator designed to generate at a low frequencies and most commonly used to modulate other parameters. Phase cancellation When two signals of the same frequency are out of phase with each other (that is, their waveforms don't start at the same time), parts of the signal will be cut resulting in a drop in amplitude. Harmonic series The harmonics produced by a fundamental note that have a simple mathematical relationship to the fundamental frequency. If the fundamental was 110Hz (the A an octave below Middle C), the harmonics would be 220Hz, 330Hz, 440Hz, 550Hz and so on. Harmonics whose mathematical relationship to the fundamental is more complex - such as 121Hz, 309Hz and 499Hz - produce a more dissonant sound.

Another popular modulation is tremolo which is often confused with vibrato. Whereas vibrato is applied to the pitch of a sound, tremolo is applied to its volume or amplitude to create small crescendos and diminuendos. Again, small and subtle is musical, large and wide is not. Tremolo is particularly effective with metallic instrument sounds such as vibes and bells.

It's worth pointing out that some instrument manufacturers confuse the two. The prime example is the tremolo arm on a guitar which stretches the strings creating a type of vibrato, not tremolo. However, other than the technical difference between the two effects, they can sound similar.

The Modulation Wheel on synthesisers and keyboards is used to apply - yes, you guessed  - modulation to a sound. On most instruments the modulation is set to create vibrato by default and the more you move the wheel the greater the vibrato effect. However, particularly on synthesisers, you may be able to assign the Wheel to one of many parameters to create other effects. In such cases, the modulation may not be acting directly on pitch or volume but on a synth parameter such as the filter's cutoff frequency to create tonal changes.

Vibrato can also be applied directly to some non-electronic instruments, primarily string instruments and the voice. Most Western vocalists, consciously or unconsciously, add a little vibrato to their voice to make it warmer and more appealing.

That's not necessarily the case in other parts of the world. Listen to African singers, particularly tribal chants, and you'll notice a marked lack of vibrato which makes the result quite distinctive. Karl Jenkins uses this to good effect in his Adiemus recordings which feature the voice of Miriam Stockley. But even if vocalists in a choir do not use vibrato, the result is still a warm sound - a chorus effect - which is still a modulation but created in a slightly different way.

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