7. Introduction to Synthesis

Basic Tutorial In Sound Design

To create your own Synth Sounds, you will need a basic understanding of Sound Design or Synthesis.

There are a variety of types of Synthesis, including…

• Subtractive
• Granular
• Spectral
• Ring Modulation
• Amplitude Modulation
• Frequency Modulation
• Re-Synthesis
• Wavetable
• Physical Modelling

A detailed description of each form of Synthesis is outside the scope of the book – For further reading, Google it.  The fundamental areas of Synthesis are explained below.

In it’s simplest form, Additive Synthesis is the addition of sine waves at various frequencies to create more complex sounds.  In theory, any sound can be reproduced using up to an infinite number of sine waves.  Square Saves, Saw Waves and other more complex waves are generated by adding Sine waves of different harmonic frequencies together.  A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. For example, if the fundamental frequency is f, the harmonics have frequencies f, 2f, 3f, 4f, etc.

Fig 1: One sine wave at 440 Hz.

Fig 2: Three addition sine waves are added at double 880 Hz, treble 1,320 Hz and Quadruple 1,760 Hz the original frequency.

Fig 3: More harmonic frequencies of the Sine Wave are added to create a Square Wave.

Introduction to Subtractive Synthesis

The most common form of Synthesis in most VSTs is Subtractive Syntheses.  Filters are used to subtract frequencies from different types of waves, e.g. Sine, Saw Tooth, Square to create a variety of sounds.  It is a 4 stage process to create an infinite array of any sound imaginable

Subtractive Synthesis = 1) VCO + 2) ADSR + 3) Filter + 4) DSP

1) VCO = Voltage Controlled Oscillator e.g. Sine, Saw or Square wave.

 Sine Saw Square

2) ADSR = Attack Sustain Delay and Release.  Attack is the time for the instrument to reach maximum amplitude, e.g. a percussive sound has a short Attack and a Pad has a longer attack.

3) Cut off Filter = A filter removes unwanted frequencies, for example a Low Cut filter removes low frequencies at a specific frequency, e.g. 410 Hz in the below diagram.

4) DSP = Digital Signal Processing.  The final stage includes adding filters that manipulate the sound, e.g. distortion, modulation, etc.  Chapter 13 covers DSP in more detail.

LFO Low Frequency Operator

A Sine Wave VCO (Voltage Controlled Oscillator) that generates a tone of A4, or “Concert A” oscillates 440 times per second or 440 Hz.  The human range of hearing is 20 Hz to 20,000 Hz (20 KHz).  A signal that oscillates below 20 Hz is called an LFO.  LFOs are used in Synthesis to apply subtle variation to sounds via modulation.  An LFO with the value of 2 oscillates 2 times per second.  An LFO with the value of 0.5 oscillates once every 2 seconds.

Fig 4: An LFO with 1 Oscillation.

Modulation

All variables in Synthesis such as pitch, cut off frequency, LFO etc are represented by numerical values.  Modulation is changing the value of one element by the value of another element.

Example

-Sine Wave at 440 Hz: Tone = x

-Low Frequency Oscillator of 0.5, or 1 oscillation every 2 seconds. Value = y

If the value of the Tone x is modulated by the value of the oscillator y, then the pitch will rise and fall like a police siren as per the diagram below.

Fig 4: The LFO pictures has a value of 0.5, 1 oscillation every 2 seconds.

Another example is to set the Mod Wheel on your keyboard to control the pitch.  The value of the pitch can be adjusted at a specific parameter to create a drastic (+12 semi tones) or subtle (+ 2 semi tones) modulation.

MOD wheel > +2 semi tones > Pitch

MOD wheel > +12 semi tones > Pitch

Modulating Expression

With analogue instruments, the timbre of the sound varies according to how soft or hard you play.  These subtle nuances can be programmed into synths using velocity modulation.  If velocity was the source and filter Cut Off was the destination, you could set it so that the harder you hit a key, the more a filter would open.