Introduction To Sound Synthesis
Introduction
Sound synthesis can be defined as the process of creating a sound using electronic hardware or software while having independent control over various aspects of the sound.
The most common tools used in modern day sound synthesis are electronic keyboard instruments aptly called “synthesisers”. These synthesisers have keyboards much like a piano. However, instead of acoustic sound production, synthesisers produce and alter sound electronically (with electricity) via various components and modules.
Although many modern synthesisers make use of hardwired components and modules in keyboards to create sound, the first true synthesisers were modular, meaning they didn't rely on the use of keyboards, but instead used various components and modules that could be linked together by cables in whatever sequence the user wanted. These tones could then be altered by turning knobs and pushing faders on the modules that were linked together, creating richer and more complex tones and sequences. The components in keyboard synthesisers on the other hand are usually hardwired in a way that the designing engineer deems appropriate or desirable. Therefore modular synth units like the “Euro Rack” or “Moog Modular” are less user-friendly but allow for more user control and unique creativity, while hardwired keyboard synthesisers like the original “Mini-Moog” allow for ease of use by musicians, who don’t necessarily have the time to learn the ins and outs of modular synthesis.
It should be noted that there are various different types of sound synthesis including but not limited to: additive, wavetable, subtractive, grain and of course modular synthesis. Each one of these forms of synthesis work from a slightly different “philosophy” in the way they create and alter the tones being generated. However, for the most part the components used serve the same basic functions, regardless of the type of synthesis being used.
In today’s music world subtractive synthesis is the most common form of synthesis. Subtractive synthesis works by attenuating (reducing) signals or tones using filters that subtract from the original sound to create a whole new set of harmonics in the signal. A good analogy is that subtractive synthesis is like carving a stone sculpture: the artist starts with a whole piece of stone and chips away until he/she is left with the image they had in their head.
The aim of this article is to de-mystify all those knobs and faders that make up a typical keyboard synthesiser, thereby giving a beginners guide to sound synthesis.
How Synthesisers Work
A synthesiser generates a tone or tones and combines them to create a new sound. In analogue synthesisers these tones are actual electrical signals and the tones are generated via components called voltage controlled oscillators (VCO) or simply oscillators for short. Oscillators are usually capable of generating one of four basic waveforms: sine wave, triangle wave, square tooth wave or sawtooth wave.
These basic waveforms have their own distinct sound characteristics and tones, represented by different wave shapes. Individual waveforms can be combined to create more complex and interesting sounds, while other modules can be used to further alter the tone of the selected waveform, be it in an emulation capacity or as unique and original synth sounds.
The name, shape and characteristics of the four basic waveforms are outlined below:
Sine Wave
Square Wave
Square waves have a distinct buzzy and hollow sound, this hollow sound is due to the fact that square waves lack even harmonics; instead they only contain odd harmonics of the fundamental frequency. Old Nintendo video games were made almost exclusively using square wave tones. Electronic music producers often use them as higher register basses and leads.
Triangle Wave
Triangle waves sound like something between a square wave and a sine wave, like square waves they too only contain odd harmonics of the fundamental frequency. However, triangle waves differ from square waves because the volume of each added harmonic drops more quickly. They also have less of a buzzy tone than a square wave. Common uses for triangle waves are pitched bass drums and synth lead reinforcement.
Sawtooth Waves
Sawtooth waves have a very strong, rich, clear and buzzy tone. This is because the sawtooth wave contains every integer harmonic of the fundamental frequency. For example, a sawtooth with a fundamental frequency of 100Hz would have harmonics of 200Hz, 300Hz, 400Hz and so on to infinity, with each progressive harmonic diminishing in volume, thereby resulting in its rich buzzy tone. A sawtooth wave tone could be compared to that of traditional brass or violin instruments, especially in the higher registers. Sawtooth waves are often used for big hard hitting synth leads in electro music. French electronic music producers like Justice are a good example of this.
Basic Synth Components (modules)
In addition to oscillators, synthesisers make use of various other components and modules to alter the tones being generated by the oscillators, some of these modules include filters, envelopes and LFOs. Understanding how these different modules work will help in emulating and creating traditional instrument sounds such as brass, piano and drums, as well as in creating new and unique sounds envisioned by the user.
Synthesisers have come to play a major role in modern pop and film music for their ability to create “out of this world” sounds as well as their ability to emulate more familiar traditional instrument sounds. Many modern film scores like the TV series “Stranger Things” and motion pictures like “It Follows” make use of synthesisers almost entirely, creating a particular atmosphere relevant to that particular genre of film in a way only synth sounds can. In addition to this many electronic synth driven music genres have risen in popularity in recent years and this is only likely to continue to grow. Therefore, having some level of understanding of how synthesisers work is a useful tool for any modern day musician or sound engineer.
Oscillators
As already stated, this is what generates a signal that we hear as sound. An oscillator generates a consistent repeating signal. Without an oscillator generating a signal, there will be no sound.
As we already know, there are four basic wave signals or waveforms that are used to generate sound. Each waveform has its own distinct timbre and associated harmonics.
Some synthesisers only have one oscillator that can generate one of the selected waveforms at a time, while other synthesisers use multiple oscillators to generate different waveforms that can then be mixed together to create more complex and “richer” sounds.
Figure 1 shows an image of a software synth oscillator. Notice the dial with changeable waveforms. In the picture the oscillator is set to produce a sawtooth tone.
Fig.1: Image of Logic Pros ES2 Synth Oscillator 1.
Frequency and Pitch
Many synthesisers make it possible to change the pitch in half steps or semitones, this is usually done with a dial or fader. Some synthesisers give more control over pitch by making it possible to micro tune, with a changeable “cc” or “cents” value (100 cents = 1 semitone). This is especially useful on synths with more than one oscillator or if creating an Instrument tuned to a non-Western scale.
Figure 2 shows an image of a software synth pitch knob. Notice the pitch is set to -12 semitones and 0 cents.
Fig.2: Image of the pitch control feature in Logic Pros ES2. Notice the semitone and cent parameters.
Labeled with an S and C, respectively
*Tip: Using different pitch settings on separate oscillators can help in thickening the overall synth sound.
Volume Control
To control the signal being generated by an oscillator, synthesisers make use of a module called a volume controlled amplifier or VCA. Without a VCA the synth will continuously produce sound, even when nothing is being played on the keyboard.
Therefore a VCA is basically a volume control that increases volume when a key is pressed and switches off the volume when nothing is being played. In most software synths the VCA is not visible. This kind of switch is sometimes called a “gate”.
Filters
Filters are tools used for manipulating signals. Filters allow you to select a range of frequencies (low, mid or high) and either amplify or reduce (attenuate) those frequencies. Different frequency filters target a different range of frequencies. This is done with a cutoff. The position of the cutoff dial will determine which frequencies are “blocked” and which frequencies are allowed to “pass” through.
These are the four basic frequency filters you should know about:
Low Pass Filter: Allows low frequencies to pass through, while blocking or “cutting” out higher frequencies. Sometimes called a “high cut filter”
High Pass Filter: A high pass filter does the exact opposite of a low pass filter and allows high frequencies to pass through, while blocking or “cutting” lower frequencies. Sometimes called a “low cut filter”
Band Pass Filter: This type of filter combines a low pass and a high pass filter to isolate groups of frequencies between the two high and low cutoff points. Therefore it retains the middle frequencies between the high and low frequencies.
Band Reject: A band reject filter has the opposite function of a band pass filter. Band reject filters cut out the middle frequencies while letting the low and high frequencies through.
Some synth filters will also allow you to select at what “steepness” level the filter cutoff is set. This is called the filter “slope” and it allows the user to set a more subtle or more blatant filter cut. The slope cuts decibels per octave, so for example a filter slope of 6dB is more subtle than a slope of 12dB or 24dB.
Some synthesisers go a step further by adding an extra filter. This simply gives more control over the sound being processed. These dual filters will usually give the option to “blend” and process the signal in “parallel” or in “series”. This is especially common with software synthesisers such as Logic Pros ES2.
Figure 3 shows an image of a software synth filter. This particular synthesiser has dual filters, set to a low cut with a 12dB slope, a 50/50 blend and is connected in series.
Fig.3: Image of Logic Pros Dual ES2 Filters Connected in Series
Envelope Generators
An envelope generator is used to control signals within a synthesiser. Unlike an oscillator, which repeats the signals generated, an envelope generator sends out its signal only once. An envelope generator can be used to control other modules in a synthesiser to create even more interesting sounds.
ADSR envelopes, which stand for attack, decay, sustain and release, are the most commonly used filter envelopes and are usually used to control the volume via the VCA within a synthesiser. However, they can be routed to control almost any other module within a synthesiser, such as pitch or cutoff.
ADSR envelopes are usually controlled using faders, whose positions correspond to time in milliseconds (ms) going from a low value to high value.
Here is an outline of what function each specific ADSR value serves, in order over time:
1st, Attack: Is how long it takes for a sound to reach its maximum volume once a key is pressed.
2nd, Decay: Is the time taken for the sound to run down, from the attack level to the sustain level.
3rd, Sustain: Is the level during the sound's duration after the decay, until the key is released. This is basically the sound’s plateau.
4th, Release: Is the time taken for the level to decay, from the sustain level to zero after the key is released.
Figure 4 shows image of a software synth’s ADSR envelope. Some synths have more than one ADSR. This particular envelope is the third one in the synth and is hardwired to control volume (VCA).
Fig. 4: Image of Logic Pros ES2 Envelope 3.
This envelope is hardwired to control volume (VCA).
ADSR envelopes can be especially useful in helping emulate the differences we hear in the sounds of different groups of instruments. For example a wind instrument might have a longer attack time, while a percussive instrument would have a considerably shorter attack time, while a string instrument might have a short attack but long release time.
Figure 5 gives a graphical representation of how ADSR envelopes alter sound over time:
Fig.5: Graphical representation of attack, decay, sustain and release over time.
Low Frequency Oscillator
An LFO or low frequency oscillator is like a normal oscillator. It can have any waveform and amplitude specified, but it has a very low frequency that oscillates very slowly. An LFO can be used to control other modules within a synthesiser. For example, an LFO can be used to move the volume level of a VCA up and down thereby creating a tremolo effect.
LFO use is common in electronic music genres like drum & bass and dubstep, giving both genres their distinctive wobbly basses and leads.
An LFO can’t actually be heard on its own; what you are actually hearing instead when an LFO is used is the effect it has on the target or source sound.
Figure 6 shows an image of a software synth LFO. Some synthesisers have more than one LFO. Notice the fader knob labeled “rate”. This changes the speed at which the LFO oscillates.
Fig. 6: Image of Logic Pros ES2 LFO 1
Conclusion
While it’s true that many synthesisers have other modules or components the ones discussed here are the most commonly found components in synthesisers. This information should give you a beginner’s understanding of sound synthesis and perhaps de-mystify what all those curious knobs and dials do.
Synthesis can be complicated and very technical. However, it involves a few basic components that once understood make it much less dubious. If sound synthesis an area that you're interested in the only way to understand it in depth is to read, research, experiment and practice as much as possible.
A good place to start is with software synths such as Logic Pros ES2 or any other DAW synth you might be able to get your hands on. Ideally we would all learn synthesis with analogue hardware synths. However, getting your hands on one is usually much more expensive and not as easy.
Hopefully this article has been helpful in “de-mystifying” some of the fundamental modules found in synthesisers and how they work. Perhaps a later article will give a step by step instructional explaining how to go about actually building a specific synth sound.
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