2. Voice Model in Subtractive Synthesis

The other day in the studio on a recording, I met with a choral vocalist and musician. During the break, he became interested in the topic of modular synthesizers and synthesis, and I briefly tried to explain some of the basics in a short time and realized that this is not easy to do in a couple of minutes. So there was a desire to write an some articles about this.


In the article, we will deal with the question: what is electronic sound and how to create sound from “nothing”. Consider the traditional model for constructing a monophonic voice in subtractive synthesis. Examples and explanations will be given in the field of a modular synthesizer, because the theory of sound synthesis for analog and virtual-analog synthesizers is the same. Perhaps it sounds scary, but it’s called that and there is nothing terrible here, let’s sort it out.




In the classical version, the electronic sound in subtractive synthesis is based on three types of devices: an oscillator, a filter, and an amplifier. The idea is to first get the oscillations with the necessary characteristics (oscillator), then adjust the spectrum (filter) and amplitude (amplifier). Each device in a modular synthesizer is an accurate and streamlined circuit with a mass of dynamically changing parameters. Manufacturers are trying to make devices flexible and versatile and integrate a voltage control circuits (CV) into the basic and additional parameters, which differ from the manufacturer to manufacturer of synth hardware. To confidently own the synthesizer, it is important to understand which of the devices form the basis and directly influence to sound creating. It is also important to understand which of the parameters of these devices determine the basic controls, and which are optional.


When there are many instruments, it is not easy to keep in mind what is happening. This is impossible. And during a live performance it is important to feel the equipment and to determine by ear what is happening and, if necessary, to make changes quickly.


So, let’s figure out how the synthesizer core works and form a complete picture of the voice model. To begin, consider each of the devices individually.


Oscillator (VCO)


Sound is a wave, pressure fluctuations. In electronic circuits, this is voltage fluctuation. An oscillator is just that device that creates oscillations or generates a wave, which, as a rule, oscillates in the range of -5V / + 5V, that is, with an amplitude of 10 volts, which is standard for a sound wave.


The analog oscillator has four basic types of waves: sinusoidal, triangular, sawtooth and rectangular. Each wave has its own sound: the sine wave and triangle sound softly, but the saw and the rectangle are more sharp.




A square wave is characterized by a pulse width parameter (PW or Pulse Width). This parameter determines the time of the duty cycle of the pulse signal or the time ratio of the positive voltage to the negative voltage in one wave cycle.


The main parameter of the oscillator is the oscillation frequency. That is, controlling the oscillator, control occurs primarily in the frequency response and only then in the rest, such as pulse width (PW) or SYNC. The second is needed to reset the oscillation cycle, which is useful when you need to synchronously connect two oscillators with each other. This approach is used to obtain musical intervals and for the harmonious sound of two sources of waves that mix into complex sounds. This technique is often used in club and popular electronic music.




It is important to remember: the oscillator determines the pitch in the voice-core of the sinthesizer.


Filter (VCF)


A filter is the soul of a synthesizer. The filter determines the nature of the sound and “artifacts” in the sound, which make the record unlike the others, recognizable and unique. This complex device affects the spectral characteristics of sound, that is, it determines which frequencies to skip and which to exclude from the audio signal. There are a lot of filter schemes and patents, but some have seriously taken root and are considered classics, such as the Transistor Ladder Filter, patented by Robert Mug in the 70s and making the name for the Moog synthesizer brand by the fatest sound.


The first main parameter of the filter is the cutoff frequency at which the excess sound is screened. The second main parameter is resonance or saturation. The fact is that it is possible to adjust the filter so that self-excitation occurs at the cutoff frequency in the circuit, thereby emphasizing and amplifying the selected frequency. It is precisely in this circuit that artifacts and features are hidden that uniquely identify each individual filter. Here «Cosmos» is located and the lion’s share of the magic of analog synthesis is enclosed. To understand how this works, first look at the types of filters. In the meantime, just remember the main parameters – cutoff frequency and resonance.




There are four types of filters in the classical theory of synthesis:

  • A low-pass filter (LPF) excludes frequencies above the cut-off frequency from the signal, that is, leaves low frequencies. It is used more often than other types. Applies to bass lines and leading instruments.
  • The high-pass filter (HPF) eliminates the low frequencies and leaves (filters) only the high frequencies.
  • The bandpass filter (BPF) is a hybrid of the first two. It leaves only a band in the sound spectrum, cutting off the low and high frequencies.
  • A Notch Filter is the same hybrid of LPF and HPF as a band filter, but works on the principle of cutting out the frequency band from the sound spectrum.




So, the magic and Cosmos of the resonance of the filter is the appearance of a natural peak at the cutoff frequency border, the rise of which is manually controlled, and in some models dynamically using the control voltage. Based on the schematic features, each type of filter is characterized by its own unique sound. In addition, the components used in the circuit also affect the sound – the same circuit is very different in sound, if, for example, you change the transistors in the heart of the filter to vintage ones or another manufacturer or class. This is an analog world and magic happens inside, in the heart of PN transitions. There, electrons move according to strict laws, but random motions are not excluded, and non-linear distortions and electromagnetic fields create that unique natural analog charm that human hearing is so fond of.


Amplifier (VCA)


With an amplifier, things are much simpler than with a filter. The amplifier works with the amplitude of the signal, that is, with volume in the case of sound. And the main parameter here will be the gain. If you look at the graph of voltage versus time, that is, how the wave changes with time, the principle of the amplifier visually looks like stretching or compressing the signal vertically, which means a change in signal strength. If we return to the beginning in the text, then changing, for example, the main parameter of the oscillator will look like stretching or compressing the signal horizontally. An amplifier, as a device, is easy to understand, but even here the magic of PN transitions in the circuit elements determines the nature of the sound.




About modulation


The oscillator (VCO), filter (VCF), and amplifier (VCA) are the three whales on which subtractive synthesis is based. Consistently combining these three devices, we get a complete controllable voice model with character and inherent features, ready to create electronic masterpieces live and unfold mysterious worlds in people’s imaginations.


Only one thing left! For this living organism to begin to sound, control is required in synthesis model. Even if you take the minimum parameters that are discussed in this article, it becomes clear that this organism cannot be controlled with two hands. This is where modulation comes to the rescue.


Each main parameter of each of the devices in the rack is controlled by external voltage, which makes the modular synthesizer more versatile than the classic one, which determines such flexibility and convenience in sound surgery. That is, there is no need to constantly twist the handles with your hands. For this, special devices have been created that control the main parameters of the basic elements of the voice. This control is called modulation. And the musician or sound engineer is left with a tidbit – the creative part.


The abbreviations VCO (Voltage Controlled Oscillator), VCF (Voltage Controlled Filter) and VCA (Voltage Controlled Amplifier) ​​just mean that the devices are controlled by voltage.


As a rule, the created classical voice models are tied up with control devices that dynamically control the characteristics when playing, creating melodies or bass lines. These are devices such as ADSR, sequencer, LFO and the like. These devices are not directly related to the sound, but strongly determine the final sound. About control devices you can read in the following articles…

Read the next article 3. Control Voltage, Triggers and Gates >>>

Plutonic Sound

"Plutonic Sound" is a Russian musician, sound engineer and founder of ILOVETECHNO.FM radio. He began to study music in early childhood, and electronic music in about 2005-2006. Currently actively engaged in the popularization of Techno music and also developments in the field of audio electronics, processing, and synthesis of sound under the "o3audio".

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