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Analog Synthesis Modules

Module by: Ed Doering. E-mail the authorEdited By: Erik Luther, Sam Shearman

Summary: Learn about analog synthesizer modules, the foundation for synthesizers based on analog electronics technology. While analog synthesis has largely been replaced by digital techniques, the concepts associated with analog modular synthesis (oscillators, amplifiers, envelope generators, and patches) still form the basis for many digital synthesis algorithms.

Introduction

Analog synthesizers dominated music synthesis technology throughout all but the last 15 years of the 20th century. Early synthesizers were based on vacuum tubes or other electro-mechanical devices, and transistor technology entered the music scene during the early 1960s. Analog synthesizers produce sound waveforms as continuous voltages. Oscillators produce basic waveforms such as sinusoids, square waves, and triangle waves, much like a function generator in the electronics laboratory. These waveforms are shaped by time-varying amplifiers to emulate the characteristics of physical instruments, e.g., loud at the beginning transient of a note, softer during the sustained portion of the note.

You probably know that synthesizers defined many of the pop music styles of the 1970s. Watch (and listen!) to the screencast video in Figure 1 to learn more about some of the common synthesizer techniques. A real-time graphical signal analyzer is used to visualize the sounds so that you can better understand what you hear.

Figure 1: [video] Examples of analog synthesizer sounds, including visualization of waveform and frequency spectra
Figure 1 (asyn_modules-synthsounds.html)

The history of electronic synthesizers is really fascinating. In particular, the following sites form an excellent starting point:

Analog Synthesizer Modules

Everything about an analog synthesizer is analog! For example, a keyboard-based synthesizer uses a control voltage (CV) to change the frequency of the oscillator; the oscillator is therefore called a voltage-controlled oscillator or VCO. The time-varying gain of an amplifier is also controlled by a CV, so the amplifier is called a voltage-controlled amplifier or VCA. The VCO and VCA are two of many types of synthesizer modules that can be interconnected (or patched together) in many different ways.

Take a look at the video in Figure 2 to find out why interconnected modules are called patches, and to learn how to put together a simple patch involving a VCO, VCA, envelope generator, and keyboard controller.

Figure 2: [video] Origins of the term "patch", and simple example of an analog synthesizer patch using a VCO, VCA, envelope generator, and keyboard
Figure 2 (asyn_modules-patches.html)

Analog synthesizer modules can be grouped into four categories: sources, processors, envelope generators, and controllers; each of these is discussed in detail in the following sections.

Sources

Signal sources include the VCO and the noise generator. View the video in Figure 3 to learn more, then quiz yourself to check your understanding.

Figure 3: [video] VCO and noise generator signal sources
Figure 3 (asyn_modules-sources.html)

Exercise 1

The amplitude of a VCO's waveform can be adjusted (true or false).

Solution

False

Exercise 2

How does a VCO interpret its control voltage to produce a desired frequency?

Solution

One octave per volt

Exercise 3

Which VCO has the richest harmonic content?

Solution

Square wave has highest amplitude harmonics, but contains odd harmonics only; triangle wave has the most harmonics (even and odd)

Processors

Signal processors include the (VCA) and the voltage-controlled filter (VCF). View the video in Figure 4 to learn more, then quiz yourself to check your understanding.

Figure 4: [video] VCA and VCF signal processors
Figure 4 (asyn_modules-processors.html)

Exercise 4

How does a VCA interpret its control voltage to produce a desired gain?

Solution

Zero volts mean zero gain, one volt mean unit gain

Exercise 5

What types of filter functions can be implemented by a VCF?

Solution

Lowpass, highpass, bandpass, etc.

Exercise 6

What VCF filter parameters can be adjusted by control voltages?

Solution

Corner (cutoff) frequency, bandwidth, resonance frequency

Envelope Generators

An envelope generator creates a CV to operate other voltage-controlled modules such as the VCA and VCF. View the video in Figure 5 to learn more about envelope generators, in particular why they are usually called an ADSR.

Figure 5: [video] Envelope generators, especially the ADSR-style envelope generator
Figure 5 (asyn_modules-envgens.html)

Exercise 7

What is the normal (un-triggered) output of an envelope generator?

Solution

Zero

Exercise 8

What does the acronym ADSR mean?

Solution

Attack - Decay - Sustain - Release

Exercise 9

Why is the exponential shape used for envelope generators?

Solution

Easy to produce with RC-networks; matches behavior of real instruments

Controllers

A controller creates a control voltage (CV) to operate other voltage-controlled modules such as the VCA and VCF. An interactive controller offers the musician direct and immediate control of the sound, such as a keyboard, knob, or slider. A programmed controller generates a control voltage in some pre-defined way, such as a low-frequency oscillator (LFO) to produce vibrato, and a sequencer to produce a repeating pattern of control voltages for the VCO. View the video in Figure 6 to learn more, and then quiz yourself to check your understanding.

Figure 6: [video] Controllers including keyboard, knobs and sliders, low-frequency oscillator (LFO), and sequencer
Figure 6 (asyn_modules-controllers.html)

Exercise 10

Which types of output voltages does a keyboard controller produce?

Solution

A control voltage to control the frequency of a VCO, and a gate voltage to control an envelope generator

Exercise 11

What is the portamento effect, and how is it produced?

Solution

A portamento is a continuous frequency transition from one note to the next; instead of producing a step-change in the control voltage connected to the VCO, the keyboard produces a continuously-varying voltage from the starting note to the ending note

Exercise 12

Which device would a keyboardist use to automatically play a repeating pattern of notes?

Solution

Sequencer

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