Audio

If we want to play some music, sound has to be generated somehow, right? Then the first family of modules that we are going to tackle is that of the sound sources: oscillators, noise sources, and samplers, mainly. Oscillators are those modules that generate a pitched tone. Their frequency content varies depending on the waveform that is generated. Historically, only simple waveforms were generated, according to the electronic knowledge available. Typical waveforms are thus triangular, rectangular, sawtooth, and sinusoidal. These are all very simple waveforms that can be obtained by a few discrete components. From simple designs come simple spectra: their shape is very straight and unnatural, thus requiring additional processing to obtain pleasant sounds. Their spectral properties are discussed in Section 2.8, after the basic concepts related to frequency-domain analysis have been discussed (Figure 1.5). 8 Modular Synthesis Figure 1.4: Connection between two modules. The output of Module 1 is connected to the input of Module 2 by a TS jack. This way, the input voltage of Module 2 follows the output voltage of Module 1, and thus the signal is conveyed from Module 1 to Module 2. Figure 1.5: Typical synthesizer oscillator waveforms include (from left to right) sawtooth, triangular, and rectangular shapes. Oscillators usually have at least one controllable parameter: the pitch (i.e. the fundamental frequency they emit). Oscillators also offer control over some spectral properties. For example, rectangular waveform oscillators may allow pulse width modulation (PWM) (i.e. changing the duty cycle Δ, discussed later). Another important feature of oscillators is the synchronization to another signal. Synchronization to an external input (a master oscillator) is available on many oscillator designs. So- called hard sync allows an external rising edge to reset the waveform of the slave oscillator and is a very popular effect to apply to oscillators. The reset implies a sudden transient in the waveform that alters the spectrum, introducing high-frequency content. Other effects known as weak sync and soft sync have different implementations. Generally, with soft sync, the oscillator reverses direction at the rising edge of the external signal. Finally, weak sync is similar to hard sync, but the reset is applied only if the waveform is close to the beginning or ending of its natural cycle. It must be noted, however, that there is no consensus on the use of the last two terms, and different synthesizers have different behaviors. All these synchronization effects require a different period between slave and master. More complex oscillators have other ways to alter the spectrum of a simple waveform (e.g. by using waveshaping). Since there are specific modules that perform waveshaping, we shall discuss them later. Oscillators may allow frequency modulation (i.e. roughly speaking, controlling the pitch with a high-frequency signal). Frequency modulation is the basis for FM synthesis techniques, and can be either linear or logarithmic (linear FM is the preferred one for timbre sculpting following the path traced by John Chowning and Yamaha DX7’s sound designers). To conclude, tone generation may be obtained from modules not originally conceived for this aim, such as an envelope generator (discussed later) triggered with extremely high frequency. Noise sources also belong to the tone generators family. These have no pitch, since noise is a broadband signal, but may allow the selection of the noise coloration (i.e. the slope of the spectral rolloff), something we shall discuss in the next chapter. Noise sources are very useful to create percussive sounds, to create drones, or to add character to pitched sounds. Finally, the recent introduction of digital modules allows for samplers to be housed in a Eurorack module. Samplers are usually capable of recording tones from an input or to recall recordings from a memory (e.g. an SD card) and trigger their playback. Other all-in-one modules are available that provide advanced tone generation techniques, such as modal synthesis, FM synthesis, formant synthesis, and so on. These are also based on digital architecture with powerful microcontrollers or digital signal processors (DSPs). 1.3.2 Timbre Modification and Spectral Processing As discussed, most tone generators produce very static sounds that need to be colored, altered, or emphasized. Timbre modification modules can be divided into at least four classes: filters, waveshapers, modulation effects, and vocoders. Filtering devices are well known to engineers and have played a major role in electrical and communication engineering since the inception of these two fields. They are devices that operate in the frequency domain to attenuate or boost certain frequency components. Common filters are the low-pass, band-pass, and high-pass type. Important filters in equalization applications are the peak, notch, and shelving filters. Musical filters are rarely discussed in engineering textbooks, since engineering requirements are different from musical requirements. Among these, we have a low implementation cost, predetermined spectral roll-off (e.g. 12 or 24 dB/oct), and the possibility to introduce a resonance at the cutoff frequency, eventually leading to self-sustained oscillation.4 10 Modular Synthesis While engineering textbooks consider filters as linear devices, most analog musical filters can be operated in a way that leads to nonlinear behavior, requiring specific knowledge to model them in the digital domain. Waveshaping devices have been extensively adopted by synthesizer developers such as Don Buchla and others in the West Coast tradition to create distinctive sound palettes. A waveshaper introduces new spectral components by distorting the waveform in the time domain. A common form of waveshaper is the foldback circuit, which wraps the signal over a desired threshold. Other processing circuits that are common with guitar players are distortion and clipping circuits. Waveshaping in the digital domain requires a lot of attention in order to reduce undesired artifacts (aliasing). Other effects used in modular synthesizers are so-called modulation effects, most of which are based on delay lines: chorus, phaser, flanger, echo and delay, reverb, etc. Effects can be of any sort and are not limited to spectral processing or coloration, so the list can go on. Vocoders have had a large impact in the history of electronic music and its contaminations. They also played a major role in the movie industry to shape robot voices. Several variations exist; however, the main idea behind it is to modify the spectrum of a first sound source with a second one that provides spectral information. An example is the use of a human voice to shape a synthesized tone, giving it a speech-like character. This configuration is very popular. Figure 1.6: A CRB Voco-Strings, exposed at the temporary Museum of the Italian Synthesizer in 2018, in Macerata, Italy. This keyboard was manufactured in 1979–1982. It was a string machine with vocoder and chorus, designed and produced not more than 3 km from where I wrote most of this book. Photo courtesy of Acusmatiq MATME. Owner: Riccardo Pietroni. Modular Synthesis 11 1.3.3 Envelope, Dynamics, Articulation Another notable family of effects includes all the envelope, dynamics, and articulation devices. Voltage-controlled amplifiers (VCAs) are meant to apply a time-varying gain to a signal in order to shape its amplitude in time and create a dynamic contour. They can be controlled by a high- frequency signal, introducing amplitude modulation (AM), but more often they are controlled by envelope generators (EGs). These are tools that respond to a trigger or gate signal to generate a voltage that rises and decays, determining the temporal evolution of a note or any other musical event. Usually, such evolution is described by four parameters: the attack, decay, and release times and the sustain level, producing an ADSR scheme, depicted in Figure 1.7. Most envelope generation schemes follow the so-called ADSR scheme, where a tone is divided into three phases, requiring four parameters: • A: The attack time. This parameter is expressed as a time parameter in [s] or [ms] or a percentage of a maximum attack time (i.e. 1–100). • D: The decay time. The time to reach a steady-state level (usually the sustain, or zero when no sustain is provided by the EG), also expressed as a time ([s], [ms]) or a percentage of a maximum decay time (1–100). • S: The sustain level. The steady-state level to be reached when the decay phase ends. This is usually expressed as a percentage of the peak level that is reached in the attack phase 1–100). • R: The release time. The time to reach zero after the musical event ends (e.g. note off event). This is also expressed in [s], [ms], or percentage of a maximum release time (1–100). Subsets of this scheme, such as AR, with no sustain phase, can still be obtained by ADSR. An EG generates an envelope signal, which is used as an operand in a product with the actual signal to shape. It is important to distinguish between an EG and a VCA; however, sometimes both functionalities are comprised in one device or module. Envelope generators are also used to control other aspects of sound production, from the pitch of the oscillator to the cutoff of a filter (Figure 1.8). Similarly, low-frequency oscillators (LFOs) are used to control any of these parameters. LFOs are very similar to oscillators, but with a frequency of oscillation that sits below the audible range or slightly overlapping with its lower part. They are used to modulate other parameters. If they modulate the pitch of an oscillator, they are performing vibrato. If they modulate the Figure 1.7: A linear envelope generated according to the ADSR scheme. 12 Modular Synthesis amplitude of a tone through a VCA, they are performing tremolo. Finally, if they are used to shape the timbre of a sound (e.g. by modulating the cutoff of a filter), they are performing what is sometime called wobble. Other tools for articulation are slew limiters, which smooth step-like transitions of a control voltage. A typical use is the smoothing of a keyboard control voltage that provides a glide or portamento effect by prolonging the transition from one pitch value to another. A somewhat related type of module is the sample and hold (S&H). This module does the inverse of a slew limiter by taking the value at given time instants and holding it for some time, giving rise to a step-like output. The operation of an S&H device is mathematically known as a zero-order hold filter. An S&H device requires an input signal and depends on a clock that sends triggering pulses. When these are received, the S&H outputs the instantaneous input signal value and holds it until a new trigger arrives. Its output is inherently step-like and can be used to control a range of other modules. 1.3.4 “Fire at Will,” or in Short: Sequencers Step sequencers is another family of modules that allow you to control the performance. Sequencers specifically had – and still have – a distinctive role in the making of electronic music, thanks to their Figure 1.8: Advanced envelope generation schemes may go beyond the ADSR scheme. The panel of a Viscount-Oberheim OB12 is shown, featuring an initial delay (DL) and a double decay (D1, D2) in addition to the usual controls. Modular Synthesis 13 machine-like precision and their obsessive repetition on standard time signatures. Sequencers are made of an array or a matrix of steps, each representing equally spaced time divisions. For drum machines, each step stores a binary information: fire/do not fire. The sequencer cycles repeatedly along the steps and fires whenever one of them is armed. We may call this a binary sequencer. For synthesizers, each step has one or more control voltage values associated, selectable through knobs or sliders. These can be employed to control any of the synth parameters, most notably the pitch, which is altered cyclically, following the values read at each step. Sequencers may also include both control voltage and a binary switch, the latter for arming the step. Skipping some steps allows creating pauses in the sequence. Sequencers are usually controlled by a master clock at metronome rate (e.g. 120 bpm), and at each clock pulse a new step is selected for output, sending the value or values stored in that step. This allows, for example, storing musical phrases if the value controls the pitch of a VCO, or storing time-synchronized modulations if the value controls other timbre-related devices. Typical sequencers consist of an array of 8 or 16 steps, used in electronic dance music (EDM) genres to store a musical phrase or a drumming sequence of one or two bars with time signature 4/4. The modular market, however, provides all sorts of weird sequencers that allow for generative music, polyrhythmic composition, and so on. Binary sequencers are used for drum machines to indicate whether a part of the drum should fire or not. Several rows are required, one for each drum part. Although the Roland TR-808 is widely recognized as one of the first drum machines that could be programmed using a step sequencer, the first drum machine ever to host a step sequencer was the Eko Computer Rhythm, produced in 1972 and developed by Italian engineers Aldo Paci, Giuseppe Censori, and Urbano Mancinelli. This sci-fi wonder has six rows of 16 lit switches, one per step. Each row can play up to two selectable drum parts (Figure 1.9). Figure 1.9: The Eko Computer Rhythm, the first drum machine ever to be programmed with a step sequencer. It was devised and engineered not more than 30 km away from where this book was written. Photo courtesy of Acusmatiq MATME. Owner: Paolo Bragaglia. Restored by Marco Molendi. 14 Modular Synthesis 1.3.5 Utility Modules There are, finally, a terrific number of utility modules that, despite their simplicity, have a high value for patching. Attenuators and attenuverters, mixers, multiples, mutes, and multiplexers and demultiplexers are very important tools to operate on signals. A brief definition is given for each one of these: • Attenuators and attenuverters. An attenuator is a passive or active circuit that just attenuates the signal using a potentiometer. In the digital domain, this is equivalent to multiplying a signal by any value in the range [0, 1]. Attenuverters, additionally, are able to invert the signal, as if multiplying the signal by a number in the range [−1, 1]. Please note that inversion of a periodic signal is equivalent to a phase shift of 180° or π. • Mixers. These modules allow you to sum signals together. They may be passive, providing just an electrical sum of the input voltages, they may be active, and they may have faders to control the gain of each input channel. Of course, in VCV Rack, there will be no difference between active and passive; we will just be summing discrete-time signals. • Multiples. It is often useful to duplicate a signal. Multiples are made for this. They provide one input signal into several outputs. In Rack, this is not always required, since cables can be stacked from outputs, allowing duplication without requiring a multiple. However, they can still be useful to make a patch tidy. • Mutes. It is sometimes useful to mute a signal, especially during a performance. Mutes are just switches that allow the signal flow from an input to an output or not. • Multiplexers and demultiplexers. These modules allow for complex routing of signals. A multiplexer, or mux, has one input and multiple outputs and a knob to select where to route the input signal. A demultiplexer, or demux, on the contrary, has multiple inputs and one output. In this case, the knob selects which input to route to the output. Mux and demux devices only allow one signal to pass at a time. Interface and control modules are also available to control a performance with external tools or add expressiveness. MIDI-to-CV modules are necessary to transform Musical Instruments Digital Interface (MIDI) messages into a CV. Theremin-like antennas and metal plates are used as input devices, while piezoelectric transducers are used to capture vibrations and touch, to be processed by other modules.

Overview

This document is intended to act as a teaching tutorial for sound terminology, theory and practice, across multiple disciplines, but focusing on acoustics, psychoacoustics, environmental acoustics, electroacoustics, speech acoustics, audiology, noise and soundscape studies. In many cases, we draw comparisons between these disciplines and attempt to explain their basic models and how they differ, beginning with the Introductory module.

INTRODUCTION: Sound is .....

A survey of basic concepts in each discipline

1: Sound-Medium Interface

ACOUSTIC

2. Vibration: Frequency and Pitch

3. Vibration: Spectrum and Timbre

4. Magnitude: Levels and Loudness

5. Sound-Environment Interaction

6. Binaural Hearing and Acoustic Space

7. Sound-Sound Interaction

8. Speech Acoustics

9. Audiology and Hearing Loss

10. Effects of Noise and Noise Measurement Systems

ELECTROACOUSTIC

11. Field Recording

12. Filters and Equalization

13. Modulation and Auto-Convolution

14. Time Delays and Phasing

15. Time Delays and Reverberation

16. Dynamic Range and Compression

17. Microsound and Granular Synthesis

18. Voice and Text-based Composition

19. Soundscape Composition

The Tutorial is divided into a number of modules which are designed to cover a particular topic similar to a lab-based class or a set of studio demos. They are divided into an Acoustic set and an Electroacoustic set. Subtopics in each module can be accessed separately by a link in the series A, B, C, etc.

Interdisciplinary Thematic Search Engine

The subject matter of this document is organized according to various themes, the first five of which are traced through various subdisciplines, each of which treats the theme differently. The relevant terms for each theme and each discipline are grouped together. The themes are:

Specific Subdisciplines

Communications Theory

The principal discipline which is the "home" for each term is indicated by an icon, as follows:

acoustics psychoacoustics soundscape Noise electroacoustics [linguistics]audiology [music] [1]

Terms that are found in more than one discipline are indicated as follows: Acoustics / Electroacoustics