Music Technology and the Project Studio

Synthesis and Sampling

Dan Hosken

• Publisher: Routledge
• Year: 2012
• ISBN:
  9781136644344
• Edition: 1
• Topic: Medien & darstellende Kunst
• Subtopic: Musiktheorie & Musikverständnis

Music Technology and the Project Studio: Synthesis and Sampling provides clear explanations of synthesis and sampling techniques and how to use them effectively and creatively. Starting with analog-style synthesis as a basic model, this textbook explores in detail how messages from a MIDI controller or sequencer are used to control elements of a synthesizer to create rich, dynamic sound. Since samplers and sample players are also common in today's software, the book explores the details of sampling and the control of sampled instruments with MIDI messages.

This book is not limited to any specific software and is general enough to apply to many different software instruments. Overviews of sound and digital audio provide students with a set of common concepts used throughout the text, and "Technically Speaking" sidebars offer detailed explanations of advanced technical concepts, preparing students for future studies in sound synthesis.

Music Technology and the Project Studio: Synthesis and Sampling is an ideal follow-up to the author's An Introduction to Music Technology, although each book can be used independently.

The Companion Website includes:

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• Audio examples demonstrating synthesis and sampling techniques

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• Interactive software that allows the reader to experiment with various synthesis techniques

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• Guides relating the material in the book to various software synthesizers and samplers

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• Links to relevant resources, examples, and software

CHAPTER 1

Sound

The focus of this book is the creation of rich, complex timbres using software instruments that employ various synthesis and sampling techniques. The output of these instruments is first carried as digital audio from the software to the audio interface and then as sound from the speakers or headphones to your ears. Sound and digital audio, then, are fundamental topics that underlie the creation of these rich, complex timbres.

Both sound and digital audio are deep subjects that would require several books many times the size of this one to fully explain. The goal of these first two chapters is more modest: to provide an overview of essential sound and digital audio concepts necessary to create sounds through synthesis and sampling.

The way the material in this book is presented assumes that you have some experience with music technology and thus some understanding of sound and digital audio. As a result, the material in these chapters is presented in a relatively brief fashion to review important concepts and provide a common basis for the terminology that will be used throughout the book.

SOUND GENERATION, PROPAGATION, AND PERCEPTION

A sound wave can be thought of in three different phases: generation, propagation, and perception. First, a sound wave is generated by a vibrating source, such as a drumhead, a string, buzzing lips, or a speaker cone, that comes in contact with an elastic medium, such as air. The vibrating source creates regions of air pressure that are higher and lower than normal air pressure as the molecules in the air are periodically pushed together. The regions of higher air pressure are called compressions and the regions of lower air pressure are called rarefactions (see Figure 1.1).

Figure 1.1 Side cutaway view of a speaker cone: a) the forward motion of the speaker cone causes a compression; b) the backward motion of the speaker cone causes a rarefaction

Once air molecules have been pushed together by the vibrating source, creating a compression, their energy is passed on to other air molecules and the compression moves, or propagates, through the medium. In its wake, the compression leaves a rarefaction, a region of lower than normal air pressure. As the vibrating source continues to move back and forth, more compressions and rarefactions are produced that also propagate through the medium. This series of moving compressions and rarefactions form a sound wave, also referred to as a compression wave or a longitudinal wave.

The actual medium through which a sound wave travels affects the propagation of the sound wave. For example, sound waves travel faster through water than through air and faster still through a metal such as steel. The speed of sound in the air is also dependent on the air temperature, with higher temperatures yielding faster speeds. Different gases besides the normal air mixture will also have different speeds of sound. The higher speed of sound in helium is responsible for the well-known helium-voice effect. Sound waves do not travel through the vacuum of space because there isn’t a sufficient density of molecules for a compression to propagate by passing its energy on to other molecules.

The final phase of a sound wave is, of course, our perception of it. In fact, it is arguable that a sound wave is not actually “sound” until someone perceives it, leading to the well-known question: “If a tree falls in a forest and no one is around to hear it, does it make a sound?” Human perception of sound waves starts with the sound waves entering our ears and leads eventually to a response from our brains. The study of perception is called psychoacoustics, and the study of our brain’s evaluation of what we perceive is called music cognition.

While we won’t spend time in this book discussing the human perceptual and cognitive apparatus in much detail, it is important to note that our ears and brain influence strongly what we actually hear. Throughout this chapter we will be discussing the relationship between the physical properties of sound and the perceptual properties of sound, and noting the many ways in which they are not the same.

SOUND PROPERTIES AND THE WAVEFORM VIEW

The view in Figure 1.1 of a speaker cone generating a series of compressions and rarefactions helps us understand the physical basis of sound generation and propagation, but it doesn’t tell us much about the sound itself. In a musical context we are interested in such perceptual properties as pitch, loudness, articulation, and timbre.

To help us see the physical properties of a sound wave that give rise to the perceptual properties we’re interested in, it is helpful to look at the sound wave graphed as the change in air pressure (y-axis) over time (x-axis). We will refer to this as the waveform view of sound, though some sources refer to this as the “time domain representation” (see Figure 1.2).

Figure 1.2 The waveform view of sound: a trumpet sound graphed as the amount of air pressure change over time. Notice the repeating pattern

Frequency and Pitch

One of the physical properties that we can see in the waveform view is the amount of time between compressions in a sound wave. This amount of time is the period of the waveform and is measured in the number of seconds per compression-rarefaction cycle (see Figure 1.3). The frequency can then be derived from the period by noting that we measure frequency in cycles per second, or Hertz (Hz), which is the inverse of the period:

Figure 1.3 Two waves with the same amplitude but different frequencies graphed on the same axis. Period T₂ is twice as long as Period T₁ resulting in half the frequency

For example, if the period was 2 seconds per cycle, its frequency would be 0.5 Hz (cycles per second) and if the period was 0.01 seconds per cycle, its frequency would be 100 Hz.

One of the perceptual properties of a sound wave with a regular frequency is pitch (frequency is also central to our perception of timbre, which we’ll discuss a little later in the chapter). While a sound wave with a period T has a frequency f calculated from the above formula, we can’t perceive all frequencies as pitch. Our perception of frequency is limited to approximately 20 Hz to 20 kHz (kilohertz), or 20,000 Hz. Below 20 Hz (infrasonic) we don’t perceive the wave as a pitch, though we may perceive some kind of pulse if the sound wave is complicated enough. Above 20,000 Hz (ultrasonic), we don’t perceive frequencies as sound, though they may still have an impact on us. For example, doctors use compression waves at ultrasonic frequencies to break up kidney stones. As we age, the upper limit of our frequency perception naturally diminishes, and people who have sustained hearing damage may have a dramatically reduced upper frequency limit.

Another distinction between the perceptual property of pitch and the physical property of frequency can be found in the way we perceive musical intervals. We hear two musical intervals as being the same size when they have the same frequency ratio between the highest and lowest pitches, not when they are the same number of Hertz apart. The simplest example is the octave. The frequency ratio between pitches that are an octave apart is 2 to 1, notated as 2:1. This ratio holds regardless of the octave. As a result, the absolute size of an octave gets larger as we go up in pitch. For example, if we start with the A that has a frequency of 110 Hz, the octave above it would be the A with a frequency of 220 Hz. The octave above that would be 440 Hz and the octave above that would be 880 Hz. Thus the octaves grow in absolute size—110 Hz, 220 Hz, and 440 Hz—but we perceive them as being the same size. The same holds true for all of the other musical intervals as well. This will be discussed further under “Tuning and Temperament” below.

Figure 1.4 shows the frequencies and pitches across the range of the piano. Pitches are often given in a pitch-class/register notation, with C4 being middle C, B3 being one semitone below that and C5 being one octave above middle C. As you’ll see in Chapter 3, many MIDI applications use C3 as middle C. Notice that the lowest frequency on a piano is near the bottom of our hearing range, but the highest frequency is only a little over 4,000 Hz. The frequencies above that are usually a part of the timbre of a pitch with a lower fundamental frequency (see below).

Figure 1.4 The frequencies associated with piano keys. C4 is middle C

Amplitude and Loudness

Another physical property that we can see in the waveform view is amplitude, which is related to the amount of air pressure change above normal in a compression and below normal in a rarefaction (see Figure 1.5). In general, the more energy you put into a sound—the harder you pluck, strike, bow, or blow—the more the air molecules are pressed together in a compression and thus the greater the amplitude. As a result we perceive amplitude as loudness.

Figure 1.5 Two waves with the same frequency but different amplitudes graphed on the same axis

As w...