Introduction
In our previous module, you mastered the art of programming rhythm, culminating in recreating and transforming complex drum patterns. You learned to arrange pre-existing sounds—samples—in time. Now, we shift our focus from arranging sounds to creating them from scratch. Welcome to the world of synthesis, the process of generating sound electronically. This is where your background in physics, particularly your understanding of waves and frequencies, will provide you with a powerful conceptual advantage.
This lesson introduces the most fundamental and ubiquitous method of synthesis: subtractive synthesis. We will explore the standard signal flow that defines this process, tracing the journey of a signal from its raw creation to the final sound you hear. By the end of this lesson, you will be able to describe the roles of the three core components in this chain: the Oscillator, the Filter, and the Amplifier. This forms the bedrock of sound design for the majority of synthesizers you will encounter.
The Source-Filter-Amplifier Model
At its heart, subtractive synthesis is analogous to sculpting. You begin with a large, unformed block of material and progressively remove parts of it until you arrive at your desired shape. In synthesis, the "block of material" is a harmonically rich sound wave, and the "sculpting tools" are electronic components that subtract parts of its frequency spectrum.
This process is typically organized into a clear, linear signal flow or path. Take a look at this fundamental block diagram:
This diagram illustrates the journey of the audio signal. Let's formally define this process before we break down each component.
[PDF] Sound synthesis and physical modeling
This excerpt from a textbook by Dr. Stefan Bilbao provides a concise, academic definition of subtractive synthesis. It establishes the core principle of starting with a rich sound and shaping it with filters.
Please read the section titled "Subtractive synthesis". Pay close attention to the description of starting with a rich sound, like white noise or an impulse train, and then using filtering to shape its spectrum. The figure shown (Figure 1.3) is another excellent visualization of this process.
Now, let's examine each stage of this signal path in detail.
1. The Oscillator: The Sound Source
The oscillator is the origin point of the sound. Its function is to generate a periodic waveform at a specific frequency. This frequency, known as the fundamental frequency (), determines the pitch of the note we hear.
However, the oscillator's true power lies in its ability to produce different waveforms. These waveforms are not pure tones (like a sine wave); they contain a series of additional frequencies called harmonics or overtones. These harmonics are integer multiples of the fundamental frequency (). The presence and relative amplitude of these harmonics determine the raw timbre, or tonal character, of the sound. This is a direct application of the principles of Fourier analysis: any periodic waveform can be deconstructed into a sum of sine waves at different frequencies and amplitudes.
The following video offers an excellent exploration of common oscillator waveforms and their harmonic content. Your physics background will make the spectrum analyzer visualizations particularly insightful.
Oscillators, harmonics, and a way to think of synthesized sound
This video from the Sound + Voltage channel demonstrates the sound and harmonic structure of the basic building blocks of synthesis.
Watch the first two sections of the video. First, observe the demonstration of the four basic waveforms: sine, triangle, sawtooth, and square. Note how their perceived loudness and character differ even at the same pitch. Next, focus on the explanation of harmonics and spectra. This is the crucial part. The spectrum analyzer shows you precisely why these waves sound different. Pay attention to which harmonics are present in each waveform (e.g., all harmonics for sawtooth, only odd harmonics for square).
From the video, we can summarize the essential waveforms used as the source material in subtractive synthesis:
- Sine Wave: Contains only the fundamental frequency. It is the purest tone but offers nothing for a filter to subtract.
- Triangle Wave: Contains the fundamental and odd-numbered harmonics, but their amplitude drops off very quickly. This results in a mellow, flute-like tone.
- Square Wave: Contains the fundamental and odd-numbered harmonics, whose amplitudes decrease more slowly. This creates a "hollow" or "reedy" sound, characteristic of many classic video game soundtracks.
- Sawtooth Wave: Contains the fundamental and all integer harmonics (both even and odd). This makes it the most harmonically rich and bright-sounding of the basic waveforms, providing the most material for a filter to sculpt.
2. The Filter: The Sculptor
Once the oscillator generates a harmonically rich signal, the signal flows to the filter. The filter's job is to alter the timbre of the sound by boosting or attenuating specific frequency ranges—it subtracts harmonics. This is the most significant sound-shaping stage in subtractive synthesis.
The most common type of filter is the Low-Pass Filter (LPF).
- An LPF allows frequencies below a certain cutoff frequency to pass through unaffected.
- It attenuates (reduces the amplitude of) frequencies above the cutoff frequency.
By sweeping the cutoff frequency of a low-pass filter, a synthesist can transform a bright, buzzing sawtooth wave into a soft, muted sine-like tone, or create the classic "wah-wah" effect. Other common filter types include:
- High-Pass Filter (HPF): The opposite of a low-pass filter. It attenuates frequencies below the cutoff.
- Band-Pass Filter (BPF): Attenuates frequencies both above and below a central frequency, allowing only a specific "band" of frequencies to pass through.
3. The Amplifier: The Final Control
After being sculpted by the filter, the signal arrives at the final stage: the Voltage-Controlled Amplifier (VCA), or simply, the amplifier. The amplifier's function is straightforward but essential: it controls the overall volume or amplitude of the signal.
Think of it as a volume knob. Without it, the synthesizer would produce a constant, unending drone. The amplifier shapes the sound's volume over time, defining how a note starts, continues, and ends. While we will cover the automatic control of the amplifier (using envelopes) in a future lesson, for now, understand it as the final gatekeeper that determines the loudness of the sound before it reaches your speakers.
Tying It All Together: Pitch, Tone, and Volume
A powerful way to conceptualize the Oscillator -> Filter -> Amplifier signal flow is to map each stage to a fundamental property of sound. The following video provides an exceptionally clear explanation of this entire process.
Logic Pro Tips: Subtractive Synthesis Explained
The presenter in this MusicTech video uses a simple analogy to explain the three essential modules of subtractive synthesis and their functions.
Please watch the introduction and the breakdown of the three core modules from the beginning to the summary. The key insight here is the mapping the presenter makes: Oscillator → Pitch: Generates the fundamental frequency. Filter → Tone: Shapes the harmonic content, i.e., the timbre. Amplifier → Volume: Controls the overall loudness.
To see how these theoretical blocks appear on a real synthesizer and how they sound in practice, you can watch the demonstration portion of the same video.
Logic Pro Tips: Subtractive Synthesis Explained
This section of the video demonstrates the concepts on Logic Pro's Retro Synth, but the layout is representative of most subtractive synthesizers.
Watch the practical demonstration from Retro Synth demo. As you watch, identify the three sections in the user interface: The Oscillator section, where he chooses and mixes different waveforms. The Filter section, where he adjusts the "Cutoff" knob to change the tone. The Amplifier section, with its simple "Volume" control.
Conclusion
You have now traced the fundamental signal path of subtractive synthesis. This Oscillator -> Filter -> Amplifier chain is the conceptual framework for the vast majority of synthesizers, both hardware and software. By understanding that sound creation begins with a harmonically rich source, which is then carved away by a filter and given its final volume by an amplifier, you have grasped the core principle of sound design.
Key Takeaways:
- Subtractive Synthesis: The process of creating sound by starting with a harmonically rich waveform and using filters to subtract unwanted frequencies.
- Signal Flow: The path the audio signal takes is Oscillator → Filter → Amplifier.
- Oscillator: The source of the sound. It determines the pitch (fundamental frequency) and initial timbre (waveform/harmonics).
- Filter: The primary sound-shaping tool. It modifies the timbre by removing harmonics. The low-pass filter is the most common type.
- Amplifier: The final stage. It controls the volume (amplitude) of the sound.
This simple, three-stage model is just the beginning. The real magic happens when we start to automatically control, or modulate, the parameters of these stages over time. The diagram you saw at the beginning had other boxes pointing towards our main components, labeled "LFO" and "ADSR".
In our next lesson, we will take a closer look at the filter, exploring the crucial parameter of resonance and the characteristics of different filter types. Following that, we will dive into the world of modulation, learning how Envelopes and Low-Frequency Oscillators (LFOs) are used to bring our static sounds to life.