Intro to Subtractive Synthesis (in progress)

Subtractive synthesis is the process of creating comparatively complex sounds using only two elements: a sound source and a filter. The sound source may be an oscillator, as in analog synthesis, or a sample (often abbreviated S&S, for sample and synthesis). The filter may be any type of frequency modicfier, but by far the most common is a low-pass (LP) filter. Because the filter is removing elements from the signal, it is known as "subtractive" synthesis.

In the early days of electronic instruments (up to about the mid 1960s), the only element was the oscillator. The sound was a pure tone generated by a wave, the shape of which was the primary characteristic determining timbre. Typically, this was a sine wave, which creates a full, even tone across frequencies. Square waves, on the other hand, sound distinctively "hollow", and triangle waves more mellow or "flutey". In addition to waveshape oscillators, noise oscillators were often used. These generate random frequencies across the entire spectrum, and so have no apparent note or harmonic emphasis- though different types of noise tended to focus on general ends of the spectrum ("white noise", for instance, covers the entire spectrum and sounds much like television "snow", wheras pink noise covers more of the lower end of the spectrum, giving a more muted static).

The oscillators alone made no sound; the electrical output needed to be fed into an amplifier circuit to create a signal that could be audible through speakers. So a basic synthesizer must contain both a sound source and an amplifier. In order to control the pitch or volume of the tone produced, an additional element had to be applied (simple synths had pitches that were either permanently fixed on one note or that rose and fell much like a police siren). In analog synths, this was implemented by voltage control, and so we have the acronym VCO for voltage-controlled oscillator, VCA for voltage-controlled amplifier, and sometimes VCP for voltage-controlled pitch. These could be controlled directly using dials, or connected to a waveform generator for automatic response over time. For instance, using a sine wave as the source of the amplifier results in a sound that repeatedly fades in and out, whereas a square wave altrenates full volume with silence for a choppier sound.

The filter was added later, to make the sound more interesting and as an attempt to mimic real instruments. A static filter removes specific frequency bands, exactly like changing one of the sliders on an equalizer or the bass/treble dials on a car stereo. A variable filter can be swept through a series of frequencies (imagine moving the EQ sliders one after another) but the effect is rather subtle. Synth engineers soon found that a much more useful sound could be achieved by emphasizing the point at which the filtering begins to occur- i.e., on an EQ that would be a bit like moving the leading slider up and the consecutive sliders down. This emphasis frequency is usually called "resonance", and can be adjusted from a very subtle coloring of the sound all the way to the point at which the emphasized frequency overpowers the original oscillator (referred to as "self-oscillation"). At high frequencies this could sound very piercing indeed, and at low frequencies the bass tends to swell and distort.

Using only these elements, the sound engineer has many possibilities. When the various elements are "modulated" (i.e., altered by a cyclic variable), a vast spectrum of sounds is possible. As mentioned earlier, one may have a sine wave modulating the VCA, for regular changes in volume. Even the oscillator wave can be modulated initially, producing more irregular waveforms that can vary tonally over time. One application of this type is known as "pulse-width modulation", a common option on many synthesizers. Often the modulations will be synchronized to the tempo of the music, but they can just as easily be left random for a more "organic" effect.

As technology progressed through the 1970s and 80s, the analog elements of the synthesizer began to be progressively replaced by digital versions. In most cases, this resulted in a much more stable and efficient (and lower-cost) synthesizer, with access to things like patch memories and sequencing that had been difficult or impossible to achieve with purely analog technology. Still, many people continue to prefer the analog sound and so many synthesizers were hybrids (for instance, digital ocillators but an analog filter). When sampling began to dominate, many designers realized that subtractive synthesis structure could be utilized, with the sample taking the place of the oscillator. The amplitude, filtering, and modulation variations could be used to increase the realism of the sampled sound, as well as opening up new creative applications. This led to the term "sample and synthesis", or "S&S", a sound technology used from the mid 1980s onward.

Note: FM (Frequency modulation) synthesis and Additive synthesis also came about with digital technology. These processes use a number of variables at the basic oscillator level (usually referred to as "carriers" and "modulators") for extremely flexible tonal control. The complexity of these synthesizer types requires a separate treatment, and are significantly different in structure from subtractive or even S&S synthesizers. However, most of the FM or additive synthesizers will also conform to the subtractive structure once the oscillator itself has been defined, allowing for more standard VCA & VCF type variations.