The science of Synthesis - Part 4 Subtractive Synthesizers

Subtractive synthesis is the basis of many forms of synthesizers and is commonly related to analogue synthesis. It is achieved by combining a number of sounds or 'oscillators' together to create a timbre that is very rich in harmonics.

Having looked into the theory of sound, we can look at how this relates to a synthesizer. Subtractive synthesis is the basis of many forms of synthesizers and is commonly related to analogue synthesis. it is achieved by combining a number of sounds or 'oscillators' together to create a timbre that is very rich in harmonics.

Layout of a basic synthesizer

This rich sound can then be sculpted using a series of 'modifiers'. The number of modifiers available on a synthesizer is entirely dependent on the model, but all synthesizers offer a way of filtering out certain harmonics and of shaping the overall volume of the timbre.

The next part of this series of posts looks at how a real analogue synthesizer operates, although any synthesizer that emulates analogue synthesis (i.e. digital signal processing (DSP) analogue) will operate in essentially the same way, with the only difference being that the original analogue synthesizer voltages do not apply to their DSP equivalents.

An analogue synthesizer can be said to consist of three components.

• An oscillator to make the initial sound.
• A filter to remove frequencies within the sound.
• An amplifier to define to overall level of the sound.

Each o these components and their role in synthesis are discussed in the sections below. 

Voltage controlled Oscillator (VCO)

When a key on a keyboard is pressed, a signal is sent to the oscillator to activate it, followed by a specific control voltage (CV) to determine the pitch. The CV that is sent is unique to the key that is pressed, allowing the oscillator to determine the pitch it should reproduce. For this approach to work correctly, the circuitry in the keyboard and the oscillator must be incredibly precise in order to prevent the tuning from drifting, so the synthesizer must be serviced regularly. In addition, changes in external temperature and fluctuations in the power supply may also cause the oscillator's tuning to drift.

This instability gives analogue synthesizers their charm and is the reason why many purists will invest small fortunes in second-hand models rather than using the latest DSP-based analogue emulations. Although, that said, if too much detuning is present, it will be immediately evident and could become a major problem! There is still an ongoing argument over whether is possible for DSP oscillators to faithfully reproduce analogue-based synthesizers, but the argument in favor of DSP synthesizers is that they offer more waveforms and do not drift too widely, therefore prove more reliable in the long run.

In most early subtractive synthesizers the oscillator generated only three types of waveforms: square, sawtooth and triangle waveforms. Today this number has increased and many synthesizers now offer additional sine, noise, tai-saw, pulse and numerous variable wave shapes as well.

Although these additional waveforms produce different sounds, they are all based around the three basic wave shapes and are often introduced into synthesizers to prevent mixing of numerous basic waveforms together, a task that would reduce the number of oscillators.

For example, a tri-saw wave is commonly a sample of three sawtooth waves blended together to produce a sound that is rich in harmonics, with the advantage that the whole sound is contained in one oscillator. Without this waveform it would take three oscillators to recreate this sound, which could be beyond the capabilities of the synthesizer. Even if the synthesizer could utilize three oscillators to produce this one sound, the number of available oscillators would be reduced. 

Here concludes the fourth part of this post, if you want to know more about acoustic science, please read Rick Snoman's Dance Music Manual (Second Edition) Toys, Tools and Techniques.