Synthesis types
Introduction
Before we can talk about synthesis types we need to define a few terms and concepts.
NOTE: If you have not already done so, you may wish to read our articles on Sound theory first.
What is synthesis?
Sound synthesis is the process of using electronics to create an electrical pressure soundwave from scratch and then controlling and manipulating it.
Oscillators
An Oscillator is an electronic sound source. It is the device which creates the electrical pressure soundwave in a synthesiser. Oscillators can be analogue or digital.
Types of synthesis
Many different types of synthesis have been invented since the first commercial analogue synthesisers in the late 1960"s. The following are some of the most common ...
- Subtractive (also know as Analogue)
- Additive
- FM (Frequency Modulation)
- S & S (Sample and Synthesis)
- Physical Modelling (also known as Soft Synthesis, or Virtual Synthesis)
- Wavetable
- Phase Distortion
- Granular
Subtractive synthesis
Subtractive synthesis involves using oscillators to create electrical pressure soundwaves which are then processed, or "modified" to alter their pitch, frequency content and amplitude over time. In essence, elements of the original waveform are "wobbled" and "subtracted". Click here for a detailed explaination of Subtractive Synthesis.
Additive synthesis
Additive synthesis emulates the way in which sound is created in nature. An additive synthesiser has multiple oscillators. Each oscillator creates a simple sine wave. The user can set the frequency and amplitude of each sine wave. By combining these sine waves together a complex waveform is created. This process emulates the process by which natural sound is created with multiple harmonics.
Commercially available additive synthesisers have tended to be complex and difficult to operate and have therefore had limited success. Examples include the Kawai K5 (1987).
FM (Frequency Modulation)
FM was the first commercially available digital synthesis type. It is similar to Additive synthesis in that it uses 6 sine wave creating oscillators, each of which can have frequency, amplitude and envelope (volume over time) set by the user. It differs in that rather than combining the sine waves together, the output of one oscillator is sent to modulate, or "wobble", the next. The second then modulates a third and so on until a complex waveform is produced by the final oscillator in the chain, the so-called Output Oscillator. The oscillators can be arranged in various "patterns" called algorithms, each with a different series of connections.
Yamaha DX7
Examples of FM synthesisers include the Yamaha DX7 (1983).
S & S (Sample & synthesis)
In the late 1980"s there was a demand for a single synthesiser which could produce a range of analogue, digital and natural sounds. This lead to the invention of S & S synthesis.
Rather than using waveform creating oscillators, an S & S synthesiser uses pre-recorded samples stored in ROM (Read Only Memory) as its sound sources. Due to the expense of ROM memory chips, only 8 to 16Mb capacity chips are fitted. It is therefore not possible to use full length sample recordings if many sounds are to be stored. To overcome this problem just the attack and a portion of the sustain of the sound is stored. The sustain section is looped to create the illusion of a longer sample.
These samples are then processed through a conventional subtractive architecture of filters and amplifiers which create the illusion of a natural decay.
Examples of S & S synthesis include the Roland D50 (1987) shown here, all multi-timbral GM synthesisers (such as the Roland XP10). the Roland JV1080 module, and the synthesiser part of a PC computer"s Soundcard.
Physical modelling (or Soft Synthesis or Virtual Synthesis)
This relatively new form of synthesis utilises fast, powerful digital signal processors (DSPs) and high quality digital to analogue converters (D.A.C.s). The synthesiser will take the form of either a program, which may run on a computer (normally called a soft synth or plug-in ), or on dedicated hardware in the form of a rackmountable synth, or a keyboard-type synth.
As a note event is triggered, the program uses DSP to mathematically calculate in real time the stream of digital data required to create the sound it will output through the D.A.C.
Consider a sampler which can convert an incoming analogue elctrical pressure soundwave into digital (binary) data, which it then stores. This sound can then be replayed with a connected MIDI keyboard. The sound can respond to performance changes in velocity and pitch bend thereby altering volume, brightness (with the use of a low pass filter) and pitch, but otherwise remains unchanged.
A physical modelling synthesiser is actually generating the binary data (sound) in real time, tailored and responsive to the nuances of the performance arriving from a controller (usually a MIDI keyboard). It can therefore sound much more organic and "realistic". Additional hardware controllers (such as breath controllers, log wheels, pressure pads etc) can also effect the way the sound is produced.
In addition, PM synthesis can model elements of instruments and combine them in ways unheard of in nature: for example, a bowed trumpet or a snare drum that alters pitch or size in real time.
Physical modelling can also be used to emulate any other form of synthesis. The range of sounds it can create is determined not by the hardware (providing the DSP is powerful enough), but by the available software models. Thus we have many PM emulations of subtractive (Logics ES1 soft synth, Propellerheads Reason), FM (Native Instruments FM7 soft synth), additive and S&S synthesis (Spectrasonics Atmosphere.) etc.
| Types of physical modelling synth | ||||
|---|---|---|---|---|
| Hardware | Hardware | Software | Software | |
| Type | Module | Keyboard | Stand-alone | Plug-in |
| Example | Nord rack | Roland V-synth | NI Reaktor | NI Reaktor |
Examples of early physical modelling hardware synthesisers include: Yamaha VL1 (1995), Korg Prophecy (1996).
Korg Prophecy
This synthesiser can model subtractive, S&S and real brass, woodwind and keyboard and string instruments.
Wavetable
This form of synthesis is ideal for slow pad-like evolving sounds. The essential concept involves sequentially cycling through (or morphing between) a number of waveforms over time. The waveforms are usually stored digitally in a so called "wave table" and may be samples or digital "models".
Popular wavetable synths inculude the Sequential Circuits Prophet VS (1996) and the Korg Wavestation.
Phase distortion (PD)
Phase distortion is a form of synthesis, which uses an algorithm to create a sine wave, and then uses a second algorithm to distort the "shape" of the sine wave to create a totally new waveform. A simple example of how distortion alters a waveform would be inputting an oscillator generating a sine wave to a mixer input channel and then overloading the channel by increasing the gain until it distorts. The resultant waveform will be similar to a sine wave, but will have added harmonics. As a result of being clipped, the wave will sound more like a square wave. If you use an algorithm to achieve this, this would be phase distortion.
Phase distortion was used extensively on the Casio CZ series of synthesisers, notably the CZ101. It is best used when added to other forms of synthesis, such as Frequency Modulation, where the ability to create waveforms which are variations of sine waves is particularly useful for creating new tones.
Granular Synthesis
This method involves analysing very small individual frequency bursts, or grains of sound energy, which when combined serially create recognisable sound. By changing the order of these grains, or combining them with grains from different sources, it is possible to create entirely new synthesised sounds. Filters and amplifier envelopes are usually applied afterwards, to affect the brightness and shape of the sound over time.
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