EE2F2 - Music Technology

8. Subtractive Synthesis

Analogue Synthesisers Analogue synthesisers attempt to produce

realistic timbres using combinations of primitive, easy to generate waveforms, e.g.

Sine waves Square waves ‘Sawtooth’ waves Etc.

Two main methods of creating timbres: Subtractive synthesis – start with a waveform with lots

of harmonics and filter out the ones you don’t want Additive synthesis – build up the timbre a component at

a time using just sine waves at the harmonic frequencies

Subtractive Synthesis Subtractive synthesis: one of the earliest techniques but

still being used today Outline:

An oscillator produces a harmonically rich waveform A filter shapes the spectrum of that waveform to create the

desired timbre (subtracts unwanted harmonics) An amplifier shapes the envelope of the sound

This is the basis of contemporary sample+synthesis techniques and is related to functional physical modelling

In each case, the sound production techniques use the source-modifier approach

NB. We’ll only look at monophonic designs (i.e. only one note can be played at a time)

Source-Modifier Model Source

A signal generator Frequency is set by the

keyboard Waveform shape is set by the

controls Modifier

Filter + amplifier Shapes the waveform in the

frequency and the time domains

Filter characteristics and amplifier gain are varied by control voltages

Source Modifier

Front Panel Controls

Output

Control Voltage Sources

The control voltages influencing the source and modifier(s) are:

Keyboard output A control voltage proportional to the pitch of the key

pressed Envelope generator(s)

A slowly changing control voltage waveform triggered at the start of a note and then evolving throughout its duration

Low Frequency Oscillator (LFO) Slowly varying periodic control voltage. Used to

modulate the pitch and/or amplitude

A Subtractive Synthesiser Control voltages

(shown in green) determine the behaviour of:

Voltage Controlled Oscillator (VCO)

Voltage Controlled Filter (VCF)

Voltage Controlled Amplifier (VCA)

V.C.O.

L.F.O.

OutputV.C.F. V.C.A.

Env. Gen.Trigger

Source Modifier

A Subtractive Synthesiser

Audio signal (shown in red) is:

Generated by the source (VCO)

Modified by the VCF and the VCA

V.C.O.

L.F.O.

OutputV.C.F. V.C.A.

Env. Gen.Trigger

The V.C.O.

The Oscillator frequency is usually controlled by the keyboard with maybe a little modulation from the LFO

The waveform produced is selected from the front panel controls:

The V.C.F. The VCF modifies the

spectrum produced by the VCO

It is usually a low-pass filter with adjustable resonance

The cut-off frequency is (partly) controlled by control voltages from the keyboard and the envelope generator

Ga

in

Frequency

Cut-off frequencySet by control input

Peak at cut-offSet by resonance control

Using the VCO & VCF

Desired Spectrumf

VCO Output (sawtooth wave)

VCF Responsef f

Output waveform spectrum is built up by multiplying the source spectrum by the filter response

The V.C.A.

The VCA is simply a voltage controlled amplifier

The gain (amplification factor) is controlled by the control voltage

In Out

Gain control

E.g. Violin ensemble

The Envelope Generator Usually used to control the VCA and VCF to

‘shape’ the sound in terms of: Amplitude shape in the time domain Spectral content (evolving over time)

The ‘envelope’ is a slowly varying waveform triggered at the start of each note

Simple envelope generators

time

am

plit

ude

Note On

Note Off

On-Off envelope

Simply turns the output on when a note is

pressed and off when its released

time

am

plit

ude

Note On

Note Off

Attack-Release envelope

Gradual attack (ramp-up) and release (ramp-down). Attack and release rate

can be adjusted

ADSR Generator ADSR envelope

generator can create a wide variety of envelopes using just four parameters:

Attack: The initial rise time

Decay: Immediately following the attack

Sustain Level: A level maintained until the note is released

Release: The rate that the sound decays after the ‘note-off’

En

ve

lop

e L

ev

el

Time

Sustain Level

Attack Rate

Decay Rate

ReleaseRate

Note Pressed Note Released

A D S R

The L.F.O.

Generates a low frequency waveform (usually 1-10 Hz)

Usually selectable between sine or triangle waveform

Used as a control input to modulate: VCO frequency: Creating vibrato effect VCA gain: Creating tremolo effect VCF frequency: Creating special effects, e.g.

‘flanging’ effects

Sound & Synthesis Lab

Part One

Sounds & Timbre

Part Two

Subtractive Synthesis

Part One

Sounds & Timbre

Subtractive Synthesis Pros & Cons

Pros Huge variety of sounds from a simple architecture Not too many parameters, fairly easy to program Can emulate real instruments or create novel sounds

Cons Not always obvious how to program a particular

sound Some waveforms can’t be synthesised with the VCO

and VCF Almost all sounds have an ‘artificial’ nature

Summary Subtractive synthesis requires

Source(s) Modifier(s)

Source: VCO

Modifiers VCA, VCF

All three are controlled by: Keyboard Envelope generator LFO Other controllers (e.g. pedals, joysticks, etc.)