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8. Subtractive Synthesis. EE2F2 - Music Technology. 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:

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analogue synthesisers
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
  • 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-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

Front Panel Controls

Source

Modifier

Output

control voltage sources
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
A Subtractive Synthesiser
  • Control voltages (shown in green) determine the behaviour of:
    • Voltage Controlled Oscillator (VCO)
    • Voltage Controlled Filter (VCF)
    • Voltage Controlled Amplifier (VCA)

Source

Modifier

V.C.O.

V.C.F.

V.C.A.

Output

L.F.O.

Trigger

Env. Gen.

a subtractive synthesiser1
A Subtractive Synthesiser
  • Audio signal (shown in red) is:
    • Generated by the source (VCO)
    • Modified by the VCF and the VCA

V.C.O.

V.C.F.

V.C.A.

Output

L.F.O.

Trigger

Env. Gen.

the v c o
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 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

Peak at cut-off

Set by resonance control

Gain

Cut-off frequency

Set by control input

Frequency

using the vco vcf

f

f

VCO Output (sawtooth wave)

VCF Response

Using the VCO & VCF

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

f

Desired Spectrum

the v c a
The V.C.A.
  • The VCA is simply a voltage controlled amplifier
  • The gain (amplification factor) is controlled by the control voltage

Gain control

In

Out

the envelope generator

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

Attack-Release envelope

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

amplitude

time

Note On

Note Off

Simple envelope generators

On-Off envelope

Simply turns the output on when a note is pressed and off when its released

amplitude

time

Note On

Note Off

adsr generator
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’

A

D

S

R

Decay Rate

Release

Rate

Envelope Level

Sustain Level

Attack Rate

Time

Note Pressed

Note Released

the l f o
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

Sound & Synthesis Lab

Part One

Sounds & Timbre

subtractive synthesis pros cons
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
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.)