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 SourceModifier 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. Output V.C.F.V.C.A. Env. Gen. Trigger SourceModifier

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. Output V.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 Gain Frequency Cut-off frequency Set by control input Peak at cut-off Set by resonance control

Using the VCO & VCF Desired Spectrum f VCO Output (sawtooth wave) VCF Response ff 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 InOut 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 amplitude Note On Note Off On-Off envelope Simply turns the output on when a note is pressed and off when its released time amplitude 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’ Envelope Level Time Sustain Level Attack Rate Decay Rate Release Rate Note PressedNote Released ADSR

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.)