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  1. 8. Subtractive Synthesis EE2F2 - Music Technology

  2. 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

  3. 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)

  4. 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

  5. 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

  6. 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.

  7. 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.

  8. 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:

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. Part One Sounds & Timbre Part Two Subtractive Synthesis Sound & Synthesis Lab Part One Sounds & Timbre

  17. 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

  18. 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.)