Electronic Music

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Electronic Music

• Dr Ian Drumm

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Subtractive Synthesis

• Aims
• An introduction to subtractive synthesis theory
  and techniques
• Learning Outcomes
• To introduce basic concepts
• Describe typical modular subtractive synthesiser
• Implement some example emulations
• Evaluation of advantages and disadvantages
• Explain simple subtractive vocoder

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Basic Concept

• Start with sources to generate a complex waveform
• Filter to attenuate the relative amplitudes of
  the constituent harmonics
• Output to amplifier

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Brief History

• Moog
• Prophet 5 (polyphony and midi control)
• Digital emulations (EMU, Virus, MS2000)
• Modular Software (Reaktor)

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Sources

• Something broadband (rich in frequency content)
• minimoog sources (sawtooth, square, pulse train)
• also has noise source and option for external
  input

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Relaxation Oscillator

• charges capacitor to a given voltage
• triggers a comparator which initiates a discharge
  
• frequency can be varied by varying the current
  used for charging.
• voltage across the capacitor gives sawtooth
• voltage across the comparator gives pulse train
• Further waveshaping for other functions

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VCO-Voltage Controlled Oscillator

• Provides simple source waveforms simultaneously
• Frequency controlled by input voltage
• Oscillator produces sawtooth and then other
  waveforms derived
• Rectify saw -gt triangle
• Hard clip saw -gt square
• Soft clip triangle -gtsine

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Building the waveform

• Typically three oscillators mixed together
• Can control tuning and volume of each oscillator
• Can synchronise oscillators (i.e. choose whether
  or not to keep phase relationships constant)

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Modern Source Options

• Digital Waveshapes (e.g. Virus)
• Additive waveshape (e.g. Oscar)
• Vox Waveforms (e.g. Microkorg)

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Filtering

• Traditional analogue filter types
• high-pass
• low-pass
• band-pass (combine low and high pass)
• Notch (combine low and high pass)
• Or a combination (e.g. voice formants look like
  multiple band pass)

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RC Filters

• Time to charge capacitor
• Voltage divided between resistor and capacitor
• At high frequencies capacitor doesnt change in
  time hence less voltage across capacitor
• Hence less signal out

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Filter Response

• Cut off when 3dB drop from pass band
• RC (one pole) circuit gives 6dB / Octave
  attenuation
• RCL circuits give 12/dB and potential for
  resonance Resonance (Q) at cut-off and self
  oscillation

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Moog ladder filter

• Current determined by a control voltage
• Current controls resistance of diodes
• Voltage across circuit filtered
• Some output sent back to circuit for resonance

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VFC-Voltage Controlled Filter

• Cutoff frequency (and perhaps Q) controlled by
  input voltage
• Typical filter has fixed -6dB/oct slope and
  constant Q
• Voltage-controlled slope useful for emulative
  synth but hard to implement

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Filter control

• Control the movement of the cutoff point over
  time using attack decay and release parameters

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Digital Filtering

• FIR, IIR options
• For example simple biquad IIR filter
• Can control type, cutoff point, resonance, etc by
  setting coefficients a0, a1, a2, b1, b2
  appropriately

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Envelope

• Attack, Decay, Sustain and Release

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Modular System

• Typical synth has several VCOs, VCAs and envelop
  generators, and one or two VCFs
• Can connect anything to anything else
• A particular set of connections to make a
  particular sound is called a patch

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VCA Voltage Controlled Amplifier

• Provides control over signal amplitude
• Gain controlled by voltage input
• Several inputs provide simple mixing

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Envelope generator

• Provides programmable control for an input type
• Usually ADSR type
• Analogue implementation usually gives
  exponential, not straight lines