The beginning of class was spent discussing the signal path and significant components used in Subtractive Synthesizers. We identified three basic types of components: Generators, Processors & Controllers. Generators include circuits that create audio signals, essentially Oscillators and Noise Generators. Processors include devices that alter audio signals such as Filters, Amplifiers, Chorus and Distortion. Controllers generate control voltages that modify the behavior of Generators and Processors. These include devices such as the Keyboard, Low-Frequency Oscillators, Envelope Generators, and Lag & Sample-and-Hold processors.
In the classic analog synths of the 1970's (such as Moog and ARP), D.C. Control Voltage (CV) was used to pass control signals between the synthesizer's components. So, for example, the monophonic keyboard acted like a voltage divider with a voltage output that was proportional to the pitch position on the keyboard, that is, the higher the key, the higher the voltage. This signal, which was a steady voltage as long as one key was held, would be fed to Voltage-Controlled Oscillator (VCO) that would put out the pitch corresponding to that voltage. The keyboard CV could simultaneously be sent to the Voltage-Controlled Filter (VCF) to raise the cutoff frequency in proportion to the VCO's pitch.
VCO's offer several waveform choices. By far, the most popular waveforms for subtractive synthesis are Square and Sawtooth. In addition, it is common to find Triangle (eesentially a filtered Square), Variable Pulse-Width Rectangular (like a square, but where the positive and negative peaks are of uneven duration. Sine waves are less common because the waveform contains only one harmonic and is therefore not rich enough to be filtered (after all, it is subtractive synthesis!).
Noise Generators come in two flavors: White Noise, which sounds like thin static, and Pink Noise, which sounds deeper like rushing water.
VCF's also come in two types: the Low-Pass Filter (LPF), which filters out high frequencies, and the High-Pass Filter (HPF), which filters out lows. Filters for synthesis have some significant differences from passive filters used on consoles or microphones. For one thing, they use steeper slopes. Synthesizers typically use slopes of 18 or 24 dB per octave for a more control of harmonics beyond the cutoff frequency. Also, filters offer Resonance control, which boosts frequencies at the cutoff, for a more dramatic filtering effect. An LPF and an HPF can be combined to get either a Band-Pass or a Band-Reject (Notch) Filter.
The Voltage-Controlled Amplifier (VCA) is used to create the dynamic envelope of the patch. Typically, this is controlled by an Envelope Generator (EG). There are a couple of EG types: the most basic is the AR Generator, which allows variable control of the sound's Attack Time and Release Time; The ADSR Generator expands on this idea by providing additional Decay Time and Sustain Level controls; even more flexible is the Levels & Rates Generator, which allows more complex envelopes to be specified as a series of vectors. EG's are triggered by a keyboard's Gate output.
The Low-Frequency Oscillator (LFO) is similar to the audio oscillator except that is able to run in the sub-audio range, typically in the range between .1 Hz to 20 Hz. Offering similar waveforms to those mentioned above, the LFO's CV is used to control other devices:
The Lag Processor delays CV changes, for example, to create a portamento (glide) effect at the VCO. Sample-and-Hold (S&H) samples an incoming voltage and holds it. When noise is the incoming signal, the S&H outputs a randomly changing signal that can be sent to control the VCO or VCF.
Be able to identify three basic types of components: Generators, Processors & Controllers. Generators include circuits that create audio signals, essentially Oscillators and Noise Generators. Processors include devices that alter audio signals such as Filters, Amplifiers, Chorus and Distortion. Controllers generate signals that modify the behavior of Generators and Processors and even other controllers. These include devices such as the MIDI Keyboard (Trigger, Note Number, Velocity), Low-Frequency Oscillators, Envelope Generators, and Lag & Sample-and-Hold processors.
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Generators: Oscillator (VCO) Noise Generator Inputs: Voltage Control Signals Outputs: Audio Signal |
Processors: Amplifier (VCA) Filters (VCF) Distortion & Overdrive Reverb & Delay Effects Flanging, Chorus & Ensemble Dynamic Effects (Compression, gating, etc.) Inputs: Audio Signal Voltage Control Signals Outputs: Audio Signal |
Controllers: Envelope Generator Keyboard pitch Keyboard Trigger and Gate Signals MIDI Velocity Low Frequency Oscillator (LFO) Lag Processor Sample & Hold Generator Inputs: User Input Voltage Control Signals Output: Voltage Control Signals |