(musTh625) Synthesis

Synthesis in outline form

Analog Synthesis -> Synthesis

Analog synthesis originally involved synthesizers that generated and modified electrical signals that fluctuated in an analogous manner to the air pressure fluctuations that we hear as sound. Most synthesizers today use digital circuits and programs to generate and process sound, but these synthesizers are usually modeled after analog devices. You will find the terms analog synthesis and synthesis are used interchangeably.

Modular

Analog synthesizers are generally modular in design. The different components are independent of each other, and audio signal can usually be routed flexibly.

Basic Components

Generators. Generators produce periodic or aperiodic fluctuating signals. The two main types are oscillators and noise generators. Oscillators generally produce one of the elementary waveforms (sine, triangle, sawtooth, or square), although digital systems can use any arbitrary fluctuating signal. Noise generators produce aperiodic, or random signals.

Modifiers or Processors. Processors effect some change on the fluctuating signal. This category is large and includes what we generally think of as effects (reverb, compression, etc.) and filters. Envelope generators and LFOs (Low Frequency Oscillators) also fall into this category.

Controllers. Controllers generally refer to devices designed to modify sound in performance, or to provide performance information to generators and modifiers.

Generators

Elementary waveforms are described by the shape of their oscillation (or wave) and have specific timbral properties.

• Sine wave: the shape of a sine (or cosine) function, containing only the fundamental frequency.
  
• Triangle wave: the shape of an equilateral triangle, containing odd partials at an amplitude ratio of 1/partial#-squared. (1, 3, 5, 7, at 1/1, 1/9, 1/25, 1/49)
• Sawtooth wave: the shape of a tooth on a saw, a ramp with instantaneous change, containing all partials at an amplitude ratio of 1/partial#.
• Square wave: a type of pulse wave, the square wave has a duty cycle of half on and half off. It contains odd partials at an amplitude ratio of 1/partial#.

A noise generator generates a randomly fluctuating signal. There are generally two types of audible noise:

• White noise: equal amplitude distribution across frequency range.
  
• Pink noise: equal amplitude distribution per octave range. 

Since higher octaves contain larger frequency ranges than lower octaves, white noise sounds brighter than pink noise.

Modifiers: Filters

Filters take a complex signal and attenuate or boost amplitudes of designated frequency ranges. Filter types are named after the type of signals that are passed through unchanged.

• Lowpass filters allow frequencies below a specified cutoff to pass through unchanged. Frequencies above the cutoff are reduced or eliminated.
• Highpass filters allow frequencies above a specified cutoff to pass through unchanged. It is the opposite of a lowpass filter.
• Bandpass filters allow frequencies around a specified center frequency to pass through unchanged.

Filters do not allow one frequency to pass through, and then fully eliminate an adjacent frequency. Instead, filters have attenuation slopes that describe their amplitude reduction over a frequency range. A first order filter (or 1 pole) has a slope of 6 dB per octave. A second order filter (2 pole) has a slope of 12 dB per octave. Bandpass filters combine the slopes on either side of the center frequency, so a second order bandpass filter has the same slope as a first order lowpass or highpass filter.

Although it would seem intuitive that you would want the steepest slope to your filter cutoff, this is not always the best musical situation. Filters work by using delay lines to mix current and past output together. The steeper the slope, the longer the delay line. Longer delay lines can start to affect transient clarity and can create phase problems.

Modifiers: Envelopes

An envelope is a function that changes over time. The most common envelope type is the amplitude envelope, which modifies the amplitude of a given note over time. Envelopes can have any number of sub-parts, but the most common design is the ADSR envelope. The ADSR envelope has an Attack time, initial Decay time, Sustain level, and Release time.

Envelopes can be applied to any synthesis parameter. Other parameters that are frequently controlled by envelopes include filter cutoff frequency and oscillator pitch.

Modifiers: LFOs

A Low Frequency Oscillator (LFO) is an oscillator that fluctuates at a sub-audio rate (0 – 20 Hz, sometimes a bit more). LFOs are used as control signals, modifying some parameter of a sound in a periodic way. They can be used to create amplitude or pitch vibrato, as well as fluctuations of any parameter that could be controlled by an envelope.