Synthesizers are electronical musical instruments that produce audio signals through several analog and digital methods. The most common techniques include frequency modulation synthesis, additive synthesis, subtractive synthesis, phase distortion synthesis, physical modelling synthesis, linear arithmetic synthesis, analysis/resynthesis, as well as sample-based synthesis. The sounds produced by synths are typically shaped and modulated by low-frequency oscillators, filters, and envelopes. Synthesizers are usually played with keyboards and controlled by sequencers via MIDI. Synthesizers are widely used in film and television soundtracks.
As mentioned before, there are several techniques through which synths produce audio signals. Frequency modulation synthesis (FM) generates sounds by modulating one waveform with the frequency of another. The complex waveform that is formed can be used to modulate other signals. Frequency modulation synthesis is capable of mimicking diverse acoustic sounds. Additive synthesis generates sounds by combining waveforms into a composite sound. Subtractive synthesis creates waveforms by using oscillators and shaping them with filters to boost or eliminate particular frequencies.
There are some other techniques through which synths produce audio signals. Phase distortion synthesis is capable of replacing the analog waveform with complex digital waveforms. Physical modelling synthesis uses a set of equations and algorithms to mimic unique sonic characteristics of musical instruments. Through various steps, the sound realistically approximates the desired instrument. Linear arithmetic synthesis uses PCM samples for the attack of a waveform, and subtractive synthesis for the rest of the envelope. Sample-based synthesis uses a digitally recorded short fragment of a sound that is taken from an actual musical instrument. When this sound is chosen, it is played back at different speeds to create different pitches. Analysis/resynthesis involves bandpass filters to analyze the harmonic content of a specific sound and resynthesize the sound with a band of oscillators.
The sounds created by synths can be controlled by the operator through virtual stages or circuits, such as oscillators, low-frequency oscillators, voltage-controlled filters, envelope generators, as well as voltage-controlled amps.
Filters are essential in subtractive synthesis, being the low-pass filter the most common one. Envelopes control the way sounds change over the course of time through several parameters, such as amplitude and pitch. ADSR is the most common envelope, since it includes attack time, decay time, sustain time, and release time. Low-frequency oscillators create electronic signals below 20Hz. Voltage-controlled amps control the volume of the audio signal.