An analog (or analogue) synthesizer is a particular type of synth that uses analog circuits and signals to produce sound along with a variety of filters. Most analog synths operate using a method called subtractive synthesis which is a foundation of sound design and electronic instruments. Early iterations of this instrument were primarily monophonic but polyphonic versions have now also been developed.
Analog synths began to emerge in the late 1920s into the 1930s with a variety of new technologies being produced. Early synthesizers used thermionic-valves (also called vacuum tubes) and electro-mechanical devices to generate sound. In 1930, Friedrich Trautwein invented the Trautonium. It is an electronic synthesizer which uses a resistor wire over a metal plate to produce sound (– later this wire would be replaced with a keyboard in analog synths). The sounds produced initially came from neon-tube relaxation oscillators which created sawtooth-like waveforms. Around 200 of the first Trautoniums were made and sold from 1933-35. Its development was later carried over by Oskar Sala, who went on to create radio, concert, folk and mixture trautoniums. This instrument and other early electronic instruments like it such as the Ondioline and the Hammond organ were precursors to the analog synthesizer.
As electronic computer technologies developed into the 1960s and 70s, analog synths started to be built with operational amplifier integrated circuits with potentiometers (type of resistor) being used to control the different parameters of sound. These synths. Were primarily modular and were made up of multiple components. Moog Music amongst a handful of other companies became the main producers of these synths and began to introduce the component of independent modules that could be patched together. Modules control and add different oscillators, effects, and filtering such as voltage-controlled oscillators (VCOs), voltage-controlled amplifiers (VCAs), voltage-controlled filters (VCFs), low-frequency oscillators (LFOs), envelope generators, ring modulators, reverb, sequencers and mixers.
In the later part of the 1980s, analog synthesizers began to be replaced by digital synthesizers which grew to be a staple in the electronic/techno music of the 1990s. The price of the digital counterpart was beyond the reach of many musicians so the demand for the cheaper analog synthesizer surged. As many up and coming artists continued to use and reproduce these analog sounds, the analog synth rose to popularity again and along with it a number of drum machines that could be created from them.
A power source provides voltage that generates a waveform by oscillating electrons– similar to when a string is plucked on an instrument, the vibration produces a signal. These waveforms can come in a variety of different shapes including sine waves, square waves, sawtooth waves and triangle waves. Each wave shape gives the sound a distinct tone that can be manipulated to create various sounds. These wave generators are called oscillators in the synthesizer. They are often called VCOs (voltage-controlled oscillators) for their ability to use the difference between voltages to create audio signals within the analog synthesizer. An oscillator controls many parameters of the initial sound including the pitch and frequency which are dependent on the speed of the vibration which is measured in Hertz (Hz).
Beginning in the 1960s/70s, analog synths primarily use one of the main types of sound synthesis called subtractive synthesis. This form of synthesis takes the waveform and filters out specific frequencies to carve out the sound. In this filtration, the harmonics of simple waveforms are run through a voltage-controlled resonant low-pass filter that attenuate the waveform. By cutting out certain frequencies and harmonics, the sound becomes richer in certain areas allowing for different timbres to be created and mimicked.
Modulators and Filtering
The waves from the oscillators can move through one or numerous signal modulators and filters to make unique sounds.
- Amplifiers control and modify the amplitude of the signal which can be heard as volume. When the signal grows, the perceived volume becomes louder and when the signal attenuates, the signal becomes quieter.
- Envelope generators work alongside amplifiers and contour the loudness of the signal. They control the attack, decay, sustain and release (ADSR) of the sound. The attack controls how fast the signal occurs once it has been triggered, the decay controls how long the signal will take to fully dissipate once it has been triggered, the sustain controls how a signal will hold once it has been triggered and the release controls the total timespan of the signal.
- Low-frequency oscillators (LFOs) are a type of oscillator used to alter other parameters including the VCO itself. With this oscillator you can achieve effects like vibrato and sweeps.
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- https://www.synthtopia.com/content/2004/03/15/modular-analog-synthesizers-return/ 
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- https://www.sweetwater.com/insync/analog-vs-digital-synthesizers/ 
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- https://www.electrical4u.com/voltage-controlled-oscillator/. Missing or empty
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