VCO, DCO and Digital Oscillators
Breaking Down the Key Differences of Oscillator Architecture
VCO: Voltage Controlled Oscillator
Classic synthesizers from the 1960s, 1970s and 1980s were built primarily from VCOs for sound generation. VCOs are "all analog": both the waveshaping and frequency clock that determines the wavelength are voltage controlled analog. VCOs have some inherent instability in them, which causes a frequency modulation / frequency jitter, intonation tuning offsets over multiple octaves, and a tendancy for the oscillators to have changing tuning characteristics depending on a variety of environmental conditions. With high voice count VCO poly synths, this results in every key press having certain offsets in tuning from the target pitch, as well as natural phasing / detuning motion between multiple oscillators on a given voice.
Classic mono and poly synths from the 1980s and before used VCOs, including MiniMoog, MemoryMoog, Sequential Prophets, Yamaha CS Synths, Oberheim SEM and OBX synths, Roland Jupiters, and a majority of the other legendary synths from the 60s through the 80s. The production of VCO based synths dipped in the late 90s and 2000s, but has seen a resurgence as of late with new offerings like the Sequential Prophet-6, OB-6, Moog One, Korg Prologue and Minilogue and countless modern monosynths.
DCO: Digitally Controlled Oscillator
In the 1980s, as demand for higher voice count polyphony synths expanded, a new type of oscillator was designed, the Digitally Controlled Oscillator. DCOs solved the problem of keeping multiple voices tuned within reasonable tolerances. DCOs have an analog waveshaping core with the rich series of harmonics associated with analog oscillators, and no aliasing artifacts. The part of the circuit that is digital is the frequency clock, which determines what wavelength/note the waveshape should oscillate at. DCOs should really be called "DCAOs" as they are digitally controlled analog oscillators.
There are many highly acclaimed and used DCO based synths, including the Roland Juno, Roland JX, Korg Poly-800, Oberheim Matrix-1000, Dave Smith Evolver Series, Dave Smith Prophet 08, Dave Smith Prophet Rev2, Mopho, Tetra, Novation Bass Stations, and others.
DO: Digital Oscillator
Digital oscillators are fully digital in both waveshaping and frequency clock generation. Digital synths have ultra accurate tuning characteristics, with no frequency jitter or drift. One of the downsides of many Digital Oscillators is that there may be aliasing aritifacts as a result of low oversampling rates with waveshaping. This aliasing may produce unwanted noise or artifacts that sounds unnatural or distorted. Modern Digital Oscillators and VST Digital Oscillators have improved over time, and the aliasing issue has become minimal. Digital synthesizers can do many advanced techniques that their analog counterparts can't. One of the first real jumps in technology occured with the Yamaha DX7 synth, which offered advanced Frequency Modulation (FM) to create new sounds that never existed before. Modern Digital Synths include Virtual Analog Digital Oscillators, and many modern digital synths include additional digital synthesis types like wavetable synthesis, granular synthesis, and sample based synthesis.
NCO: Numerically Controlled Oscillator
Numerically Controlled Oscillators, in terms of synthesizers, generally refers to the latest evolution of Digital Oscillators that use cutting edge FPGA architecture and ultra high oversampling rates to eliminate aliasing artifacts.