a DCO is not a digital oscillator

VCO vs. DCO vs. Digital Oscillator

I keep hearing and seeing people referring to DCOs as digital oscillators, which is not correct, and a sad insult to DCOs in my opinion as a lover of DCO synths. So I decided to fight misinformation by making a post here explaining the actual difference between VCOs, DCOs and digital oscillators.

Analog vs. digital, actually

What do the words analog and digital even mean? Their meanings are more general than some people realize. Analog means continuously changing, like a circuit in which potentials (voltages) and current flow are always present and always changing.

Digital refers to any system in which there are two states (which can be thought of as 1 and 0, high state and low state, or on and off) and the outputs can only switch immediately between them, with no slopes transitioning between them gradually.

VCOs – Voltage Controlled Oscillators

Most synth people are probably familiar with the idea behind a VCO (voltage controlled oscillator).  The frequency is controlled by the amount of control voltage being put in, following the basic rule of electricity that a higher applied voltage yields a higher frequency. The most important sonic characteristic of a VCO is the fact that it is typically somewhat unstable.

A VCO will always be experiencing slight fluctuations in temperature which change the behavior of its components.  And since it will always be sharing a power supply with other circuits (the other parts of the synth), there will be slight fluctuations in the voltage it can use due to the changing voltage and current needs of its neighboring circuits.  These factors lead to slight fluctuations in the frequency of the oscillator’s output.  This can show up as pitch drifts over the span of minutes or seconds, warbles or tiny irregularities in the waveform.

graph of vco waveform
VCO with fluctuating frequency

T represents the period (amount of time it takes for a full cycle) at the the target frequency (e.g., 440hz for A4).

In this graph, the VCO is oscillating at the correct frequency in the long run, but there are slight short-term fluctuations in its frequency (first too fast, then too slow).

Digital Oscillators

A digital oscillator is very different. A digital oscillator has a processor or other digital data source spitting out a series of discrete values, each value represented by a series of 1s and 0s, at a standardized time interval to create an approximation of a continuous waveform.

digital oscillator triangle wave graph
a triangle wave as produced by a digital oscillator

This is an exaggerated depiction of a digitally generated waveform, though many years ago there were digital oscillators this crude.  Digital oscillators designed decades ago sounded gnarly or noisy, or just sounded different from analog oscillators because of all the information they naturally had to leave out. But we’ve finally gotten to the point that digital oscillators designed today can have such a high resolution (resolution represented on this graph by the number of “steps” making up each “triangle wave”) that they don’t sound any different from analog waveforms.

DCOs – Digitally Controlled Oscillators

Some people describe DCOs as “somewhere in between” VCOs and digital oscillators or “hybrid digital/analog oscillators” but I take exception even to those characterizations.  DCOs are true analog oscillators that are just synced to a digital clock source.  The part of the DCO that is doing the oscillating is analog– producing an analog waveform! They were invented in the 80s as a way to avoid the pitch instability of VCOs.  A DCO’s digital clock source can produce the exact correct frequency for each note, ensuring that the oscillator is perfectly in tune across all notes. (I should probably mention that because a DCO is an analog oscillator, it is still very difficult for it to produce a perfectly symmetrical triangle wave like the one shown in the graph, and the same goes for a VCO. I just used triangle waves because they’re easy to draw in MS Paint.)

dco triangle wave graph
DCO triangle wave perfectly synced to note’s target frequency

Some people prefer VCOs because their pitch drifts and variations can make a synth sound more colorful and naturalistic.  Some people are excessively sensitive to intonation and can’t stand very unstable VCOs.  I’ll stand by DCOs until the end.

Thanks for reading, especially if you are just a casual synth user– you are now one step closer to being a synth nerd.

5 thoughts on “a DCO is not a digital oscillator”

  1. Sorry but I just don’t get it , how does the waveform can be analog if it’s not created by the voltage ( I mean how the waveform is created , if there is a sample rate it’s not analog right ?

    1. Hmm, let me see if I can explain it another way. A DCO-based synth does have digital “oscillators,” typically generated by a digital IC or even the CPU itself, but they are never heard and function only as a clock source. The analog oscillator circuits that they are used to clock, which are the ones that you hear, are actually very similar to those that might appear in a fully analog monosynth. The way that they are clocked is similar to how you can sync two analog oscillators by forcing one to reset to the beginning of its waveform’s period whenever the other one returns to the beginning of its own. Similarly, the analog oscillators of a DCO polysynth are synced to the digital clock source “oscillators.” Does that help?

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