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.

12 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?

      1. Yes, …I think. I could never figure that out either. Electricity is analog. So the speaker moves continuously. I think some people picture the speaker/voltage moving in discreet steps, like the graph. But that is impossible. So where does the digital info transform into the analog world? And how? Is it like a wave table synth, spitting out analog waves, triggered by a digital clock?

        1. There is DA-converter in a synth with digital oscillators. Like you need a DA-converter when you stream music. “Line out”-signal is allways analog.

        2. VCOs and DCOs basically are the same except for the control signal.

          In both designs the oscialltor is controlled by voltage (aka fully analog). But while in a VCO the voltage is controlled by analog components only (mainly a capacitor and comparator), in a DCO the voltage is controlled by an IC via an digital-analog convertor. It’s important to understand, that the “digital part” only sits in the control circuit, not in the audio circuit, meaning also a DCO delivers a fully analog signal.

          The main (or only) reason why DCOs had been introduced was because (specially early day’s) VCOs easily de-tuned, some of them just after a couple of minutes while playing, which made them hard to use on stage (the Jupiter-8 i.e. had this problem).

  2. In Kawai k5000 oscillators also called DCO, but oscillators is digital. Perhaps a description of the DCO should be added – DIGITAL oscillators the same called the DCO. (like Alpha Juno, k5000 еtс).
    In Service Manual of Alpha Juno oscillators is also called DCO. Probably … these chips should have been called – Direct digital synthesis (DDS) .. based on umerically-controlled oscillator (NCO). But I have not seen that digital oscillators so called in the service manual. And you have them in turn call Digital Oscillators. Why did you decide to use just such a term? This is the correct name? Source? And what name will be the most correct? What do you think about this? Thanks! 😉

    1. You bring up a funny point, that DIGITAL oscillators are certainly DIGITALLY CONTROLLED as well, so if you wanted to be very literal you could insist that they should be called DCOs too. And there are certainly times when manufacturers chose to use different terms for types of oscillators other than what was the convention, but most commonly, “DCO” refers to a digitally controlled ANALOG oscillator.

      1. It does not matter – more or less frequently, usual or not usual. The definition must be comprehensive and unambiguous. But what about those synthesizers that digital oscillators historically called DCO – in the menu, in the manual, on the panel, in the diagram in the service manual, and later on hundreds of sites that take information from manuals – Roland Alpha Juno, Kawai k4, Kawai k5000, Korg Poly-800, …. etc. What is now a blind eye to the existence of such synthesizers and under DCO mean only analog because these synthesizers more? If all of these synthesizers do not take into account, and try to define that DCO is only an analog oscillator – that is the definition of misdirecting users who read the panel next digital synthesizer “DCO”, search into Google, and you answer them – DCO it is only an analog oscillator.

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