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.
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.
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.)
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.
Thanks for the helpful explanation!
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 ?
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?
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?
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.
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).
A digital synth always has to have a DAC (digital to analog converter) at the output stage.
The control voltage and the signal voltage are separate functions of the circuits in an analog synth.
Very useful explanation here, many thanks.
Wow thank you for this clarification
Why in Roland Alpha Juno oscillators named DCO? They are digital! And … What about Poly-800? (also DCO in manual)
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! 😉
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.
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.
I just stumbled upon this article and although BeerMans comments are already more than a year old, I felt the need to correct some of the statements for others who might read it.
BeerMan, I’m afraid to say that you are mixing up different technologies.
Kawai K4 and Kawai K5000 are fully digital synthesizers. The K4 uses PCM sample playback and you could argue that because of that it has no oscillators at all. The K5000 uses a mixture of PCM and additive synthesis using digital sine oscillators. There is the term DCO used by Kawai but it‘s a little misleading as this does NOT refer to the oscillators. Kawai calls its ENVELOPES DCO-Envelope.
The Roland Alpha Juno and the Korg Poly-800 however do use DCOs in the sense if this article. Both are ANALOG synthesizers.
She is talking about standard accepted TERMINOLOGY and you’re talking about nuances and interpretation and what should or shouldn’t be.
From all of my research, she explains the DCO perfectly. It’s a VCO paired up to a digital clock source. The digital clock source prevents the pitch from falling out of tune. The old analog synths were hideous to play live because of all of the tuning that was required. A digital oscillator many years ago was a bad thing, they sounded bad. Today the fidelity of the digital oscillator is so high, they sound virtually identical to their analog peers. Go on YouTube and listen to a Roland Jupiter-X and a Roland Jupiter-8. The X can mimic that 8 for most waveforms identically or at least barely indistinguishable and/or totally indistinguishable to your ear.
Interesting post.
Wikipedia: DCO:
“Many voltage-controlled oscillators for electronic music are based on a capacitor charging linearly in an op-amp integrator configuration.[1] When the capacitor charge reaches a certain level, a comparator generates a reset pulse, which discharges the capacitor and the cycle begins again. This produces a rising ramp (or sawtooth) waveform, and this type of oscillator core is known as a ramp core.
A common DCO design uses a programmable counter IC such as the 8253.
This provides stable digital pitch generation but square waves only. Further analog waveshaping is provided afterward. Sawtooth generation is formed by amplifying the integration of the original signal. A control voltage is used to control the sawtooth amplitude since no reset comparator and feedback loop are provided as with an actual analog oscillator core.
This method differs from an analog core (which dictates pitch through oscillatory feedback) to a pitch directly produced by a counter and controlling CPU.
So the source square wave of say an 8253, used in a bit one synth for example etc is a digital chip. You can argue that the shaping IC, which are analogue ‘make the sound’ of the oscillator and therefore it’s analogue.
However DCOs have more characteristics in common with a digital oscillator than they do with an analog one.
So it’s neither fully digital or analogue. You mention part you ‘hear’. That’s very subjective and could be argued .. is it he quartzy crystal & 8253 you are hearing or the analogue inverter afterwards. Each to their own. My view is that the 8253 is intrinsic part of the DCO sound rather than just some timer controlling device.
In the end a lot of semantics and and definitions are used.
Since it’s frequency scale is discrete, the DCO is hybrid. it’s an oscillator under digital control, but, without that digital control, it won’t oscillate. This is true for any practical DCO
Many DCOs depend even further on microprocessor algorithms for correct function, so, in fact, a DCO is a hybrid oscillator. Without the digital part, you don’t have an oscillator that you can use.
Just my 2p worth.
just as an update to my comment in this fascinating discussion. It seems that the BIT one, the synth I was referring to is actually a DIgital Oscillator rather than a DCO like with Rolands etc. This is why I was not finding the analogue oscillator component . I assumed all synths that had a 8253 IC generator were the same this is not the case.
So yep a DCO is not a digital oscillator
The BITone is..it seems
What Frank and Allison said!