Building a Synthesizer, 9

A Field Guide to Oscillators

If you own a synthesizer, or have worked with software synthesizer plugins, you have no doubt encountered the term “VCA” or “Voltage Controlled Oscillator,” and indeed we have seen it before in this series. Today most synthesizers are fully digital and just have “oscillators,” which produce arbitrary waveforms using a computer, but you may also encounter “DCOs” or perhaps just “Oscillators,” and wonder if these names actually mean anything distinct.

In this post I’ll explain the “oscillator” part of how synthesizers produce sound, look at three designs for actually building an oscillator, and talk about the differences in sound that you may hear from these designs.

Voltage Controlled Oscillators

I won’t spend a ton of time explaining how a VCO works because I’ve already done that. For the sake of this post I’ll just say that the entire oscillator, including both the control of the tuning/timing as well as the shape of the wave produced, is analog.

The earliest commercial synthesizers,¹ such as Moogs, ARPs, and Buchlas from the 1960s all used VCOs.

One of the major downsides of a VCO is you have to tune it. A lot. Some later commercial synths with VCOs introduced an automatic tuning feature which helps considerably, but they will still drift as you play them due primarily to temperature changes but also due to aging of electronic componaents and the inherent differences between two “identical” components. Two transistors from the same manufacturer with the same model number and made in the same production run can have very different electrical characteristics, for example. Also, as the name suggests, they are Voltage Controlled, that is, controlled with an analog voltage which might be produced by a keyboard or a sequencer, and there is no single standard for control voltages, so you have to tune the VCO to produce the correct note when you, for example, press keys on your keyboard.

On the other hand, VCOs often sound amazing, and the tuning drift may be part of the reason! One surefire way to fatten up nearly any synthesizer sound is to add another oscillator and detune the second oscillator so it doesn’t produce precisely the same frequency as the first. Make it “fatter” still by using a LFO (Low Frequency Oscillator) to vary the detuning over time just a bit. Our ears hear this difference in frequencies as a beating sound which might be pleasant or unpleasant, depending upon the amount of difference between the two oscillators. Do this with two VCOs and there will be a certain amount of drift between the two oscillators as you play, and this can be a very pleasant sound. For a while, anyway, until it goes way out of tune and you have to stop playing and retune it.

VCOs are said to have a characteristic sound due both to a limitation in the waveshapes which are used (typically, square, sine, sawtooth/ramp, triangle, and a few others) and also due to the variations in tuning. Happily, though, many people really like the sounds that they produce.

Digitally Controlled Oscillators

A DCO still produces an individual cycle of oscillation using analog electronics, but uses a digital timer (typically controlled via a microcontroller) to trigger each cycle. This allows the DCO to always have “perfect pitch.” Like a VCO, a DCO is also an analog oscillator which uses analog circuits to produce the wave shape that you hear.

A DCO will never require tuning. The note “A 440” will always sound at 440 Hz. You can detune a DCO, which as above you might do if using two oscillators together to produce a fatter sound. But there will not be any random “drift” in the tuning.²

The first synth with a DCO was the Roland Juno-6, which was available in 1982.³ DCO-based oscillators are not so common today. The majority of commercial synthesizers are digital, and folks looking for an “authentic” analog sound often want a VCO in spite of tuning headaches.

Essentially, the way a DCO works is the microcontroller sends a signal to the analog circuit when it’s time to produce each cycle of the waveform. You can think of a microcontroller sending a pulse every 1/440th of a second to produce an “A” note. From there an analog circuit produces a single cycle in almost exactly the same way that a VCO does, and then it stops and waits for the next pulse from the digital controller.

I would explain how a DCO is implemented in hardware, but Thea Flowers has already done a much better job of this than I ever could, so if you’re interested (and it is very interesting, I think!) then I recommend you go read her article!

Conceptually, the only “audible” difference between a DCO and a VCO is that the DCO will never drift in the tuning of its fundamental frequency. From a practical, real-world standpoint, however, they might be implemented with different components and hence sound very distinct, and even two “identical” analog oscillators can sound different because of the difference between “identical” analog parts. Also, VCOs, as noted, do drift in pitch, and this also makes them sound different than a DCO.

Digital Synthesis

Digital Synthesis produces a sound by using a computer program to produce a waveform digitally. This includes samplers/sample players, all software synthesizers, and “virtual analog” synths which use clever programming to try to reproduce the sound of a VCO or DCO. Some of them do it quite well!

The first mainstream digital synthesizers were the Casio VL-1 in 1979, which was something of a toy but considerably cheaper than the other digital synthesizers available at that time which were > $10000 in 1980 dollars. Other notable milestones in making digital synthesis a mainstream technique were the E-mu Emulator, released in 1982, and the Yamaha DX-7, in 1983.

The sound of a digital “oscillator” will be precisely what its programmers specify. This could be a perfect sine wave, playback of a sample, a modeled recreation of an analog oscillator, small slices of a sample as with a wavetable synthesizer, or many other things. A digital synthesizer will have a Digital to Analog Converter (DAC) in its signal path; a VCO or DCO based synth will not require this.

Some people will tell you that they can hear the digital “stepping” as, for example, a 16 bit, 44.1 kHz digital oscillator changes its output through its 65,536 possible amplitudes over every 1/44100s. I will tell you that these people are wrong, and that they cannot hear this.⁴ However, they may in fact hear the difference between a VCO and a “perfect” digital recreation of the “same” VCO, because, once again, even two “identical” VCOs may sound different because of differences between the analog parts, their variance with temperature, etc.

While it’s both theoretically and practically possible for a digital oscillator to reproduce the sound of an analog oscillator well, there’s no denying that many synth manufacturers have not bothered to do it well. Sometimes this is on purpose; a wavetable synth, for example, is not trying to reproduce the sound of a VCO. But even in cases where a synth manufacturer explicitly claims to have an “analog sound” from their digital oscillators, well, some of them do it better than others.

Synthesizers Today

The majority of hardware synthesizers, and all software synthesizers, sold today are digital. Digital oscillators are quite common even in the modular synthesis world. However, there are still fully analog hardware synthesizers produced, in keyboard, desktop, and modular forms, generally with VCOs.