SOUND SWINGERS
Jan 1, 2007 12:00 PM, By Doug Eisengrein
If you are an electronic musician and don't yet know what the letters VCO, LFO or DCO stand for, we have to talk. Those ubiquitous three-letter acronyms all stand for a particular type of oscillator, which is the most basic of sound sources and exists on classic and most modern synthesizers, as well as many drum machines, soft synths, hardware, software effects processors and so on. According to Encyclopedia Britannica, an oscillator is a “mechanical or electronic device that produces a back-and-forth periodic motion,” and according to Merriam-Webster's online dictionary, the definitions of oscillate include “to swing backward and forward like a pendulum” and “to vary above and below a mean value.” In the case of musical oscillators, those “back-and-forth” or “above-and-below” descriptions may be said to refer to the circuits or lines of code that cycle and generate periodic sound waves of vastly differing frequencies and shapes, thus generating notes and timbres.
CHARGED UP
Analog or digital, sinusoidal- or square-producing, oscillators come in several different types; the three discussed in this column are not the only kinds. They are, however, the classics and still the most commonly found in electronic-music gear. The acronyms VCO, LFO and DCO stand for voltage-controlled oscillator, low-frequency oscillator and digitally controlled oscillator, respectively. They are all undoubtedly different creatures. Early analog synths had no digital components and incorporated analog VCOs. As the name implies, a voltage-controlled oscillator essentially uses a regulated (“clean”) DC voltage applied to a diode or capacitor, which in turn varies a tuned circuit, resulting in a specific desired frequency. In short, by altering the applied voltage (usually via potentiometers in synths), you directly alter the frequency of the resulting sound. The catch here is that “clean” is a relative term with regard to voltage, and heat will cause oscillators to become unstable as well. Thus, you have the characteristic (and sometimes maddeningly unpredictable) signal drift inherent in most purely analog synthesizers. The type (or shape) of the waveforms generated by oscillators can vary from one oscillator to another; some of the most common waveforms seen are square, sinusoidal (sine), sawtooth (or saw), triangle and random. Besides being used as basic sound sources, VCOs are also sometimes implemented as a means to modulate other sources in electronic music gear, such as other oscillators.
THE SILENCE OF THE LFOs
Low-frequency oscillators operate in much the same way as VCOs, yet as their name implies, they bear the notable character trait of operating at much lower frequencies. In fact, they often produce signals near or below the 20 Hz threshold, which is approximately the lowest frequency audible to most human ears. Although LFOs are sometimes used for the express purpose of generating just-audible, extreme sub-bass frequencies, more often they are used as control signals. Like the output of a standard VCO, these control signals typically assume the form of periodic waves such as square, sine or saw, but are instead used as modulation sources for all manner of targets such as other oscillators, synthesizer envelopes, audio-signal panning and much, much more. Their advantage as controllers is precisely in their ability to exist beyond the human ear's reach. Beyond hardware synthesizers, LFOs (or digital emulations of them) have been incorporated for sound-enhancement purposes in a swath of other electronic gear, such as soft synths, samplers, effects processors and drum machines.
DIGI OR DODGY?
Digitally controlled oscillators are another beast altogether. They were introduced into synthesizers much later than VCOs and LFOs, and are icons of the digital age — the age of precision. Alternatively referred to as numerically controlled oscillators (NCOs) within the worlds of physics and electrical engineering, DCOs do not rely on voltage to generate or control their resultant tones. Instead, they use precise math in the form of bits going in and coming out that are phase-locked to a CPU clock; the clock is known as a fixed timebase. DCOs are capable of generating a wide range of precise frequencies that are not subject to the voltage and thermal fluctuations that analog VCOs are, making them musically stable. But DCOs have their own set of issues. For one, their cold, hard precision — some call it a lack of character — is exactly what makes some people prefer true analog synths with VCOs. Besides precision, in digital synths that provide controls for sweeping “smoothly” across the frequency spectrum, a staircaselike sound can sometimes be heard in the sweep; that is a sure indicator of the bits, and not pure persistent voltage, involved in the sound-generation mechanism. Also, due to a mathematical principle known as Nyquist's theorem, DCOs are limited to producing frequencies that are below half the timebase frequency, though with modern computer CPU clock speeds, that is hardly an issue. More notable is that, in the process of producing a waveform, DCOs often introduce undesirable digital artifacts such as “aliasing” (a form of overlapping duplicate signals), which require extra filters to clean up the resulting sound.
WHOLE LOT OF SHAKING
Whether you work in the digital or analog realms, consider this: Each discrete oscillator can output a discrete waveform; most synths, whether hardware or software, will have two or more oscillators onboard, and oftentimes each oscillator is capable of generating numerous different waveform shapes (and therefore different tones). In many cases, one or more of these oscillators can also be used as a modulation source. With all that, you begin to see how programmable synthesizers have such enormous sound-creating potential thanks in large part to oscillators.
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