Analog Boot Camp

Dec 1, 2000 12:00 PM, John Duesenberry

Learn the true meaning of the word "analog" and other '70s arcana.

Analog synthesizers, real and emulated, are enjoying renewed popularity today. Indeed, analog devices of all types might be more numerous now than they were during the heyday of ARP, Moog, and Buchla. At the dawn of the 21st century, there is steady demand for the technologies of yesteryear. This article will help you understand some basic analog-synthesis concepts and introduce you to some of the key features of analog synthesizers. Then you'll be able to better evaluate the analog and pseudoanalog products flooding music stores.

The term analog has nothing to do with resonant filters, ring modulators, or any other widely advertised "analog" features. Vintage analog machines did have these features, but so do digital devices. Analog applies properly to signals and the devices that generate or process them, not to particular synthesizer options.

ANALOG DEFINED
Consider the quintessential analog device: the microphone. A mic responds to fluctuations in air pressure (that is, sound). It puts out corresponding fluctuations in electrical "pressure" (that is, voltage). If you were to plot both air pressure and voltage variations over time, the graphs would look very similar (see Fig 1.).

The fluctuating voltage at the mic output is a signal, not a sound. The signal is an electronic representation, or analog, of the original sound. That's all the word analog means - there's no mystery to it.

Whether it originates in a mic or electronic circuitry, an analog signal represents sound directly and continuously. The voltage does not move in discrete steps from one level to another; it flows smoothly in an infinite continuum of voltage levels. In the analog universe, even stepped waveforms such as square waves move continuously. A square wave's leading and trailing edges aren't truly "square." Examine a square wave closely on an oscilloscope, and you'll see slightly rounded edges, because the voltage takes a finite time to rise and fall. No physical oscillator can achieve the infinitesimal rise time of the ideal square wave.

Analog signals have two key properties: (1) they are continuous, and (2) their parameters - frequency, amplitude, and phase - are continuously (and infinitely) variable. These properties make analog-synthesizer signals profoundly different from digital synthesizer signals. By definition, digital synths represent signals as numbers. Digital signals are quantized into a finite number of discrete steps, and there are no levels between steps. Likewise, parameter values on a digital synthesizer are quantized into a finite number of steps. Smaller step sizes give a digital synthesizer higher resolution; the higher the resolution, the better the synth can approximate the infinite resolution of analog devices.

INFINITE CONTINUUM
It follows from properties 1 and 2 that between any two positions on an analog oscillator's frequency knob, there are an infinite number of other settings. As a result, tuning two analog oscillators to the same frequency is virtually impossible.

This statement is not as absurd as it seems. Oscillators resemble the strings that produce individual piano notes. Any piano tuner knows that two or three strings can be tuned to approximately, but not exactly, the same frequency. This slight detuning between strings produces an amplitude/ timbral fluctuation called beating, which is considered musically desirable.

You tune analog oscillators as you do piano strings, listening for beats. Yet even if you did tune two analog oscillators to an exact unison or octave, they'd soon drift out of tune. Even the best analog oscillators are inherently unstable, unlike digital oscillators, which are referenced to a highly accurate system clock. Over time, analog oscillators will drift randomly up or down in frequency. Beating is practically unavoidable on true analog synthesizers, unless the oscillators are synchronized or otherwise artificially stabilized.

The warm sound of beating oscillators with complex waveforms is a popular characteristic of analog synthesizers. To get a similar effect on a digital synthesizer, you must detune the oscillators, preferably with a little randomized frequency variation.

These concepts also apply to filter cutoff and Q, signal gain, and all other parameters: if you want analog sounds from a digital machine, you need the highest resolution you can get. Manufacturers seldom reveal how they quantize parameter values. When you check out an analog-modeling synth, turn the knobs slowly and listen carefully for discontinuities.

TAKE CONTROL
Just as with audio signals, control signals determine how an analog synthesizer sounds. To understand this, you need to understand the concept of voltage control. Synthesizer modules such as voltage-controlled oscillators (VCOs), filters (VCFs), and amplifiers (VCAs) have one or more control-signal inputs. A varying voltage (signal) applied to a module's control input causes a particular parameter - a VCO's frequency, a VCF's cutoff frequency, or a VCA's amplitude - to vary in a similar manner. Analog control signals are just as continuous as audio signals. Any parameter under voltage control fluctuates continuously in proportion to the control signal.

Take amplitude envelopes, for instance. An analog envelope generator outputs a smoothly varying voltage. If the gain of a VCA is controlled by this envelope, it rises and falls just as smoothly.

Digital control signals work quite differently. Again, I'll use envelopes as an example. The function of a digital envelope generator is to generate a stream of numbers at some periodic rate. These numbers are used to scale the instantaneous amplitude values of audio samples. The rate at which the envelope generator produces its values is called the control rate. It's usually a fraction of the audio sample rate. For example, if the audio sample rate is 44,100 Hz, the envelope generator might produce values at a control rate of 441 Hz. In this case, there would be a level change every 100 samples. If the control rate is too slow, there can be an audible "staircase" effect.

MODULARITY
A modular synthesizer is constructed from an expandable number of functionally separate components called modules. Interconnections between modules are pretty much unrestricted. Many famous analog machines (such as the Moog 55 and ARP 2500) were modular, so the term is often associated with analog synthesizers. However, a digital synthesizer can be modular as well. (Plug-ins are the "modules" of today.)

A well-designed modular synthesizer makes almost no electrical or logical distinction between audio and control signals. Analog modular synthesizers let you control almost any module with the output of any other module. For example, you can "play" a VCA's gain with a keyboard voltage (analog synths use keyboards that generate different constant voltages for each note). You can also control a VCO's waveform with a pink-noise generator's filtered output or create complex control-signal hierarchies with feedback loops.

Most hardware synthesizers today are constrained by preset signal paths. If you want a real modular architecture, you'll have to look among the many software synthesizers currently available (see "Going Soft" in the July 2000 issue of EM).