Analog Boot Camp

Dec 1, 2000 12:00 PM, John Duesenberry

TYPECASTING
Several categories of synthesizers contend in the analog marketplace. A true analog synthesizer fully lives up to properties 1 and 2: it boasts continuous signals and infinitely variable parameters. Companies such as Serge Modular and Technosaurus build these system types, and a new, all-analog synthesizer has been announced by Big Briar, Bob Moog's company.

A hybrid synthesizer employs some combination of digital and analog techniques. For example, it might have true analog oscillators, filters, and envelope generators controlled with quantized knobs. Quantization makes it possible to store parameters in memory, but it also sacrifices some degree of analog resolution. The famed Sequential Circuits Prophet-5 was an early hybrid synthesizer, and several products from Studio Electronics fall into this category.

An analog-modeling synthesizer is all digital. To emulate the characteristics of true analog synthesizers, it implements mathematical models of analog circuitry. Analog modeling is a type of physical modeling, but it imitates electronic hardware instead of mechanical or acoustic systems. Software synths that employ analog modeling include BitHeadz's Retro AS-1, TC Electronic's Spark Modular, and Native Instruments' Reaktor. Hardware devices offering analog modeling include the Clavia Nord Lead and Korg's OASYS PCI system and MS2000 synth. Guitarists are mad for devices that model tube amps, speaker cabinets, and spring reverbs, such as the Line 6 Pod.

The term virtual analog seems to refer to any digital synthesizer that imitates analog features. These days, any machine with a resonant filter or a bunch of knobs is marketed as "virtual analog," a term I'll avoid.

ANALOG TECHNIQUES
Now that I've covered some of the general characteristics of analog synthesizers, I'll turn to some specific features and synthesis techniques associated with analog machines. You'll find that many of these features are implemented on analog-modeling machines. If you need a quick review of modulation synthesis basics, refer to "Square One: Modulation Synthesis Methods" in the March 1999 issue of EM and "Square One: FM Basic Training" in the April 1999 issue.

Analog distortion. The way in which analog synthesizers distort a signal contributes significantly to their sound. A good analog model offers a choice of distortion characteristics, reflecting the operational differences among different brands of analog synthesizers.

For example, if you set up the simple patch in Fig. 2 on an ARP 2600 and a Moog 55 modular system, you'd be surprised at how different they sound. On the ARP, you'd hear a shrill sound with audible harmonic and intermodulation distortion. The Moog would sound warmer and less piercing.

The difference stems from the VCAs' behavior. On both machines, the summed sawtooth signals exceed the maximum input level of the VCA. This produces severe clipping in the ARP VCA. But the Moog VCA's "soft" distortion characteristics somewhat reduce the signal's harmonic content. Fig. 3 shows a soft-distorted sawtooth wave; notice the rounding of the sawtooth ramp.

The ARP 2600's high internal signal levels and its clipping characteristics made it difficult to create any patch that was free of hard clipping. This is why many '70s records made with the 2600 sound cheesy and edgy, whereas Moog recordings usually don't.

Oscillator synchronization. Designed to lock VCOs in tune, this feature soon became a desired timbral effect in its own right. In hard sync, a slave VCO is forced to conform to the frequency of a master VCO. The slave resets to the beginning of its cycle at each period of the master. This distorts the slave waveform, usually producing a brilliant, overdriven effect. Soft sync uses a phase-locked loop technique to lock oscillators into harmonic frequency ratios without distortion.

Pulse-width modulation (PWM). A pulse wave alternates periodically between a high-voltage value and a low one. Pulse width is defined as the percentage of each cycle that the pulse wave stays at the high value. For example, a square wave is a pulse wave with a width of 50 percent. In PWM, an audio-frequency modulator varies the pulse width, producing many harmonic components that sound like a complex form of amplitude modula-tion. With a low-frequency (below 20 Hz) modulator, PWM sounds a bit like phasing. PWM is often implemented in analog-modeling synthesizers.

Filter modulation. This type of synthesis requires a filter with a continuously variable cutoff frequency that can be controlled by an audio-frequency modulator. Most analog VCFs meet these requirements, but digital implementations of audio-rate filter modulation are rare. This is too bad; it's an interesting technique, often sounding like amplitude modulation with strong peaks in the spectrum.

Step sequencers. Analog sequencers were inspired by tape loops. They were designed to produce short, looping patterns of melodies, rhythms, and accents. An analog sequencer is driven by a clock signal and repeatedly steps through a series of individually tuned voltages. Generating pitch sequences was the most common application of analog sequencers. The stepped voltages were applied to the pitch-control input of a VCO, producing a repeating melodic sequence that was typically about 16 notes long. Most users of step sequencers never figured out how to vary the steps' duration, although it was quite possible to do so.

A few analog masters, such as Morton Subotnick and Roger Powell, devised amazing variants of this essentially boring procedure. Digitally implementing step sequencing is trivial, and it's making a comeback due to its kinship with fashionable looping devices such as samplers and drum machines.

Sample and hold. Also driven by a clock, a sample-and-hold circuit periodically takes a "snapshot" of its input signal and holds that value until the next clock pulse. This process is similar to that of a sampler's analog-to-digital (A/D) converter, which takes a snapshot of the incoming analog signal's instantaneous level many times per second and stores that value until the next snapshot. However, a sample-and-hold circuit takes its snapshots much more slowly than an A/D converter.

On many vintage machines, such as the ARP 2600, the preset input to the sample and hold was noise. Thus, the output was a series of random voltage values, the forerunner of the random LFO waveforms seen on almost every modern synthesizer. Fortunately, you can do many more things with a sample and hold. For example, the patch in Fig. 4 uses a step sequencer with a sample and hold, each independently clocked. A sequence of five voltages is sampled, and the output is applied to a VCO. This produces a pattern of five pitches that varies continuously as the two clocks go in and out of phase.

FURTHER READING
If you want to delve into analog lore, you'll have to hunt for old, out-of-print publications. Check out these titles:

- James Michmerhuizen's user manual for the ARP synthesizer (originally published in 1971; reprinted in 2000 by the author). You can order it at world.std.com/~jamzen/ theBachWorks/arpman.html.

- Analog Electronic Music Techniques (Schirmer, 1985) by James Wagoner and Joel Naumann.

- The Technique of Electronic Music (Schirmer, 1981) by Thomas Wells.

- Electric Sound (Prentice Hall, 1977) by Joel Chadabe. A historical overview of electronic musical instruments.

Frankly, the analog synthesizers of the '70s were a headache to work with, compared with today's imitations. If you find a vintage machine, play with it for fun, but don't buy it unless you're a historian or nostalgia addict. If you seek analog or analog-like sounds, you're much better off with a modern hybrid or analog-modeling synthesizer. Although it might not meet true analog specs, it will be pitch-stable and have patch memory and MIDI compatibility.