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Contemporary Keyboard

June 1977
Synthesizer Sound Systems, Part II: Front Ends
Bob Moog

The front end (circuitry preceding the power amplifier) of a synthesizer sound system provides level adjustment (for loudness balance), some equalization (for tonal balance), and a means of mixing all the keyboard signals into one or more output lines. Many keyboard/synthesizer sound systems also provide additional goodies such as reverb, compression, and frequency-dividing crossover networks for biamping. I'll discuss each of these functions in turn. First however, let's talk about requirements for the basic characteristics: overall frequency response, distortion, and noise.
There is no reason for a synthesizer sound system front end to be short on either low-frequency or high-frequency response. Unlike power amplifiers or speakers, low-level front-end circuitry can easily be designed to cover the entire 20-20,000Hz audio-frequency spectrum to within one or two dB. Distortion is another matter. The most common kind of distortion is simple clipping, where the peaks of the signal waveform are literally clipped off because they are too high for the circuitry to handle. Clipped signals are characterized by an obviously bright, raspy quality, and by an accompanying muddiness if the signal happens to be polyphonic. In addition to simple clipping, many popular integrated circuits that are used in front ends will actually overload when called upon to reproduce waveforms with sharp edges. This type of overload gives rise to a type of distortion called transient intermodulation (TIM), which can be bothersome when raw sawtooth or square waveforms are passed through the front end circuitry, either alone or in combination with other signals. TIM produces an elusive muddiness that may not be immediately obvious (as old-fashioned clipping is), but that definitely detracts from the fat, transparent sound of a clean system. Finally, noise, like distortion, has the effect of muddying an otherwise clear sound. Like overall frequency response, noise is easy to control through proper front-end design. Remember that audible 'bumps' in frequency response can sometimes be compensated for, but noise and distortion are 100% undesirable in keyboard sound systems, and cannot be filtered out or otherwise reduced, once they are generated by a poorly designed front end.
When the time comes to select a synthesizer sound system, or just the front end for one, use your ears to check overall frequency response, distortion, and noise. Use one or two high, unfiltered sawtooth waves from a synthesizer as a test signal to detect TIM. Use the same power amplifier and speaker to evaluate all the front ends. Compare the loudnesses of the difference frequencies that arise from distortion as you switch from one front end to another. Next, use a low-pitched sawtooth wave or pink noise as your test signal to evaluate smoothness of overall frequency response. Finally, play your synthesizer as you would during a performance, and reject any front end that doesn't seem smooth and distortion-free. This type of listening test is much more useful than spec sheets in evaluating front ends. Front end spec sheets always look good.
Now for the specific features of a front, end. Most keyboard systems have four or six inputs. As a rule, each input section has its own level control and at least two tone controls. These controls are intended to provide loudness and tonal balance among the instruments in your ensemble. An important function of the input level control is to attenuate the input signal so that it doesn't overload the tone-control circuitry. Overload indicator lights are helpful in setting input attenuators so that signal levels within the front-end circuitry stay below the level at which clipping distortion begins.

The simplest tone control complement is the two-knob bass and treble cut/boost. These perform in the same way home music system tone controls do, and are capable of correcting most tonal balance problems. A third control, for midrange cut/boost, is often handy for controlling "honk" in wideband sonorities. Parametric equalizers are the most versatile and sophisticated tone-balance equalizers. A typical parametric equalizer has one knob for setting the frequency range and another knob for determining the amount of cut/boost. Most professional recording consoles have input sections with two or three parametric equalizers. Parametrics enable you not only to fine-tune the tone balance of your ensemble, but also to actually alter the perceived timbre of an instrument to suit your taste. If you want a little extra sparkle from your strings, or blare from your horn sounds, parametric equalizers will get you closer than regular cut/boost controls.
The mixer portion of a front end follows the input sections and combines all the signals into a composite mix. Some front ends include a graphic equalizer for processing the mix signal. this is a useful feature in compensating for room acoustics. "Full feature" front ends may also include a compressor, which is basically an amplifier circuit whose gain is automatically reduced when its input signal exceeds a preset threshold. A compressor in this application serves to keep the power amplifier and speaker from overloading. A well-designed, properly adjusted compressor prevents much audible distortion when you are playing at high volume, but has no audible effect when you play at lower volume.
Reverberation is often useful in livening up electronic sounds, especially those that are supposed to simulate acoustic instruments. Most built-in reverbs are actually electroacoustical delay lines that create a series of closely spaced echoes. They are also capable of producing noise, "twang," and generally undesirable sounds. A smooth reverb section will allow sounds to decay without flutter or metallic coloration. Very few built-in reverbs will respond smoothly to highly percussive sounds.
Next month's column will delve into some of the more common sound-modifier boxes of the sort favored by guitar players, and will discuss how these devices can be used with synthesizers.