Moog Monday - On Synthesizers: Control Voltage Routing

Image placeholder title

(This column originally appeared in the December 1978 issue of Contemporary Keyboard magazine.) 

ONE OF THE GREAT MUSICAL advantages of voltage control is its inherent flexibility. In principle, any control signal source, or combination of sources, can be used to vary any voltage-controlled device (receptor). Similarly, a control signal or combination of control signals can be applied in any proportion to any number of receptors. My past five columns provide many illustrations of the musical efficiency and versatility that is directly attributable to voltage control's interconnection flexibility.

The synthesist generally thinks of a particular source-destination connection (routing) as a specific musical operation. For instance, vibrato is the routing of a slow sine wave to the frequency control input of a VCO, while the ability to play discrete pitches, which is completely different from imparting vibrato, is the routing from a keyboard controller to the VCO. In a given synthesizer system, the total number of possible individual control signal routings is equal to the number of control sources multiplied by the number of control receptors. In a typical modular system with, say, a dozen control signal receptors and a dozen control sources, well over a hundred separate control signal routings are possible: The use of patch cords or matrices allows the musician to select a) which routing he or she wishes to use, and b) what sort of attenuation or other control signal processing will be in each of the routing signal paths. Since it is unusual for a musician to need more than a few of the many possible control signal routings at any one time, patching in attenuators and control processors enables him or her to fill a great many musical needs with a minimum amount of hardware.

Compact performance synthesizers generally have no or limited patching capability. However, since these smaller instruments have fewer control signal sources and receptors than modular systems have, it is practical in these instruments to provide front panel controls or switches that establish the most musically important routings without additional patching. Two panel control systems are frequently used. Each has its own strong and weak points.

One system is called receptor-oriented, and is shown in Fig. 1. In this approach, each receptor (voltage-controlled device) has its own bank of control attenuators. Control sources often feed several attenuators. Since each attenuator is committed to one particular control routing, the musician associates a given musical effect with the attenuator that "brings the effect in." The advantages of receptor-oriented control routing are a) immediate accessibility of many effects, and b) ease of mixing the effects. Since many control attenuators are used in a receptor-oriented system, each attenuator must be small and therefore limited in how accurately it can be set.

Image placeholder title

The other routing system is called source-destination routing, and is shown in Fig. 2. Here, two selector switches select the control source and the receptor to be connected together. A single system attenuator is connected between the two switches. Since only one (or perhaps two) attenuators are used in this system instead of a separate attenuator for each desired routing path, it is feasible to make this attenuator large and easy to manipulate, or to provide an electronic attenuator whose gain is controlled by a device such as a keyboard touch sensor. However, control signal mixing and simultaneous voltage-controlled effects are difficult or impossible to achieve with a source-destination system.

Image placeholder title

Thus, in a receptor-oriented system, many effects are instantly and simultaneously available. In a source-destination system, individual effects may be more precisely and conveniently controlled. Many performance synthesizers combine the two systems to provide both accurate nuance control and rapid access to a large number of effects.

Bob Moog Takes A Vacation

This will be my last column for several months. Our family has moved to the mountains of western North Carolina. During the past summer we shared a century-old log cabin with a few hospitable mice and snakes. Now we are in our new house, where much of the interior carpentry remains to be done, fuel for our wood-burning furnace has to be cut, and similar mundane amenities need to be taken care of. For the next few months I will be deeply engrossed in studs, sheet rock, and similar unelectronic materials. Later next year I plan to be "back in the saddle" in a small laboratory next to our house, building custom electronic musical instruments and writing a new CK column series. In this new series, I plan to write on a variety of topics of interest to the growing ranks of knowledgeable, serious synthesists. If you have any ideas for this new column series, please send them to me, care of Contemporary Keyboard.

Y'all have a happy holiday!

For more articles by Bob Moog, please visit

Image placeholder title