On Synthesizers: Modulation Part II: Vibrato

Synth pioneer Bob Moog's original column for Keyboard explored issues affecting musicians and technology developers.
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Continuing our celebration of 40 years of Keyboard, we are presenting Bob Moog's original "On Synthesizers" columns in their entirety. This week, see also the 1980 NPR interview with Dr. Robert Moog on Electronic Musician


Bob Moog


Modulation, Part II: Vibrato

This column deals with some of the fine points of vibrato, an important musical effect that may be achieved with frequency modulation. The basic patch is shown below. If you have a synthesizer, hook it up to match the block diagram. As you do, visualize how the signals are flowing during the modulation process. After a while, setting up modulation will become as natural as using a swell pedal. 

A violinist produces vibrato by rapidly moving his left hand back and forth to vary the string's length, and therefore the tone's pitch. His left hand is the modulating oscillator and the string is the tone oscillator. 

To produce vibrato on a synthesizer, you need at least one voltage-controlled tone oscillator, one non-voltage-controlled modulating oscillator, and an attenuator between them. 

[Ed. Note:Synthesizer block diagrams are simply flow charts that indicate the path of control and audio signals in a patch. All that is required to read them is a basic understanding of input, output, and module functions. In general, modules are indicated by blocks, and are usually labeled as to what their function is (see A in the diagram given below). Waveform indications (drawings of the waveshape), filter cutoff frequency notations, oscillator tunings, and so on are writ ten inside the module's block. The output of a module is drawn coming from its right side (B), regardless of whether the signal is functioning as a control voltage or an audio signal that is to be processed later. Control signals are drawn entering a module from its underside (C). Input signals that are being processed and modified by a module are shown entering it at its left side (D).

Attenuators are depicted by the electrical symbol for a variable resistor (E). The understanding of block diagrams is probably easier for those who have dealt with large modular synthesizers whose functions need to be patched together externally. However, understanding the modularity of even the smallest prepatched synthesizer is of paramount importance to the understanding of the theory behind their use.)

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We will be concerned with two basic controls that are on all synthesizers except those which are wholly preset: modulation rate (or modulation speed) and modulation amount (or modulation intensity). In addition, there is usually a selector for picking a specific modulating waveform.

There are certain practices associated with vibrato that musicians tend to employ. A musician typically starts a tone with no vibrato, and builds the vibrato up as the tone progresses. They do not use vibrato on short notes. They tend to use vibrato on high notes with greater speed and intensity than on low notes. And, to a lesser degree, they speed up the vibrato as they build the intensity, and ease off the vibrato as the note ends.

A synthesizer performer can exercise the same sort of control over vibrato. First, select the appropriate modulating waveform, either sine or triangular. (Sawtooth and rectangular waveforms have sharp edges and are therefore unsuitable for vibrato.) Next, be sure that the control signal path from the output of the modulating oscillator through the modulation amount attenuator to the control input of the tone oscillator is set up. Play a tone on the keyboard. As you listen to the tone, turn up the modulation amount control (a slider, rotary knob, or wheel, depending on your brand of synthesizer). When you hear a slight bit of modulation, carefully set the modulation-rate control for the correct speed. The setting of this control is critical—about six times per second. Fine-tune the modulation speed while playing the keyboard. The exact setting for the best-sounding vibrato will depend on what range you're playing in, what tone color you're using, and, of course, your own personal taste.

When playing modulated tones, you will probably want to keep your "panel hand" near or on the modulation amount control. Determine how far you have to move this control for maximum usable vibrato, and then place your hand so you can accurately feel the control's maximum position. At first, practice some standard violin, cello, and trombone licks, using a tone color that is appropriate to the music being played. By referring to well-known playing patterns like this, you will efficiently develop technique. Of course, once you have some "chops" on the modulation amount control, you can apply your skill to controlling any type of modulation, or develop new, distinctively electronic ways of applying vibrato.

Next month, I'll discuss the use of frequency modulation in producing clangorous sounds.

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