Bob Moog - On Synthesizers: Modulation, Part I

May 23, 2016

Modulation in electronic music is the use of one waveform (the control) to shape or texture a property of another waveform (the modulated signal). Modulation may be classified according to the type of control signal (for example, a trill is square-wave modulation, vibrato is sine-wave modulation, the sample-and-hold effect is stepwise modulation), or according to the parameter of the modulated signal, which is being varied (tremolo is amplitude modulation, automatic wah-wah is spectrum modulation, and so on). In a typical performance synthesizer there are several sources of control and several voltage-controllable parameters (or control-signal receptors). The number of possible source-receptor combinations is typically a dozen or two on a modest-size synthesizer, and several hundred on a large modular instrument. Indeed, it would not be difficult to fill a whole book with a list of all the types of modulation and their characteristics.

How about the musical importance of modulation? Does an electronic musician have to have a thorough knowledge of the technical details of all types of modulation before he can modulate in public (just as keyboard players are taught to practice the scales in every key), or will a more general understanding of modulation suffice to begin with? Well, a general understanding is sufficient, but it is essential, I believe, for a synthesist to understand what properties all types of modulation have in common. I will devote this column to exploring a few ways of understanding any type of modulation.

Modulation As A Carrier Of Musical Expression.
Sound is not music. Sound is merely a carrier of information. Our ears and brains are very good at detecting changes in sound. When these changes carry a message with artistic content and are in forms that do not abuse our built-in sound-receiving and processing equipment, then we have music.

Modulation is the means of defining sound changes. This is not to say that modulation must always be precise and unvarying. One of the most valuable means of control that an electronic musician has is the ability to continuously vary the strength, shape, and speed of the modulating control signal. Furthermore, a control signal itself may be modulated by a second control signal, giving rise to a hierarchy of sound changes. A very simple example of this is delayed vibrato. Vibrato is produced when a relatively slow (six vibrations per second or so) sine wave is used to control the frequency (pitch) of an audio tone. If a slowly rising voltage is used to control the amplitude (strength) of the 6Hz sine wave, the vibrato will build in intensity. In delayed vibrato, this second, slowly rising control voltage is started by the musician when he presses a key. Thus the vibrato builds up gradually, whenever a new note is played. The musician exercises a great deal of control over the changes in the sound; simply by determining when the second control voltage starts and stops. This is an example of how synthesizers have expanded the concept of musician control from direct physical production of sound (as on acoustic instruments) to control over multiple layers of sound processing. The modulations then become carriers of information, along with the tone itself; the musician exercises control by determining how the modulation changes.

Modulation, The Ear, And The Brain.
Our ear-brain networks have a well-defined characteristic speed of response. If a sound pattern consists of one click per second, we hear the clicks separately. We can actually keep track of them and count them. On the other hand, if we speed up the clicks so that there are one hundred per second, we no longer hear individual clicks. We hear a steady low-pitched tone. Somewhere between these two repetition rates is a gray area where our ear and mind struggle to "track" sound changes. Within the pattern repetition rate range of, say, three to fifteen per second, we are aware of the periodic nature of the modulation, but we perceive it as a pattern or texture rather than as a succession of discrete sound changes. Vibrato and trill are examples of modulations in this speed range.

From the musician's point of view, the classification of modulation into these speed ranges is more important than a classification according to the type of control signal or according to the varied parameter. The slow modulations (less than two or three pattern repetitions per second) like siren, slow sample-and-hold, and so on, generally have a rhythmic feel, and may actually be an integral part of the rhythmic structure of the music. Medium-speed modulations (between three and fifteen repetitions per second) are effective carriers of nuance, since they are easily heard without having a strong rhythmic feel. The exact speed and regularity of speed of such modulations are extremely important. There is a big difference, for instance, between a 5Hz vibrato and a 6Hz vibrato, or between a perfectly steady electronic vibrato and the uncertain "wavering" vibrato of a trumpet player playing a very high note.

Modulation And Sidebands.
At speeds above 15Hz or so (fast modulation) the modulated sound loses its 'pure pitch' quality, and its timbre becomes thick and clouded. This is because the modulation process gives rise to new pitches which engineers call sidebands. Sidebands are generated in any modulation process. When the modulation speed is slow or medium, the sidebands are generally below the frequency range of human hearing, and are therefore not heard by themselves. In fast modulation, however, the sidebands are in the audio frequency range, and are heard as out-of-tune harmonics. The most important fact, though, is that the sideband frequencies generated during modulation depend only on the control signal frequency and the frequency of the modulated tone, and not on the type of modulation. In other words, in fast modulation the shape of the modulation itself and its frequency relative to that of the modulated wave are more important than the choice of modulated parameter in determining the resultant tone color.

Next Issue: Frequency And Waveform Modulation.
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