In the 1960s Robert Moog developed a voltage-controlled modular synthesizer. It consisted of a collection of individual electronic modules designed to synthesize sound or alter sound, and those modules could be interconnected in any way desired by using patch cords to connect the output of one module to the input of another module. The user could thus establish a “patch”—a configuration of interconnections for the modules—to create a unique sound, and could then play notes with that sound on a pianolike keyboard that put out voltages to control the frequency of the sound.
I’ll describe a few of the module types that were commonly found in such a synthesizer.
An oscillator is a circuit that creates a repetitively changing signal, usually a signal that alternates back and forth in some manner. That repeating oscillation is the electronic equivalent of a vibration of a physical object; when the frequency of the oscillation is in the audible range it can be amplified and heard. The oscillators in the Moog synthesizer were voltage-controlled, meaning that the oscillation frequency could be altered by a signal from another source.
Electronic oscillators often created certain specific types of waveforms, which were so common that they have come to be known as “classic” waveforms. The most fundamental is the sinusoid (a.k.a. sine wave), which is comparable to simple back-and-forth vibration at one particular frequency. A second classic waveform is the sawtooth wave, so named because it ramps repeatedly from one amplitude extreme to the other, and as the waveform repeats it resembles the teeth of a saw. It contains energy at all harmonics (whole number multiples) of the fundamental frequency of repetition, with the amplitude of each harmonic being proportional to the inverse of the harmonic number. (For example, the amplitude of the 2nd harmonic is 1/2 the amplitude of the fundamental 1st harmonic, the amplitude of the 3rd harmonic is 1/3 the amplitude of the fundamental, and so on.) A third classic waveform is the square wave, which alternates suddenly from one extreme to the other at a regular rate. A square wave contains energy only at odd-numbered harmonics of the fundamental, with the amplitude of each harmonic being proportional to the inverse of the harmonic number. (For example, the amplitude of the 3rd harmonic is 1/3 the amplitude of the fundamental, the amplitude of the 5th harmonic is 1/5 the amplitude of the fundamental, and so on.) A fourth waveform, the triangle wave, ramps back and forth from one extreme to the other. It contains energy only at odd-numbered harmonics of the fundamental, with the amplitude of each harmonic being proportional to the square of the inverse of the harmonic number. (For example, the amplitude of the 3rd harmonic is 1/9 the amplitude of the fundamental, the amplitude of the 5th harmonic is 1/25 the amplitude of the fundamental, and so on.)
An amplifier, as the name implies, serves to increase the amplitude of a signal. In a voltage-controlled amplifier, the amount of signal gain it provides to its input signal can itself be controlled by some other signal.
In a modular system, an oscillator might be used not only to provide an audible sound source, but also to provide a control signal to modulate (alter) some other module. For example we might listen to a sine wave oscillator with a frequency of 440 Hz and hear that as the musical pitch A above middle C, and then we might use a second sine wave oscillator at a sub-audio frequency such as 6 Hz as a control signal to modulate the frequency of the first oscillator. The first, audible oscillator is referred to as the carrier signal, and the second, modulating oscillator is called the modulator. The frequency of the carrier is modulated (altered up and down) proportionally to the shape of the modulator waveform. The amount of modulation up and down depends on the amplitude of the modulator. (So, an amplifier might be applied to the modulator to control the depth of the modulation.) This control of the frequency of one oscillator with the output of another oscillator is known as frequency modulation. At sub-audio rates such as 6 Hz, this frequency modulation is comparable to the vibrato that singers and violinists often apply to the pitch of their tone. The depth of the vibrato can be varied by varying the amplitude of the modulator, and the rate of the vibrato can be varied by varying the frequency of the modulator. When, as is often the case, the modulating oscillator is at a sub-audio control rate, it’s referred to as a low-frequency oscillator, commonly abbreviated LFO.
The diagram below shows schematically how a set of synthesizer modules might be configured to produce vibrato. The boxes labeled “cycle~” are sine wave oscillators, the box labeled “*~” is an amplifier, and the box labeled “+~” adds a constant (direct current) offset to a signal. In this example, the sine wave carrier oscillator has a constant signal setting its frequency at 1000 Hz, and that signal is modified up and down at a rate of 6 Hz by a sinusoidal modulator which is amplified to create a frequency fluctuation of + and – 15 Hz. The result is a sine tone centered on 1000 Hz with a vibrato going up and down 6 times per second causing a fluctuation between 985 and 1015 Hz.
vibrato by means of frequency modulation
A modulating LFO can also be applied to a voltage-controllable amplifier to create periodic fluctuations in the amplitude of a tone. This low frequency amplitude modulation is known in electronic music as tremolo. In electronic music we distinguish between the terms vibrato to describe periodic low-frequency frequency modulation and tremolo to describe periodic low-frequency amplitude modulation. (The Italian word tremolando means “trembling”. In instrumental music the words tremolo and tremolando are used to mean fast repetitions of a note, rather than subtle amplitude fluctuations. However, in electronic music the word tremolo is used to mean periodic change in loudness, similarly to the way vibrato is used to mean periodic changes in pitch. In vocal and instrumental music, vibrato is almost always synchronized with a corresponding fluctuation in loudness—which would be called tremolo in electronic music—thus further complicating the use of the term.)
Very slow frequency modulation can create a sense of the pitch gradually sliding. Sliding pitch is called glissando (an Italian musical term derived from the French word for sliding/gliding, glissant). In the sound example below, a sawtooth modulator LFO at a rate of 0.5 Hz is used to modulate the frequency of a carrier sawtooth oscillator, causing it to glide from 220 Hz to 440 Hz every 2 seconds. The same modulator also controls an voltage-controlled amplifier, which shapes the amplitude of the carrier so that it goes from loud to soft with each two-second glide.
An interesting possibility when using oscillators for modulating the frequency and/or amplitude of another oscillator is that the modulating oscillator might be amplified to produce very extreme fluctuations of pitch or loudness. Perhaps even more importantly, we can also vary the rate of the modulator from sub-audio rates (for classic LFO effects such as vibrato and tremolo) up into audio rates. When this happens, the modulation becomes so fast that we can no longer hear the effect as individual fluctuations, and the frequency modulation and/or amplitude modulation actually produces what are called sidebands—new frequencies that are related to the sum and the difference of the carrier and modulator frequencies, which are not present in the original oscillators themselves but which are generated by the audio-rate interaction of the modulator and the carrier. This can result in various strange and uniquely electronic sounds.
In the sound example below, the frequency of a sawtooth carrier oscillator is modulated by a sine wave LFO passing through an amplifier. At first the frequency of the LFO is sub-audio and its amplification is very low, so the effect is one of very subtle vibrato. Then the amplifier is turned up until the vibrato is extremely wide, making an inhuman warbling. Then the rate of the LFO is turned up until it is well into the audio range, so that we no longer hear it as a frequency fluctuation, but instead we hear many sidebands, creating a complex, inharmonic tone.
A filter is a circuit that can alter the timbre of a sound by reducing or increasing the amplitude of some of the sound’s frequencies. Common filter types are low-pass (pass low frequencies through unchanged, while reducing the amplitude of high frequencies), high-pass (pass high frequencies while reducing the amplitude of low frequencies), band-pass (pass or even emphasize a certain region of frequencies in the spectrum while reducing the amplitude of frequencies below and above that region), and band-reject (a.k.a. notch, to reduce the amplitude of a range of frequencies while passing through the frequencies above and below that region). Such filters can often be varied in terms of the frequencies they affect. Low-pass and high-pass filters are characterized by their cutoff frequency, the frequency at which they begin to substantially alter the frequency spectrum of a sound. For band-pass and band-reject filters, one more commonly refers to the center frequency, the frequency at the center of the affected band. A voltage-controlled filter can have its center frequency or cutoff frequency modulated by another signal source, such as an LFO. This can result in interesting and uniquely-electronic effects of timbre modulation, such as a a periodic filter vibrato or a gliding filter sweep. The sound example below is a 110 Hz sawtooth oscillator being passed through a low-pass filter; the cutoff frequency of the filter is at first being modulated by a sine wave LFO varying the cutoff frequency between 1540 Hz and 1980 Hz, and then the cutoff frequency is slowly swept down to 0 Hz.
We have discussed a few of the most significant types of voltage-controlled modules in the Moog modular synthesizer, such as oscillators, amplifiers, and filters. We’ve discussed four of the classic waveforms produced by the oscillators: sine, sawtooth, square, and triangle. These waveforms can be amplified and listened to directly, or they can be used as control signals to modulate the frequency of another tone. When we modulate the frequency of a tone at a sub-audio rate with a low-frequency oscillator (LFO), the resulting effect is called vibrato; when we modulate the amplitude of a tone (by modulating the gain level of an amplifier) at a sub-audio rate with a low-frequency oscillator (LFO), the resulting effect is called tremolo. Audio-rate modulation of the frequency and/or amplitude of an oscillator results in complex tones with sideband sum and difference frequencies. We’ve also discussed four common filter types: low-pass, high-pass, band-pass, and band-reject. Just as the frequency and the amplitude of a tone can be modulated, the cutoff frequency or center frequency of a filter can likewise be modulated by an LFO to create variations of timbre.