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Everything Audio

August 1975
Operations of Moog Synthesizer Modules And Components
Familiarity with the functions of synthesizer modules enables the user to select a custom-planned instrument.
Robert C. Ehle

IN TEACHING the operation of synthesizers in several schools, I have found that a desirable first step is to take each module in the synthesizer and describe its operation. Thereafter, the different interconnections can be studied after the student has a working knowledge of each module.
Here we describe the typical modules found in a medium-sized synthesizer, in this case, one manufactured by the R. A. Moog company, although other brands are very similar. The modules are as follows: oscillators, mixers, keyboard, voltage-controlled amplifier, envelope generator, filters, ring modulator, envelope follower, and sequencer. The last three modules are extras in the Moog synthesizer. The ring modulator and the sequencer are usually mounted in separate cases. They are included in other manufacturer's synthesizer models, however, and are basic components in electronic music procedure. For that reason they are included here.
A purchaser of a modular synthesizer may select any set of modules he prefers and thus have a custom synthesizer from "off-the-shelf" components. Most customers prefer to take the advice of experts and accept a standard complement of modules in a specially built cabinet. Such a standard complement may be purchased by number from most manufacturers

THE MOOG OSCILLATORS

The Moog model 901-A oscillator controller is, in effect, an amplifier with the special capability of taking the algebraic sum of all of the inputs presented to it. There are five such inputs: the three jacks marked control voltage and the two controls marked fixed control voltage. The two controls are a range switch and a vernier, both of which are marked off in volts. The summing amplifier takes the algebraic sum (accounting for the sign and the magnitude of each component) and then generates a current proportionate to the sum. The current is a voltage-controlled current.
It is this voltage-controlled current that is actually fed to the model 901-B oscillator. The oscillator contains a capacitor which stores this current as it is fed in. As the capacitor fills or charges, the level of electrons or charge increases in the capacitor. A level detector senses when the capacitor is full and rapidly discharges the capacitor. This slow charge and rapid discharge produces the saw-tooth wave, the basic waveform in the Moog oscillator. The switch on the panel of the 901-B selects one of six different capacitors, thus selecting one of six different charge rates. The knob beneath the switch is a vernier for that control.
The other three waveforms (sin, triangular and pulse waves) are generated by waveform modifying circuitry from the basic sawtooth waveform. Thus all waves have exactly the same frequency and are in phase. They may be used simultaneously for performing synchronized control functions.
Different models of synthesizers have slightly different oscillators. The difference is more in appearance than operation.

In the Synthesizer 2, the oscillator is a one-piece unit. In the Synthesizer 1, however, the oscillator appears to be constructed from three modules. That is not actually the case, as the modules are internally wired together and are inseparable. Thus, the 901-A oscillator controller, the 90l-B oscillator and the 901-C output stage are all parts of one oscillator and may not be separated. Note that only the left oscillator has an output stage; the right oscillator has none.
The function of the oscillator controller is to provide a voltage-controlled characteristic to a semiconductor oscillator which would normally be sensitive to changes in current rather than voltage. Signal connections between the oscillator controller and the oscillator are internally made. The function of the output stage is to provide an amplitude control, a switch selectable set of waveforms, and a pair of complementary outputs. Thus the output signals from the 901-C are identical to those from the 901-B with these exceptions.
The four waveforms made available are the sin wave, the square wave, the triangular wave, and the sawtooth wave. The square wave may be converted into a variable width pulse wave with the pulse-width control on the 901-A. FIGURE 1 shows various waveforms found in the synthesizer, as displayed on an oscilloscope. An oscilloscope is an ideal tool for presenting a visual picture of the characteristics of electronic music. The drawings illustrate various waveforms and illustrate the concepts of variable-width pulses.

OSCILLATOR TERMINOLOGY

Various names are often used for the component part of the different waveforms. When the complex but repetitive wave is analyzed by means of Fourier techniques, the results are a series of harmonically related frequencies called harmonics. The fundamental frequency (the lowest frequency) is the first harmonic. The sin wave contains only the fundamental, while the square wave and the triangular waves contain only odd-numbered harmonics (1, 3, 5, 7, 9, etc.) and the sawtooth wave contains both odd- and even-numbered harmonics.
The components of a wave that are present are called partials; thus, the second harmonic is the second partial of a triangular wave, but the third harmonic is the second partial of a square wave (because the second harmonic is absent). But partials do not have to be harmonic (in which case the wave is not periodic and cannot be analyzed by Fourier analysis). For example, the piano tone contains frequencies which are not harmonic; these are called partials but not harmonics. They are overtones, however, because they are generated by subdivisions of the same string. They are non-harmonic, due to string stiffness and the end effect, which makes the overtones higher than true harmonics. For that reason, the piano cannot be synthesized by subtractive synthesis, but only by additive synthesis.
The three terms-partial, overtone, and harmonic-have different but related meanings, and their differences are worth noting. To sum up, any component of a complex sound is a partial. Partials having small whole number ratios are overtones, and partials generated by the same string or pipe are overtones (a generic term).

THE MIXERS

The Moog model 982 mixer is necessary for nearly all work done with the synthesizer. FIGURE 2 illustrates a block diagram of model 982. Note that the two input controls act as panners between the two inputs. The tone controls and level controls apply only to their particular channel marked A or B. Also, the earphone jacks are stereo and have inputs from both channels. That permits the synthesizer to be used as a self-contained instrument with a pair of earphones. The center of the tone controls (twelve o'clock) is the flat position.

Other types of mixers will be useful in electronic music. One of the most useful is the panner. The panner has special controls allowing one output to be reduced while another is increased. That may be used to produce the effect of moving sounds when the two outputs feed the two channels of a stereo sound system.
Simple mixers may be either "wet" or "dry," meaning powered or unpowered. Powered mixers may provide amplification and absolute independence of control of one channel from the others. They have no cross-coupling. Unpowered mixers may only provide a limited amount of isolation, and turning a control on one input will have some effect on the others. Unpowered mixers may provide only loss and no amplification. They are, however, inexpensive. The less expensive powered mixers may, in fact, be less desirable because they introduce noise or hum.

THE KEYBOARD-PITCH CONTROL

The Moog keyboard contains two types of control outputs, one operated by the keyboard and the other by the internally triggered envelope generators. Here we discuss the keyboard-controlled pitch voltage outputs.
This voltage output (actually two outputs in parallel, having exactly the same output signal) has a range of zero to 6 volts. Normally it is used to control the voltage-controlled oscillator but may control any voltage-controlled
device with this voltage range.
The controls affecting this device are the scale, range, hold and portamento. These labels apply to the effect achieved when controlling the voltage-controlled oscillator and indicate the specific parameter of the sound thus controlled. Thus, scale determines the scale produced; range, its tessitura; portamento, the connection between pitches; hold, the effect when all keys are released (with the switch on. The internal circuitry remembers the last key depressed and continues to produce that control voltage).
Of course, these controls have related effects when the pitch-control voltage is used to control, for example, the voltage-controlled amplifier. That is possible since all Moog modules have exactly the same control voltage range and are interchangeable in this respect.
By inserting a resistor, used as a voltage divider, between the output of the pitch-control section and the voltage-controlled module, the range may be reduced farther than the internal range control allows. When controlling the voltage-controlled oscillator, this permits "micro-tonal" scales and other unusual effects.

THE VOLTAGE-CONTROLLED AMPLIFIER

The Moog voltage-controlled amplifier is a singly balanced (control input only) modulator with a resemblance to the ring modulator to be described later. The difference is in the specific circuitry used to balance both inputs in the ring modulator.
Normal operation involves connecting the voltage-controlled amplifier to the envelope-control voltage output from the keyboard, thus using the voltage-controlled amplifier for envelope generation triggered by the keys on the keyboard. For this type of operation, the fixed-control voltage control on the amplifier is set to zero. The exponential/linear switch may be set either way, although an exponential curve duplicates the type of envelope produced by an acoustical instrument such as a piano. (Such an instrument seems to have a linear decay due to the compensation by the ear which has a logarithmic characteristic.) A linear decay is one of those sounds which may be synthesized, but is never heard in nature. Thus it makes an interesting resource for original music.

The voltage-controlled amplifier may also be used as an amplitude modulator. In this type of arrangement, one signal feeds the signal input while the other feeds the control input. The amount of intermodulation can be controlled by the setting of the fixed-control voltage which sets the operating point on the amplifier's characteristic curve. The voltage-controlled amplifier, when used as an amplitude modulator, produces more harmonic distortion than a well-adjusted ring modulator and is, therefore, less desirable for live-concert modulation of voices and instruments than that of instruments alone.

THE KEYBOARD-ENVELOPE GENERATION

The basic Moog keyboard contains two envelope generators which are internally connected to the keys of the keyboard. Thus, every key triggers both envelope generators in an identical fashion.
Interconnections from the envelope generators is by means of two patch cords, one for each envelope generator. The output control voltages again have a range of zero to 6 volts. They may be used to control any of the Moog voltage-controlled units. Normal use is to connect them to the two voltage-controlled amplifiers for generation of actual envelope control. Thus the controls are labeled for this application. The four controls on either envelope generator are labeled attack speed, attack height, decay time and sustain level. When the generator is used to control an envelope with a voltage-controlled amplifier these are the actual parameters controlled.
Of course, as with other Moog voltage-controlling modules, the generator may be used to control the oscillator or the voltage-controlled filter as well. In these cases an analogous effect is produced by the controls, which may not be the exact effect described by the label of the control.
The larger Moog synthesizers also contain independent envelope generators with a somewhat wider variety of control. They have a continuously variable attack time rather than the two-position switch and they have a trigger input.
An interesting variation in envelope control is to drive one signal input of the vca with one envelope generator and the control input with another. The resulting envelope will be the difference between the two envelopes. This technique may be used to produce very slow attack times on the simple envelope generator synthesizers.

THE FILTERS

Two types of filters are employed in the Moog synthesizers. These are fixed band-pass filters and variable voltage-controlled filters. The voltage-controlled filters are of two types, low-pass and high-pass. When used with an auxiliary coupler, the combination of a low-pass and a high-pass filter results in a voltage-controlled band-pass filter or a voltage-controlled notch filter, depending on how they are connected.
Fixed filters are usually supplied in banks which may be specified as quarter-octave, third-octave or half-octave, depending on the sharpness of the filter band characteristic. Narrower band filters require more units to cover the same total range. Third-octave filters require three per octave, half-octave filters two per octave, and so on.
Voltage-controlled filters are usually provided with a regeneration control which is used to regulate feedback around the filter. When turned up, the regeneration converts the filter into a resonant filter, similar to the acoustical formant filter. That is useful for musical coloration. Note that the resonance occurs at the point of maximum amplification or minimum filtering.

The Moog voltage-controlled filters have the same control voltage range as all the other Moog modules and may be controlled from all the common control devices. They may also be modulated, producing spectrum modulation.

THE BODE-MOOG RING MODULATOR

An instrument designed by Harald Bode and manufactured by the R. A. Moog Co. is the multiplier-type ring modulator with squelch. Producing a much lower distortion output than equivalent circuits set up on the Moog synthesizer, this unit is ideal for live performance and produces a particularly attractive and musical output. The inputs may be any combination of audio signals. If one of the two is a "live" acoustical instrument, and one an electronic sound source, attractive variants on the acoustical signal result. The technique is desirable for use in real-time electronic music performances.
The multiplier-type ring modulator generates very small amounts of harmonic and intermodulation distortion (other than the desired sum and difference). Thus, when two sin waveforms are presented to the two inputs, the output is also a pair of sin waves, the sum and the difference of the two inputs. Other ring modulators may generate harmonics of the modulation products as well as products of the harmonics of the input signals. These roughen the sound quality of the output and make it much less attractive to the listener.
The reader is referred to the Electronic Music Review,
vol. 1, no. 1, for detailed discussion of techniques that may be performed with the ring modulator. Some of these techniques will be repeated here for convenience.
If the two signal inputs are complex waves, such as square or sawtooth waves, having an infinite series of partials, the resultants will be two infinite series of partials, one the sum series and the other formed by the difference.
If one input is a band of white noise, and the other a sin wave, the band of white noise will appear in the output but twice as wide, or in two segments formed by the sums and the differences, respectively. If the sin wave is made to sweep, the white noise in the output will also sweep, producing the effect of "howling wind."
Sounds picked up by a microphone may be ring-modulated with themselves if the overtones are filtered from one input and the entire spectrum is presented to the other input.