The Rock Synthesizer Manual: A Revised Guide for the Electronic Musician 0914283251, 9780914283256


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Synthesizers generate two kinds of electrical flow: audio signals and control voltages. The big difference between the two is how they're used. To find out if a signal is one or the other, you've got to see where it's going, rather than where it's coming from. An oscillator doesn't care if it's generating an audio signal or a control voltage. If its signal is routed to the control input of another circuit, it's a control voltage.

Because a synthesizer oscillator's frequency can be controlled by an external voltage, it's called a voltage-controlled oscillator, or VCO for short. Most VCOs have a sensitivity of one volt per octave. That means the pitch changes one octave for every one volt change in the control voltage. If you increase the control voltage exactly one volt, the frequency doubles, and the resulting pitch goes up an octave. Decrease the control voltage by two volts, and the pitch drops two octaves. Any change in the control voltage changes oscillator frequency. i

1

In theory, just about any function that can be controlled manually can be controlled by a voltage. In most analog synthesizers, voltages control four parameters: 1)

2) 3) 4)

oscillator frequency oscillator pulse width the cutoff frequency of a voltage-controlled filter the gain of a voltage-controlled oscillator

Only four parameters? How much can you do with just four voltagecontrolled functions? Believe me, you can do plenty, due to the sophistication of control voltage sources. Occasionally, you see functions like voltage-controlled reverb or resonance, but very seldom.

Every circuit in a synthesizer is electrically independent of the others, Playing a synthesizer is but the interaction among them makes it all work. more than just playing a keyboard. It involves connecting a number of circuits, a process called patching, by providing paths for both audio signals and control voltages. There are several ways that patching is accomplished. One It's often as simple as tweaking a few knobs and flipping a few switches. method of patching uses lengths of electrical cable with plugs at either end In most synthesi(patchcords), and another requires placing pins in a grid. zers, the most commonly used paths are provided by internal wiring among its circuits. The keyboard is connected to the oscillators to control their frequency. The oscillators are connected to the filter, and the filter is connected to the amplifier, providing a path for the audio signal. Other interconnections route other control voltages to their intended destinations. Such a synthesizer is said to be normalized.

16

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Notice that the attack, decay, and release stages all govern lengths of time, but the sustain setting determines a control voltage level. When the sustain is turned all the way up, the initial decay has no effect.

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If the sustain level is zero, the amplifier closes at a rate governed by the decay setting, and the release stage is effectively bypassed (unless the gate ends before the decay begins).

The simplest type of envelope generator has just two stages, attack and release. It's called an AR generator, and it operates like an ADSR generator with full sustain. After the attack stage, the level is sustained at its peak amplitude until the gate ceases, and the release stage begins.

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Another simplification of the ADSR generator uses the same pot to control Since one knob controls both both the initial decay and the final release. parameters, the decay and release times are equal. Some of these envelope generators also feature a switch to bypass the release stage altogether. The decay pot controls only the initial decay, and the release time is zero. This type of arrangement may save time in live performance, but it sacrifices the flexibility of a true ADSR generator.

A five-stage envelope generator, the DADSR, introduces a delay between the trigger and the beginning of the attack stage. DADSR generators are useOnly a few syntheful when you want to offset the attacks of two envelopes. sizers have DADSR generators.

24

FILTERIKG

We've learned that the timbre of a sound is determined by the number and relative strengths of its harmonic frequencies. A portion of the frequencies that make up a complex waveform may be either weakened or emphasized by passing that waveform through a filter. Filtering gives you a great deal of It actually alters the shape of the control over a signal's harmonic content. waveform being filtered. Electronically producing tone colors using filters is a process called subtractive synthesis, because you subtract portions of the harmonic spectrum from complex waveforms. The human voice is a type of subtractive synthesizer. The band of frequencies that's passed by a filter is called its passband, for obvious reasons. The attenuated portion is the filter's stopband. All analog synthesizers have at least one voltage-controlled filter, or The most common and useful VCF is called a lowpass filter. A lowpass VCF. filter passes all frequencies below and up to a certain point, and gradually attenuates all frequencies above that point. This cutoff frequency (abbreviated ¥q) can be changed manually, or it can be voltage-controlled. If you turn a lowpass filter's cutoff frequency all the way down, you close the filter and nothing can pass. When you manually open it just a little at a time, you slowly raise the cutoff frequency. At first you only hear the lowest frequency components of the signal being filtered, and the timbre grows gradually brighter. With the filter fully open, the entire waveform can pass unattenuated. By changing the filter's cutoff frequency, you can continuously alter the spectrum of any complex waveform. A lowpass filter doesn't pass all frequencies up to its cutoff point and completely eliminate all frequencies above that point. Frequencies above the cutoff are usually rolled off at a fixed slope of either -12 or -24 dB per octave.

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If the sampling rate is faster than the frequency of the input, the output rises and falls with the output.

Sampling a low-frequency sawtooth wave can produce an output called a staircase wave. When a low-frequency staircase wave controls oscillator frequency, the oscillator generates a series of ascending pitches (or descending pitches, depending on whether it's a positive or negative sawtooth). In a limited way, a sample & hold may substitute for a sequencer. Unfortunately, most sample & hold circuits only sample noise, so their only output is random.

FOOTPEDAL CONTROLLERS & FOOTSWITCHES Footpedals are very handy synthesizer controllers. An active synth footpedal generates a control voltage that's proportional to the angle of the treadle. If your synth has appropriate inputs, this voltage can be used to alter any voltagecontrolled parameter. If you apply it to the filter, for example, you can sweep the maximum cutoff frequency. Another type of pedal is a simple attenuator that can control volume or modulation depth. A pitch bending pedal leaves your left hand free for other activities.

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PEDAL A footswitch may be used to shift octaves, hold a note or chord, turn the portamento on and off, or step through a memorized sequence of patch programs. Another footswitch might extend release time, like a piano sustain pedal. Its function depends on the synthesizer's inputs. Sometimes footpedal controllers and footswitches are included with a new synthesizer. Often, they can be purchased separately from the manufacturer.

42

PROGRAMMABLE PATCH MEMORY Many synthesizers feature a microcomputer that memorizes patch parameters. Once you get a sound just the way you want it, just press a couple of switches and it's stored permanently. By assigning a program number to that sound, you can retrieve it at any time. V/hen the patch is recalled, any parameter can be changed temporarily, or the changes can be memorized as part of the program. Some synths have an edit button which allows you to alter patch parameters. Others let you modify parameters at will, simply by changing pot positions. Most programmable synthesizers contain a collection of programs from the manufacturer. You may wish to keep some and discard others. Some factor)^ programmed patches are excellent, because they're often programmed by top synthesists. Just be sure you have space in the memory to store your own programs.

You needn't be limited to a fixed number of programmed tone colors. Most instruments with patch memory include a cassette interface. You can dump, or transfer, an entire set of programs to a cassette recorder, leaving space in the memory for new information. If your synth has a verify function, you can recall a single program from a cassette without disturbing what's in the synthesizer's program banks. To replace a whole set of memorized patches, simply load new programs from a cassette to the synthesizer.

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This is where we begin our hands-on, guided tour of your synthesizer. We'll examine the function of every knob and switch, and discover how each circuit affects the others. (Some synths have slider pots instead of knobs, but to save words, let's assume yours has knobs.) We'll create sounds stepby-step, parameter-by-parameter. Now that you're familiar with how and why synthesizers make music, it's time to turn yours on and get to know your way around

46

WHERE TO BEGIN Like most electric musical instruments, synthesizers must be amplified to heard. Guitar amplifiers, home stereos, PA systems, and modular keyboard be amplifier systems are suitable sources of amplification. Many synthesizers feature an extra output jack for stereo headphones, and some models even have small amplified speakers built right in. With the appropriate patchcord, plug your synth into some kind of sound system (the best quality you can afford), and turn on the power. Adjust the S3mthesizer's output volume to about half. 1.

If your synth has programmable patch memory, put it in manual mode by switching off the memory bank or pushing the "manual" switch.

2.

Open the voltage-controlled amplifier. This may be done with a switch in the amplifier section labelled "hold", "drone", or "VGA bypass", or by turning up the initial gain or level attenuator. On some models, you may need to turn up the sustain in the amplifier envelope generator section. If there's no provision to leave the amplifier open, just pick any key and hold it indefi3.

nitely. 4. Open the lowpass filter. Turn the initial cutoff frequency all the way up, and turn the filter resonance (emphasis) all the way down. If any control inputs are turned up, turn them off.

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47

THE SAWTOOTH WAVEFORM Route a signal from oscillator one (VCO 1, oscillator A, whatever it's 1. called) to the filter. The oscillator signal attenuator can usually be found in either the mixer section or the filter section. Turn it up full, completely clockwise. Switch the oscillator waveform to sawtooth.

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If there's a switch in the oscillator section labelled "audio" (normal) and "sub-audio" (low frequency), switch it to the audio range. Patch in a control voltage from the keyboard, if necessary. Adjust your sound system's volume to a comfortable level. 2.

Turn the oscillator tuning controls to approximately center. You should be hearing a constant sawtooth waveform. Notice how bright and brassy it sounds. 3.

Some oscillators have two frequency pots, one for coarse tuning and another for fine tuning. Turn them fully counterclockwise. If there's a master tuning pot, turn it down. In addition, some oscillators have a rotary pot for selecting the footage, or frequency range. Turn that down to its minimum position.

4.

48

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Switch the keyboard to its lowest octave, if possible, and strike the lowest key. You should hear the deepest pitch your instrument can produce. Maybe you can hear the vertical edges of the individual sawtooth waves thrusting the speaker back and forth, almost slowly enough to perceive each cycle. 5.

Slowly turn the oscillator tuning controls clockwise. Listen to every frequency range along the way, noticing any subjective changes in tone color.

6.

Switch the keyboard to its upper octave and depress the highest key. The highest harmonics of the resulting sound are beyond the range of human hearing, and probably beyond the range of your loudspeaker.

7.

TUNING TO STANDARD PITCH To play with other instruments, your synthesizer should be tuned to standard pitch. If you have automatic tuning, just press the autotune switch, and you're all tuned up. If you don't, just follow these instructions:

Turn the oscillator tuning controls to approximately center. If there's 8'. range selection it its a pot, turn to Switch the keyboard to middle octave and depress A above middle C on the keyboard. 1.



Find a reliable pitch source a piano, guitar, pitch pipe, tuning fork, anything you can trust to be in tune and play an A above middle C. If your synth has an internal A-A40 pitch reference, that will do nicely.

2.



Adjust the oscillator frequency pots or the master tuning pot until the pitch closely matches your reference pitch. Raise or lower the octave, if necessary. 3.

Carefully adjust the fine tuning pot to match the pitches exactly. If your reference pitch is correct, oscillator one is now in tune. However, if the instrument hasn't warmed up sufficiently, the pitch may drift slightly. If it does, wait a few minutes and make the necessary adjustments.

4.

49

THE SQUARE WAVEFORM Turn off the sawtooth input from the oscillator to the filter, 1. pulse wave from oscillator one in its place.

Patch a

Find the oscillator's initial pulse width (PW) control. Select a 50% pulse, usually by setting the pot at 5, or at its halfway position. As you adjust the pot, listen for the tone to take on a hollow quality as the second harmonic (one octave above the fundamental) drops out. 2.

3.

Notice how the square wave's timbre differs from the sawtooth's.

Listen

to the square wave in several registers.

ALTERING PULSE WIDTH Every pulse width has its own characteristic sound, because each has a unique harmonic structure. Thus a variety of basic timbres is possible from Alas, a variable pulse wave oscillator, not all synthesizers feature continuously variable pulse width. A few allow you to switch between two or three preset duty cycles, but some only offer square waves. Depress middle C on the keyboard. 1. Turn the initial pulse width pot to its minimum audible setting, probably about Listen to this waveform's high 10%. overtone content. If the pulse width falls below 1%, the wave is never "on". Consequently, there's no frequency or amplitude; therefore, no sound. Turn the pulse width to its maximum. As you slowly rotate the pot, notice the If the pulse width exceeds 99%, there's no "off gradual change in timbre. portion of the wave. Again, no sound. 2.

Turn the control back and forth at different rates, rapidly, harmonics momentarily shift out of tune.

3.

50

When it's turned

OTHER WAVEFORMS If you have an oscillator that can generate a sine wave or a triangle 1 . wave in the audio range, listen to it. These waveforms resemble one another and are useful in similar ways. Because they lack any harmonic complexity, sine and triangle waves sound much duller than pulse or sawtooth waves. Many synthesists don't consider them necessary for subtractive synthesis.

Some oscillators have a sub-octave 2. divider, which provides a tone (usually below the fundamental, to fatten up the square waveform from oscillator one and

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switch. It turns on a simple octave a square wave) one or two octaves sound. If you have one, select a

switch on the sub-oscillator.

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MANUAL FILTER CONTROL 1.

Route a sawtooth wave to the lowpass filter.

Slowly turn the filter's initial cutoff frequency pot counterclockwise. The oscillator signal loses its brightness as the upper harmonics disappear. If the fundamental frequency is below the minimum cutoff point, you may still hear it, even with the filter completely closed. 2.

3.

Play a few notes, manually opening and closing the filter to shape each

note. If there's a multimode filter, check out the other modes. highpass, etc., as you vary the cutoff frequency.

4.

51

Switch to

THE NATURAL HARMONIC SERIES In the lowpass mode, open the filter just over halfway, resonance (emphasis) about halfway up. 1.

Turn the filter

Very slowly, turn down the filter cutoff frequency. Listen as the filter emphasizes whatever harmonics are closest to the changing cutoff frequency.

2.

Close the filter completely. Now turn up its frequency just enough to pass the second harmonic, then the third, and so on, until you've swept the entire harmonic series. Repeat the procedure with various pulse waves. 3.

FILTERING NOISE

Turn off the oscillator input and route a signal from the noise generator 1. to the filter.

Slowly open and close the filter, altering the spectrum of the noise 2. waveform. Turn the filter resonance halfway up, emphasizing the component frequencies closest to the filter frequency. As you turn the filter control, note that the emphasized component changes evenly, rather than jumping from one harmonic to the next. That's because noise contains every frequency, not just multiples of a fundamental frequency. 3.

JetIf the filter has other modes, listen to noise at various settings. plane sounds are possible by manually shifting the stopband of notch-filtered noise.

A.

If the noise generator can be switched from white to pink, flip the switch and repeat the entire procedure.

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Open up the lowpass filter and turn dovm the resonance.

Turn off the signal input from the noise generator. Route a sawtooth 2. wave from oscillator one to the filter. Depress a key in the middle range. Route another sawtooth wave from oscillator two to the filter. If there are selection switches, switch the oscillator to the audio range, under keyboard control, sync off. Turn the signal attenuator up to match the level of oscillator one. 3.

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Adjust oscillator two's frequency pots so that its pitch is very close to the pitch from oscillator one. Don't touch the master tune or the frequency pots on oscillator one it's already in tune.

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As oscillator two's frequency approaches the first oscillator's frequency, you hear a pronounced pulsation as each signal alternately cancels and reinforces the other. Whenever two audio signals of nearly the same frequency or closely related frequencies are sounded together, they produce a low^ frequency sideband, called beats, equal to the difference between the two Barely turn signals. If you hear one beat per second, they're 1 Hz apart. the fine tuning control on oscillator two until no beats can be detected. 5.

Oscillators one and two are now in unison. If you have additional audio oscillators, tune them by the same method, one at a time, using oscillator one as a reference.

6.

53

OTHER INTERVALS It's not always necessary to tune the oscillators in unison, but it's a good place to start. Some sounds require tuning to other intervals.

Raise the frequency of oscillator two until its pitch is exactly one 1. octave above oscillator one. As their frequencies converge, the beating will be less pronounced than with unison tuning, but it'll be there. Tune as precisely as you can.

Now tune the second oscillator a perfect 5th above the first. Play a few notes on the keyboard and listen. Try other intervals, like a perfect 4th, major 3rd, minor 3rd, and larger intervals. When you're finished, return to unison tuning. 2.

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DETUNING Many sounds require that the oscillators are slightly out of tune with one another. Deliberately detuning one of the oscillators thickens the sound, sometimes creating the illusion of multiple instruments. Oscillator two is detuned 1 Hz or less, causing a beat every second or more. Most often, the oscillators are just a few cents apart. (A cent is 1% of a semitone. A semitone, or half step, is the interval between two adjacent keys.)

54

PHASE

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When two signals are out of phase, they begin their cycles at slightly different times. Even with the greatest care in tuning, you might still hear two oscillator signals slowly drifting in and out of phase with one another. Phase is defined as the position of a wave in time, or the instantaneous relationship between two waves in time. The phase angle is the degree of one cycle's completion at a given point. Phase shift is the difference in the phase angle of two waves.

Whenever two waves are even slightly detuned, their phase relationship is constantly changing. They alternately cancel and reinforce each other. When one wave reaches its maximum positive value at the same instant another wave reaches its maximum negative value, the result is zero amplitude. Their opposite polarity makes them cancel each other Conversely, when two waves match out. up, in phase with each other, their amplitude is increased. When the phase relationship of two waves is between these extremes, various harmonics are emphasized and deemphasized in sequence.

55

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OSCILLATOR PHASE SYNC Your synth may have a phase sync switch for synchronizing two oscillaVflien It's on, the signal from one oscillator remains perfectly In phase with the other oscillator's signal. If the oscillator that's forced Into sync Is tuned an Interval above the other, It will phase-lock Itself to the closest harmonic from the lower oscillator. tors.

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The oscillator to be synched (usually oscillator two) has a switch 1. labelled "sjmc". Switch on the sync and turn off the signal from the other oscillator (one) to the filter.

Turn the synched oscillator's frequency up and down over Its entire range. Notice that the harmonics are Individually emphasized, but the fundamental frequency remains unchanged. 2.

3.

Now switch off the sync and re tune to unison.

56

ENVELOPE GENERATION Up to this point, we've left the amplifier at unity gain as we learned to manipulate the manual controls. Now let's examine the voltage-controlled functions, beginning with the envelope generators. 1.

Open up the lowpass filter (VCF) and patch in an oscillator signal.

Close the amplifier (VGA) by switching off the hold, drone, or bypass, or If you don't have any of by lowering the initial gain or level attenuator. these, let go of that key you've been holding since we began. 2.

Find the pot that attenuates the control input to the amplifier from the 3. envelope generator, if there is one. It's usually labelled "env amount" or "intensity". Turn it all the way up. If you don't see it, don't worry about it.

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Set the envelope generator (ADSR) attack, decay, and release at minimum and the sustain at maximum. When you depress any key, that note is instantly heard at its peak amplitude. When the key is released, the amplifier closes immediately, and the note ceases. The envelope has instant attack, full sustain, and instant release, much like the contour of an organ note.

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Advance the position of the attack pot, and listen to its effect when you play the keyboard. Try to find the setting that gives a one-second attack time. How far you have to turn the attack pot depends on the envelope generator's design. The difference between pot positions below midpoint is slight, but as you rotate the attack knob clockwise, the attack time increases exponentially. The first half of the pot's rotation may control less than one second's duration, but the second half may increase the attack time tenfold. This gives you precise control over attacks that are less than a second long. 5.

Turn the attack back down to minimum and increase the release time. Strike a note and notice how it fades away. The range of the release control is often equal to or greater than the attack control's range.

6.

With the attack and release pots fully counterclockwise, turn both the decay and sustain pots to midpoint. When a key is depressed, the note begins at its peak gain, then decays to a 50% sustain level at a rate determined by the initial decay setting. When the key is released, the amplifier closes instantly. 7.

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What happens when you turn the . sustain level all the way down? If the x^^ attack and decay are up halfway, and you i depress a key and hold it, those two a It stages make up the entire envelope. decays to nothing after its attack. How ^ 505TAJN long you hold down the key after the note has decayed makes no difference, because On the sustained gain factor is zero. the other hand, if you play a staccato note, the attack stage doesn't have a chance to complete itself before the release stage takes over. If the release time is longer than the decay time, you could actually play longer notes by playing staccato, and shorter notes by holding the keys down.

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