Several programming techniques are commonly used with analog-type oscillators. Some of these techniques are equally applicable to sample playback, which is discussed in the next section, but let’s take a break and do a little programming before we dive into the next chunk of theory.
Recipe 2
Synth Strings: For a classic synth string sound, use the following settings:
• Two or three sawtooth wave oscillators, slightly detuned from one another.
• A lowpass filter with a low to moderate cutoff frequency, a shallow rolloff slope (6dB per octave if possible), and no resonance.
• Little or no envelope modulation of the filter cutoff frequency.
• An amplitude envelope with moderate attack and release times and full sustain.
• If desired, modulate amplitude and/or filter cutoff from aftertouch so you can play swells.
• Try substituting a pulse wave for one of the sawtooth waves, and add some pulse width modulation from a slow LFO.
Detuning. One oscillator, by itself, tends to produce a tone that’s static — rigid and unmoving.
Sometimes that’s what you want, but often a richer, more animated tone is called for. A classic solution is to have two oscillators play the same waveform and then detune one from the other using the fine-tune parameter. (An example can be heard in CD track 3, tone 2.)
When two oscillators playing sustained tones are detuned from one another, their combined tone will have a chorused type of sound. This effect is called beating. Beating is caused by phase cancellation, which is discussed in Chapter Two.
The speed of the beating is dependent on the frequency difference between the two oscillators. For instance, if one oscillator has a frequency of 800Hz and the other a frequency of 802Hz, the difference between them is 2Hz (two cycles per second), so you’ll hear two beats per second. As the two oscillators are detuned further, the beats will speed up.
Both slow beating and rapid beating can be musically useful. But if your synth’s detuning parameter is designed to change the frequency of an oscillator by some fraction of a half-step, as most detuning parameters are, the speed of the beating will increase as you play up the keyboard. This is because the half-steps are much closer together in frequency at the low end of the keyboard than at the upper end, because we perceive pitch in a logarithmic rather than a linear manner. The distance between the low C and C# on a five-octave MIDI keyboard (assuming the keyboard is tuned to concert pitch) is less than
3.9Hz. The distance between the high B and C on the same five-octave keyboard is almost 117.5Hz. So if we detune two oscillators from one another by 5 cents (½0 of a semitone), when we play the low C on the keyboard we’ll hear beating at a rate slower than 0.2Hz, while at the top of the keyboard it will be about 5.9Hz.
If your synth offers a parameter with which an oscillator can be detuned in constant Hz, you can avoid this discrepancy. Detuning an oscillator by a constant 2Hz, for example, will cause it to beat against a non-detuned oscillator at a rate of 2Hz from the low end of the keyboard to the high end. The disadvantage of this technique is that at the low end of the keyboard, 2Hz is a fairly large fraction of a half-step. The detuned oscillators may sound as if they’re playing different pitches, and the sense of a unified center pitch may be lost.
MAKES CENTS TO ME: On some synths, the fine-tuning parameter for each oscillator is calibrated in cents. A cent is 1/100 of an equal-tempered semitone (half-step).
Detuning is most often used with oscillators whose coarse tuning is set to a unison. When two tones an octave or two apart are detuned from one another, they tend simply to sound out of tune. Detuning is also useful, however, when the coarse tuning is set to an interval of a third, fifth, or sixth. These intervals produce beats in equal temperament — the beating of thirds is quite severe — and the detune parameter can be used to slow or eliminate the beats.
In some two-oscillator synths, only oscillator 2 has a fine-tune parameter, because the manufacturer assumes that oscillator 1 provides the unvarying base pitch of the sound, from which oscillator 2 can deviate. This is a poor assumption. When two oscillators are detuned from one another by a few cents, the perceived center frequency of the sound will tend to be halfway between them. In other words, the composite tone will sound “in tune” when one oscillator is tuned a few cents sharp and the other oscillator a few cents flat. If you’re forced to detune oscillator 2 while leaving oscillator 1 at concert pitch, the tone will tend to be perceived as either flat or sharp. You may be able to compensate by adjusting the instrument’s global tuning parameter — but if you’re using the instrument in live performance, this is not an efficient workaround, because the setting of the global tuning parameter can’t be stored in a patch.
Using three or more detuned oscillators rather than two will add to the thickness of the sound. In fact, a few analog-type digital synths include a “waveform” in which three or more detuned signals, usually sawtooth waves, are produced by a single oscillator. If your synth has one of these “triple saw” or “six saws” waves, you’ll also find a detune amount parameter. The larger the detuning amount, the thicker the sound. Synths in which several voices can double one another in unison mode tend to have a unison detune parameter, which does much the same thing.
Oscillator Sync. Oscillator sync has been used since the early days of analog synthesis to add more tone colors to the synthesist’s palette. I’m not aware of any sample playback synths that do true oscillator sync (at least, not with their sampled waveforms; the Kurzweil K2000 series does both sample playback and oscillator sync, but not with the same voice at the same time). You may find a sampled “osc sync”
waveform in a sample playback synth, however. True oscillator sync is available only in analog-type oscillators and those that play single-cycle digital waves.
Please don’t confuse oscillator sync with timing synchronization. Except for a brief discussion in Chapter Six, timing sync is not covered in this book. In oscillator sync, one oscillator (often called the master) is used as a timing reference for the waveform of another oscillator (often called the slave). In essence, the slave oscillator is forced to restart its waveform from the beginning each time the master oscillator starts a new waveform. The result is that their frequencies will always be the same.
Described this way, oscillator sync may not seem too useful. After all, if you want oscillator 2 to be
tuned to the same frequency as oscillator 1, you can just tune it. Why sync it?
To see why, take a look at Figure 4-3. When the slave oscillator’s frequency is modulated (often from an envelope or LFO, though driving it from the mod wheel is also a useful expressive technique), its base frequency can’t change. Instead, its waveform changes. For instance, if the slave oscillator is trying to produce a tone a perfect fifth above the master oscillator, it will play through 1 ½ cycles of its waveform and then have to restart. If it’s trying to produce a tone an octave and a fifth above the master, it will play through 2½ cycles before restarting.
The tone of a synced oscillator has a prominent overtone at or near the pitch it would produce if it weren’t synced. As we attempt to increase its pitch, either from modulation or simply by adjusting its coarse tune parameter, this overtone will become more prominent. Tuning a synced oscillator below the frequency of the master, by contrast, does nothing very interesting to the sound.
A synced oscillator often has a biting, metallic tone color. Moving its pitch rapidly through a couple of octaves with an envelope generator will produce a dramatic sweep. If your synth allows oscillators to be synced, try a few experiments. It may not be clear from the panel which oscillator is the master and which is the slave; you may simply have a button labelled “sync.” To hear the effect of oscillator sync, the slave oscillator must be turned up in the mixer. The effect will be more dramatic if the master oscillator is turned down in the mixer, but there are times when you want to hear the master oscillator, because it will add more fundamental.
Figure 4-3. When oscillator 2 (b and c) is synced to oscillator 1 (a), oscillator 2’s waveform has to start over each time oscillator 1 starts a new waveform cycle. Thus their fundamental frequencies will always be the same. By modulating the frequency of oscillator 2, however, we can change its waveform. In (b), the frequency of oscillator 2 is only slightly higher than that of oscillator 1, so their waveforms are similar. (The dotted line shows what the waveform of oscillator 2 would be if it weren’t synced.) In (c), the frequency of oscillator 2 is more than an octave higher than that of oscillator 1, so the synced waveform has many more peaks.
Pitch Modulation. Most synths provide some method for modulating the pitch of an oscillator while it is sounding. (For details on various types of modulation routings, see Chapter Eight.) The most common types of pitch modulation use an LFO (see Chapter Six), an envelope generator, or MIDI pitch-bend data as a source. In almost all synths, the amount of LFO modulation can be controlled from a secondary modulation source, such as a MIDI modulation wheel/lever. Such a lever normally sends MIDI continuous controller 1 (CC1) messages. By controlling the amount of LFO modulation from CC1, you can add vibrato to a note. Vibrato can also be pre-programmed, so that it is applied to every note. In this case, the secondary modulation from CC1 is bypassed, and the LFO’s output modulates pitch directly.
Pitch-bend. MIDI pitch-bend is one of several types of channel messages. Pitch-bend is unusual in the MIDI world in that it’s a bidirectional message: The zero value is in the center of the control range rather than at the lower end. As a result, a pitch-bender can either raise or lower the pitch of the oscillators.
The raw value of a pitch-bend message (which can range from -8192 to +8191 ) says nothing about the amount of pitch change that will be produced by a given pitch-bend message. The maximum pitch-bend depth is set within the receiving synthesizer, either in the global area or in a particular patch. Typically, the depth is set as a number of equal-tempered half-steps. (If you’re a devotee of alternate tunings, you may find this fact somewhat irksome. I’m not aware of any commercially available synth that allows its pitch-bend depth to be set as a number of scale steps in the currently selected tuning scale. It’s not terribly difficult to create your own pitch-bend algorithm, however, in a software-based modular synth such as Native Instruments Reaktor.)
MERCEDES BENDS: MIDI pitch-bend messages can be transmitted and responded to with 14-bit precision, which means that the maximum range of the data is from -8192 to +8191. However, not all pitch-bend hardware actually transmits data with 14-bit precision — nor are all synthesizers
equipped to respond with 14-bit precision. In practice, you may find that your pitch-bends are
restricted to a range from -64 to +63. Strange as it may seem, this range is not any narrower than the
-8192/+8191 range; it simply has less resolution. The -64/+63 range is defined by the seven most significant bits of the pitch-bend message. In a synth that uses the more restricted -64/+63 range, the seven least significant bits, which are present in all pitch-bend messages, are simply ignored. A low-precision hardware pitch-bender will most likely transmit the seven least significant bits as a string of zeroes. (In fact, not all hardware pitch-benders are even this sensitive. Some of them can only sense between 32 and 64 discrete positions.)
In some instruments, both the maximum range of an upward bend and the maximum range of a downward bend are set with a single parameter. For instance, if you set the pitch-bend depth to two half-steps, you’ll be able to bend the pitch upward by two half-steps by pushing the pitch-bend wheel or lever forward (or to the right), and downward by two half-steps by pulling the wheel or lever back toward you (or to the left). In other instruments, upward and downward bend depth can be programmed separately.
If your instrument allows pitch-bend depth to be programmed in individual oscillators, you can program a sound so that the oscillators play in unison when the pitch-bender is centered and some interval (or, with three oscillators, a chord) when the bender is pushed forward or pulled back. This technique can be especially effective if the patch uses a distortion effect, because the distortion will magnify the beating between the oscillators as they go out of tune with one another.
Using a Pitch Envelope. Several musical effects can be achieved by modulating the pitch of an oscillator with an envelope generator (EG). Your synth may allow any envelope to be used as a pitch modulation source, or it may have an envelope generator dedicated to pitch modulation (or one for each oscillator). The features of envelope generators are covered in detail in Chapter Seven. The most significant fact at the moment is that a dedicated pitch EG may have fewer stages than a multi-purpose EG. Pitch envelopes with only two parameters — starting pitch and decay time — are fairly common.
This is because pitch EGs are most often used to add pitch changes to the attack of each note. A simple pitch EG will let you program the starting pitch (which may be above or below the oscillator’s base pitch) and the amount of time required for the pitch to rise or fall from this starting value back to the base pitch.
When the decay time of the pitch EG is quick (in the 1-50ms range), you’ll hear a quick “blip” on the attack of each note. The exact timbre of the blip will depend on the starting pitch and on what the other envelopes are doing during the attack of the note. If your amplitude envelope takes 100ms to open, you’ll never hear a 5ms pitch envelope. Another factor you may have to work with is how often your synth allows its oscillator pitch to be updated. If the oscillator pitch can be changed only in blocks of 20ms or so, as was fairly common on older digital synths, a quick pitch envelope will produce a “chirp” rather than a click.
Slower pitch envelopes are useful for modulating a synced oscillator (see above) and for special effects such as spacey sighing noises.
Controlling the depth of the pitch envelope from velocity is a useful technique, as it lets you add a more percussive attack when you play harder.
Fractional Scaling. Normally, the pitch of an oscillator will track the keyboard. That is, the pitch will increase by one equal-tempered half-step per key as you play up the keyboard. Many synths give you some form of control over the oscillator’s keyboard tracking, however.
The simplest way to control keyboard tracking is to switch it off. The Minimoog, for instance, had a switch for choosing whether oscillator 3 would track the keyboard or would have a fixed pitch. The fixed pitch was controlled by the oscillator’s tuning knob and switch, but it was not affected by the key you played. Turning off keyboard tracking entirely is mainly useful for creating drones (in programming a bagpipe patch, for instance) and when the oscillator is being used as a modulation source rather than being listened to directly.
An oscillator that is programmed to track the keyboard at less or more than 100% (100% being one equal-tempered half-step per key) is sometimes said to be using fractional keyboard scaling. If the scaling parameter is set to 50%, for example, the oscillator will respond to the keyboard by playing a scale of equal-tempered quarter-tones. If the scaling parameter is set to -100%, the keyboard will be inverted. Low keys will produce high pitches and vice-versa.
Using fractional scaling for one oscillator in a multi-oscillator synth is an important programming technique. Typically, the fractionally scaled oscillator is programmed to play only a brief, percussive attack transient. This transient will typically become only slightly higher in pitch as you play up the keyboard. This mimics the way attack transients behave in acoustic instruments such as piano and guitar.
(For more on how the sound of a piano hammer varies across the keyboard, see below under
“Multisampling.”)
JUST FOR KICKS: Just intonation is not a single tuning. It’s a system of tunings in which the frequencies chosen are based on intervals that can be expressed as whole-number fractions (such as 3/2 for a perfect fifth) rather than on intervals based on irrational numbers. All intervals in the familiar 12-note equal-tempered tuning are based on the 12th root of 2, which is about 1.059463.
Tuning Tables. Many synthesizers, though by no means all of them, are equipped with tuning tables.
When a tuning table is active, the oscillator(s) to which it is applied will respond to the keyboard by playing whatever set of pitches is stored in the tuning table. Some tuning tables are pre-programmed at the factory, usually with historical tunings (also called temperaments) used in Medieval, Renaissance, and Baroque music. Others are user-programmable.
In many instruments, tuning tables are chosen at the global level. When a tuning is selected, it will be used by any sound the synth plays. In other instruments, you may be able to select a tuning table for a given patch, or even for a single oscillator within a patch.
Most user-programmable tuning tables consist of a set of 12 pitch offsets (typically ± 50 cents), which are applied to the notes in each octave of the keyboard. You may also see a parameter for choosing the starting key of the tuning — very useful if you’ve programmed a scale in just intonation in order to play a piece in the key of C, and later want to play a piece in A, B , or some other key using the same tuning.
A few instruments, such as the long-obsolete Yamaha TX802 and the E-mu Proteus 2000 series, provide full-range tuning tables. With such a tuning table, you can tune each MIDI key to any pitch. Full-range tuning tables are good for creating microtonal tunings in which you need more than 12 notes per octave, such as 19-tone equal temperament. Playing the keyboard after setting up such a tuning is not easy, however.
The frequency resolution of most tuning tables is limited. For instance, if you can tune a MIDI note up or down by a half-step using a parameter whose range is -64/+63, the nominal resolution of the tuning table is about 0.78 cent. With some modular software synths, such as Native Instruments Reaktor, you can create tuning tables that have finer resolution, but both the process of doing so and the reasons why you
The frequency resolution of most tuning tables is limited. For instance, if you can tune a MIDI note up or down by a half-step using a parameter whose range is -64/+63, the nominal resolution of the tuning table is about 0.78 cent. With some modular software synths, such as Native Instruments Reaktor, you can create tuning tables that have finer resolution, but both the process of doing so and the reasons why you