In the next couple of sections, we’re going to look at how synthesizer designers address (or, on occasion, fail to address) a fundamental issue. Since LFO modulation causes the values of certain sound parameters to rise and fall during the course of each note, the issue that arises is, what will be happening to those parameters at the very beginning of a new note? Will they start at their base level and then rise? At their base level and then fall? At some higher level and then fall? At a level below the base level and then rise?
When described this way, the problem may seem like nitpicking or mere semantics, but it isn’t. It has immediate practical consequences for the sound of the music. In order to explain the tools synthesizers give sound designers with which to control what happens, we’re going to have to weave back and forth a little, so please read the whole discussion first and then get confused.
In a polyphonic synth, each voice will usually have its own LFO(s). By default, these LFOs are independent of one another: The LFOs for each voice produce their own signals, which modulate only the other modules within that voice.
In a true analog synth, be it monophonic or polyphonic, the LFO, because it’s a piece of hardware, will always be cycling through its selected waveform, whether or not it’s being used. Even when it’s not modulating anything, or when you’re not playing a note at all, the LFO is still cycling in the background.
Polyphonic digital synths, however, are often designed so that, by default, the LFOs only cycle when the voice is actually producing a sound. Each time you play a note, the LFO(s) in the voice to which the note is assigned will start a new wave cycle.
You may be able to set an LFO to either free-run mode or key trigger mode. (Your instrument may use different names.) In free-run mode, the LFOs for each voice will operate independently, and should be cycling in the background even when the voice is not producing any sound. Even if the LFOs for all of the voices are running at the same speed, in free-run mode the relationships among their wave cycles will be undefined. In key trigger mode, when you play a block chord, all of the LFOs should begin at the same point in their cycle, producing a modulation pattern in which all of the voices are modulated in the same way at the same time. Unless some type of secondary modulation (such as velocity-to-LFO-rate) causes the various LFOs to get out of sync with one another as the note sustains, it should sound as if all of the voices are being modulated by the LFO(s) from a single voice. Another way to accomplish this, found on some synths, is a global LFO, a single LFO that can be used as a modulation source for all of the voices.
IDEA FILE: Assuming your synth’s LFOs can be set to key trigger mode, try setting up a patch in which a square wave does something interesting, such as changing the filter cutoff radically. Set the LFO to key trigger mode, program the LFO to a slow-to-medium rate, and then play some chords on the keyboard. But instead of playing block chords, start each note at a different time and observe the changes in the composite rhythm. For extra juice, add a digital delay whose delay time is
synchronized with (or simply matches) the LFO rate. Now add a second oscillator and LFO, also synchronized but running at a different speed.
In free-run mode, even if the LFOs are all running at the same rate, when you play a block chord the modulation for the various otes won’t necessarily be coherent. In the case of vibrato, one oscillator’s pitch modulation may be at the top of its travel, another oscillator’s may be at the bottom of its travel, and a third may be near the middle. With any appreciable amount of pitch modulation, the lack of coordination among the LFOs will tend to blur, if not entirely obliterate, the harmonic identity of the chord (see Figure
6-5). This may be what you want musically, but it may not be. Free-run mode can be useful because it makes the LFO’s cycle less predictable, and therefore less monotonous from note to note.
Figure 6-5. Three LFO sine waves in free-run mode (top) and in key trigger mode when all of them have been triggered by notes that start at the same moment (bottom).
Phase
If the LFOs in your instrument provide a key trigger mode, they may also have a start phase parameter.
With this parameter, you can control the point at which the LFO waveform will start its cycle each time you play a new note. Phase is usually described in terms of degrees of arc: A full cycle of a wave encompasses 360 degrees of phase. (For reference, please refer back to Figure 2-3 on page 26.) If the sine wave in Figure 2-3 is coming from an LFO, setting the LFO’s start phase parameter to 0 degrees will cause the sine wave to begin at the zero-point on the Y axis and then rise toward its maximum level. If the phase parameter is set to 180 degrees, the LFO will begin at the zero-point and then fall. If the phase is set to 90 degrees, the LFO will begin at its maximum output level and then fall.
In the case of other waveforms, the precise behavior of the output may be different, but the meaning of the start phase parameter will be the same. A setting of 180 degrees will cause the LFO to begin halfway through its waveform, and so on. A setting of 360 degrees is equivalent to a setting of 0 degrees.
In case it’s not obvious, changing the LFO’s rate (frequency) won’t have any effect on a start phase setting. If the start phase is set to 45 degrees, for example, the wave will always begin of the way through its full cycle, no matter how fast or slow the LFO happens to be cycling.
An LFO start phase parameter should have no effect when the LFO is in free-run mode, because the LFO should ignore key-down events.
CALCULATOR FORMULA: If you want the speed of an LFO or the delay time of a digital delay to match the tempo of your song, but the LFO or delay can’t be synchronized, you can always set the parameter by ear. But if the parameter values are displayed in real-world form — as Hz or
milliseconds — rather than in arbitrary units (0-127, for instance), you can use a calculator to find the correct setting.
Your sequencer will undoubtedly display the tempo in beats per minute (bpm). We’re going to assume that “beats” is synonymous with “quarter-notes,” which is usually a safe assumption. Here’s how to do the conversion. If the parameter is shown in milliseconds (ms), the formula to use is:
60,000 / bpm = ms per beat
For example, if the tempo of the song is 100 bpm, you would divide 60,000 by 100, which gives a value of 600 milliseconds per beat.
But what if the LFO’s speed is displayed in Hertz? Hertz, you’ll recall from Chapter Two, is the same as cycles per second, so the first step is to replace milliseconds in the formula above with seconds:
60 / bpm = seconds per beat
The second step is to invert the fraction. We’re looking for an LFO setting in cycles per second, not in seconds per cycle:
bpm / 60 = cycles per second
So if you want one LFO cycle per quarter-note beat at 100 bpm, the LFO needs to be set to 100 divided by 60, or 1.667Hz.
To arrive at the correct numbers for longer or shorter rhythm values, multiply or divide as needed.
If an LFO at 1.667Hz produces one cycle per quarter-note, sixteenth-notes will be produced at 1.667 x 4 = 6.667Hz. If you want the delay line to produce triplet eighths rather than quarter-notes, you’d divide the millisecond value by 3. If you’re using milliseconds, smaller rhythm values are found by dividing and larger ones by multiplying; if you’re using Hertz, smaller rhythm values are found by multiplying and larger ones by dividing.
Synchronization
Because of the importance of rhythm in pop music, many synthesizers have LFOs that can be synchronized to outside clock sources. Typically, the clock source can be either the synth’s own internal clock or an external MIDI clock signal. This type of synchronization is extremely useful if you’re programming an effect that you want to coordinate with a song’s tempo — for example, if you want an LFO to create a filter-based tremolo in an eighth-note rhythm, or if you want a triangle wave to sweep the filter up and down over the course of two bars. If you later decide to adjust the tempo of your song, or if you happen to write a song with tempo changes, a synced LFO will automatically adjust to the correct tempo.
To learn how (or whether) your instrument will sync its LFOs, consult the owner’s manual. In some instruments, the LFO rate parameter can be scrolled up or down until you reach a group of synced settings. In others, switching on LFO sync will replace the normal rate parameter values with synced settings. Most often, these settings are indicated as fractions, with or without additional abbreviations.
For instance, a setting of should produce an eighth-note rhythm, ¼ should produce quarter-notes, and so on. Settings followed by “t” produce triplets. Some instruments also have settings indicated by “d” or
“.” which produce dotted rhythms (“ .” would produce a dotted-eighth value, for instance). More complex rhythm values, such as quintuplets, are not usually supported by synced LFOs. Nor are swing/shuffle rhythms usually provided.
LFOs that can be synchronized to MIDI clock don’t usually respond to MIDI start commands. I can’t help but feel this is a significant shortcoming in contemporary instrument design. The problem arises because MIDI clock is a “dumb” signal. A clock byte is transmitted by the sync master device 24 times for every quarter-note — but there’s nothing in the clock signal to indicate which byte in any given series of 24 bytes falls on the quarter-note beat. As a result, if you’re using a synced LFO but you happen to start sequence playback at some other point than the beginning of the song, the LFO may sound different than it did before. It will be running at the correct tempo, but it will have the wrong phase.
Here’s the workaround: Set your synced LFOs to key trigger mode, and then always start the sequencer at or just before the beginning of a note that the LFO will be modulating.