Secrets of the Arpeggiator
Pages: 1, 2, 3

Bells & Whistles

Every synthesizer designer seems to put a bit of fresh spin on the arpeggiator concept. While almost all arpeggiators have the basic features discussed above, you may conceivably find some fascinating extras.

These days, there's some overlap between the arpeggiator and a related device called a step sequencer. To some extent it's a matter of semantics whether a device is called an arpeggiator or a step sequencer, and some devices can function in either manner (see Figure 2). In a step sequencer, it's always possible to program a series of steps so that they'll have different pitches—that is, different transpositions of the MIDI notes. Some arpeggiators will do this; others won't.

Figure 2. The arpeggiator in VirSyn Tera is also a complex step sequencer. You can choose patterns from the palette at upper left and insert them in the sequence grid at upper right. When a channel is set to "arpeggio," as with the top line here, Tera steps through the pattern whenever one or more keys are pressed. The pattern editor has data fields for a controller output, note transposition, chords, step mute and skip, and other functions. (Click to enlarge.)

Generally speaking, a step sequencer responds to only one key on the keyboard at a time, or responds to the keys separately: When you play a key, the sequence will be transposed so that it starts on that key. If you play multiple keys, either the step sequencer will ignore all but one of them (the more common situation), or it will play a separate sequence based on each key (as in Steinberg Xphraze). In contrast, an arpeggiator generally plays only one pattern at any given time, but that pattern can utilize (in some manner) two or more notes that are being held on the keyboard.

For example, let's suppose you've programmed a four-step pattern with note transpositions of 0, 2, 5, and –2. If you hold only one key on the keyboard, such as a D, the pattern will produce the series D-E-G-C. But when you hold a D minor triad (D, F, A), the arpeggiator will apply a different transposition to each note in the arpeggio. This table shows the result:

This type of transposition can be heard at the end of the audio example "steps.mp3" (see the Examples sidebar).

Figure 3. In the arpeggiator in Applied Acoustic Systems UltraAnalog, the arrow in the top line controls the length of the pattern. Clicking in any of the green boxes causes a step to be a rest. The Range parameter is for choosing the number of octaves, and the Latch checkbox puts the arpeggiator in hold mode.

Arpeggiators are sometimes tricked out with features such as the following:

• Step pitch sequence. Individual steps in an arpeggiator pattern can sometimes be transposed up or down in half-steps. Transposing arpeggiators respond somewhat differently to chords than step sequencers do, however, as shown in the table above. You'll find it easier to grasp this concept if you experiment with whatever arpeggiator you have handy. The arpeggiator in U-He Filterscape VA (available as a downloadable demo) has this feature.
• Chord steps. Instead of cycling through the notes you're holding on the keyboard one at a time, the arpeggiator may be able to combine two or more of them on a single step.
• Gate time. By giving individual steps within the pattern a longer or shorter gate time, you can create phrasing. You may also find an option to make the gate for a given step a tie, which will cause the note to sustain, or perhaps glide smoothly to the following note. Steps may be programmable as rests (see Figure 3).
• Step time. Instead of marching at a steady rhythm, the arpeggiator may be able to make some steps longer and others shorter. (For an example, listen to "rhythm.mp3".) You may be able to define a rhythm that uses quarter-notes, eighth-notes, and sixteenths in various combinations. You may also be able to define a step as a rest.
• Velocity modes. The velocities of the notes in the arpeggio may be fixed at one level, as played, or defined according to some pattern.
• Controller output. An arpeggiator or step sequencer may have an extra output (or more than one) that sends controller data to the synth voices rather than note numbers. With this output, you may be able to open and close the filter, for example, or change the envelope attack or decay time (see Figure 4).
• Pattern length. When any of the features in this list are implemented, you'll probably be able to set the length of the arpeggiator pattern, exactly as if it were a step sequencer.

Figure 4. The arpeggiator in LinPlug Sophistry provides velocity/modulation values for each step as a series of numbers. The VEL knob mixes the programmed step velocities (A) with the keyboard velocities (K), but the values in the series are always available as a separate modulation source independent of velocity. A special "mod-only" mod allows keyboard performances to be played normally while the arpeggiator produces a cycle of modulation values.

Some arpeggiators, such as the one in Cakewalk Z3ta+, can be programmed with complex patterns by recording and editing short Standard MIDI Files (SMFs). After saving the SMF, you can access it in the arpeggiator's pattern list (see Figure 5).

Figure 5. The arpeggiator in Cakewalk Z3ta+ shares panel space with the low-frequency oscillators (LFOs). You can store patterns as Standard MIDI Files and load them from the Pattern menu.