Introduction to Sound Synthesis Sequencers
Trigger sequencers
A trigger sequencer stores only whether each step fires. It lays the steps out in a grid with one row per sound, and on each step it sends a trigger to every sound whose row is switched on there. It is the usual way of programming drums.
Most drum sounds need only a start time from a sequencer. Their pitch is usually set on the drum voice itself, and their length is set by how quickly the sound decays rather than by a gate. A trigger sequencer stores one on or off value for each step of each sound. Drum machines such as Roland's TR-808, released in 1980, laid the sixteen steps of a bar out as a row of buttons, and the same layout appears in most drum machines, grooveboxes and the drum editors of music software. On a modular synthesiser a trigger sequencer has one output per row, each patched to a drum voice or to anything else that responds to a trigger.
In the tool below, the grid has rows for a kick, a snare and a hi-hat, and an accent row. Click any cell to switch it on or off, or choose a preset pattern to start from.
A trigger sequencer
A sixteen-step grid plays a kick, a snare and a hi-hat, with an accent row that makes every sound on that step louder. Swing delays every second step.
Listen for With four on the floor, the kick on every beat sets a steady pulse that the hi-hats fill in. The breakbeat preset moves the kicks off the beat. Accents change which hits stand out without changing which steps play. Swing delays every second sixteenth, so it turns the breakbeat's sixteenth-note hi-hats into a shuffle; the other two presets have no hits on those steps, so swing does not change them.
Rows and steps
Each row is independent, so the grid shows at a glance how the sounds fit together: which steps they share, where they leave space, and where the pattern repeats. Sixteen steps is one bar of sixteenth notes in 4/4 time, and most patterns in popular music fit in it, although many sequencers allow other lengths and step sizes, such as twelve steps for a bar of triplets. Giving one row a different length from the others, as described in step sequencers, makes that sound drift against the other rows.
Accent
An accent row marks steps that play louder. On many drum machines, including the TR-808, accent applies to every sound on that step, so it emphasises a moment in the bar rather than one drum. Accents let a pattern vary in loudness from step to step without changing which steps play. Swing, described in clocks, changes the timing in a similar way: the same steps play, and every second one is played late.
Euclidean rhythms
A number of hits is spread as evenly as possible across a number of steps, drawn as a circle, and played against a kick drum on every beat. Rotation moves the pattern's starting point.
Listen for Three hits across eight steps gives the tresillo, the 3-3-2 rhythm common in Latin American music. Five across eight, rotated by 2, gives the cinquillo, 2-1-2-1-2. Rotating a pattern keeps its spacing and moves it against the kick, which changes which of its hits land on the beat. Patterns whose length is not a multiple of four drift against the kick and line up with it again only when the two counts meet, after 20 steps for a five-step pattern.
Rhythms from an algorithm
Spreading a given number of hits as evenly as possible across a given number of steps is the same problem as dividing one number by another with the remainder spread out, and it can be solved with the procedure that the ancient Greek mathematician Euclid described for finding the greatest common divisor. The computer scientist Godfried Toussaint showed in a 2005 paper that the patterns it produces include many rhythms found in traditional music around the world, among them the tresillo, three hits across eight steps. These are now called Euclidean rhythms, and many sequencers can generate them from two numbers and a rotation. Like any trigger sequencer, a Euclidean generator stores only when each sound fires, so the pitch of each drum is still set on the drum voice. A piano roll stores pitch as well as timing, and each note in it can have any start time and length.
Common questions
What is a trigger sequencer?
A trigger sequencer stores only whether each step fires, usually in a grid with one row per sound. Each row sends a trigger on the steps that are switched on. It is the usual way of programming drums, because most drum hits need only a start time.
What is an accent on a drum machine?
An accent is a step that plays louder than the others. On many drum machines accent is a row of its own, and any sound that plays on an accented step plays at a higher level.
What is a Euclidean rhythm?
A Euclidean rhythm spreads a number of hits as evenly as possible across a number of steps, for example three hits across eight steps. The computer scientist Godfried Toussaint showed in 2005 that many traditional rhythms from around the world are Euclidean, including the 3-in-8 pattern known as the tresillo.
Why do drum machines have sixteen steps?
Sixteen steps is one bar of sixteenth notes in 4/4 time, which covers most drum patterns in popular music. Many sequencers allow other lengths and other step sizes, such as twelve steps for a bar of triplets.
Related modules
Published Glossary