Introduction to Sound Synthesis Sequencers
Piano rolls
A piano roll stores notes as bars on a grid of pitch against time. Each note has its own start time and length, several notes can sound at once, and a grid sets where new notes snap to, although notes can also be placed between its lines.
The name comes from the rolls of punched paper that drove player pianos from the late nineteenth century: one track of holes for each key, each hole as long as its note. Music software lays notes out the same way, with pitch running up the screen and time running across it, and the piano roll is the main note editor in most digital audio workstations. When the part is sent to an instrument or saved as a MIDI file, each bar becomes a pair of MIDI messages, a note-on with the key number and velocity at its start and a note-off at its end.
In a step sequencer each note starts on a step and lasts a whole number of steps, and only one note plays at a time. In a piano roll a note can start at any time and last any length, and notes can overlap, so it can hold chords and several lines at once. The tool below is a two-bar piano roll. Click an empty space to add a note and drag to set its length, or click a note to remove it.
A piano roll
Two bars of notes play in a loop. Snap sets the grid that new notes start on and Quantize moves every note onto it. With Snap off, notes can start anywhere.
Listen for The preset holds a chord under a melody, which a single-line step sequencer cannot play. Set Snap to Off and add a few notes slightly off the beat: you can hear them land early or late against the click. Press Quantize with Snap at sixteenths and every note moves onto the nearest sixteenth.
The grid and quantizing
Snap sets where a new note's start lands when you place it, and with snap turned off a note can be moved to any position. Notes played in from a keyboard arrive with the timing they were played with, slightly early or late. Quantizing moves each note's start to the nearest grid line, so that every note starts exactly on the grid. Most software can also quantize partially, moving notes part of the way towards the grid, or add a swing setting so that the grid itself is uneven.
What else a piano roll stores
Each note carries a velocity, the strength it was played with, which is usually shown as a separate lane of bars underneath the notes and often sent to the voice's level or filter. The same editor normally shows other controller data as lanes too: pitch bend, a modulation wheel, or any setting that has been automated. A piano roll therefore holds the part's controller changes over time as well as its notes. To play a modular synthesiser from a piano roll, a MIDI-to-CV interface converts those messages into a pitch voltage, a gate and further control voltages. The next part of this guide is about processing control signals on their way to a destination, starting with slew, which smooths the steps between notes into glides.
Common questions
What is a piano roll?
A piano roll is an editor that shows notes as bars on a grid, with pitch on the vertical axis and time on the horizontal axis. Each bar's position sets when its note starts and its length sets how long the note lasts. It is the main note editor in most music software.
Why is it called a piano roll?
Player pianos of the late nineteenth and early twentieth centuries played music from rolls of paper punched with holes, one track of holes per key and each hole as long as its note. A software piano roll lays notes out in the same way.
What does quantize do?
It moves each note's start time to the nearest line of a chosen grid, such as sixteenth notes. Notes played slightly early or late then start exactly on the grid. Many programs can also quantize partially, moving notes only part of the way, to keep some of the original timing.
What is the difference between a piano roll and a step sequencer?
A step sequencer has a fixed number of equal steps, each holding at most one note. A piano roll stores notes with any start time and length, and several can sound at once, so it can hold chords, overlapping lines and timing that falls between the grid lines.
Published Glossary