Introduction to Sound Synthesis Time and dynamics
Gates and triggers
A gate is a signal that is high for as long as a note should last and low the rest of the time. A trigger is a pulse a few milliseconds long that marks only the moment something starts.
The chapter on clocks covered when notes start. How long each note lasts is set by a gate, which a keyboard produces along with a pitch: the gate goes high when a key goes down and stays high until you let go. On a modular synthesiser a gate is a voltage, usually 5 or 10 volts while it is high and 0 volts while it is low. Over MIDI the same information arrives as a note-on message and a note-off message.
A trigger is a pulse a few milliseconds long. Parts of a synthesiser that act once per event, such as a drum voice or a sample and hold, respond to triggers, and a clock is usually a stream of them. In the demonstration below, the oscillator runs through an amplifier that the gate or the trigger switches straight on and off.
A gate and a trigger
Hold the gate down for as long as you like, or fire a trigger. The plot records both over the last few seconds.
Listen for The gate opens the oscillator for exactly as long as the button is held, and both edges of each note are clicks. The trigger is too short to hear as a note: a few milliseconds of open gate is a click with almost no pitch.
A gate on its own
In this chapter the gate is connected straight to an amplifier, so the oscillator is either fully on or fully off, and that causes the click at the start and end of each note. The waveform jumps from silence to full level in an instant, and a jump that fast spreads energy across a wide range of frequencies, which the ear hears as a click.
Switching the amplifier on and off also gives every note the same shape: full level from the first instant to the last. A plucked string or a struck bell starts and dies away over a span of time, and the shape of that span is a large part of what makes it recognisable. An envelope produces that shape, and the Envelopes chapter covers it.
Gate length
A clock plays a four-note sequence, and the gate stays high for a set fraction of each step.
Listen for Short gates give separated, staccato notes and long gates run them into each other. The tempo and the pitches stay the same while the slider changes how much of each step each note fills.
Gate length
A sequencer has to generate its own gates, because no key is being held to set how long each note lasts. It does that with a gate length setting, usually a percentage of the step, for each step or for the whole pattern. At 50% the note sounds for half the step and the oscillator is silent for the other half. Near 100% the notes almost join, which is the sequencer's version of legato. Many sequencers can also tie a step to the next one, holding the gate high across both so that the note is not restarted.
Gate or trigger
Anything whose behaviour depends on note length needs a gate. An ADSR envelope is the usual example, because its sustain stage lasts exactly as long as the gate is high. A drum voice or a sample and hold acts only at the start of an event, and most will fire from either a gate or a trigger, because a gate begins with the same rising edge a trigger has. A trigger sent to an ADSR envelope gives a note with no sustain, because the pulse falls again a few milliseconds after it rises and the envelope moves straight on to its release.
Common questions
What is the difference between a gate and a trigger?
A gate stays high for as long as a note should last and drops when it ends, so it carries both when a note starts and how long it lasts. A trigger is a short pulse, a few milliseconds long, that marks only the moment something starts. Anything whose behaviour changes with note length, such as an ADSR envelope, needs a gate.
What voltage is a gate signal?
On a modular synthesiser a gate is usually 5 or 10 volts while it is high and 0 volts while it is low. Over MIDI the same information travels as a note-on message and a note-off message, and in software it is usually a value that switches between 0 and 1.
Why does my synthesiser click at the start and end of notes?
If a gate goes straight to an amplifier, or an envelope's attack or release is set to zero, the level jumps between silence and full in an instant, and you hear the jump as a click. A few milliseconds of attack and release removes it.
What is gate length on a sequencer?
Gate length is how much of each step the gate stays high for, usually given as a percentage of the step. At 50% each note sounds for half its step. Near 100% the notes almost join, which gives a legato line.
Published Glossary