Introduction to Sound Synthesis Mixing and dynamics
Envelope followers
An envelope follower turns the level of an audio signal into a control signal that rises and falls with it. That control signal can be sent to a filter's cutoff, a level or another setting, so one sound can raise another's filter cutoff or lower its level.
An envelope follower measures the level of a sound as it plays and produces a control signal that follows it, whereas an envelope generator produces its shape from a gate. It rectifies the audio, so that the negative half of each cycle counts the same as the positive half, and then smooths it with separate attack and release times, so its output follows the outline of the sound's level rather than every cycle of its waveform. Its output can be sent wherever a control signal can go, such as a filter's cutoff, a level or a pitch. With it, an external sound such as a drum machine, a voice or a guitar can move a synthesiser's settings.
The tool below follows a drum loop. Its output can open the filter of a sustained pad or duck the pad's level. The plot shows the drums' level in grey and the follower's output in blue. With the follower connected to the pad, an orange line shows the result: the pad's level, which drops on each hit when it is ducked, or the pad's cutoff, which rises on each hit when the follower opens the filter.
An envelope follower
A drum loop feeds an envelope follower whose output opens a pad's filter or lowers its level. Attack and release set how quickly the follower responds, and amount sets how far it moves the pad.
Listen for With the follower sent to the filter, each drum hit raises the pad's cutoff for a moment, so the pad brightens in the drums' rhythm. With it sent to the level, the pad drops on each hit and rises again between them, which is the pumping sound of sidechain ducking. A long release smooths the follower's output, so the pad follows how busy the drum pattern is rather than each separate hit.
Attack and release
The two times set what the follower responds to. With a very short attack and release, the output follows almost every cycle of the waveform and becomes a buzzing, distorted copy of it. With an attack of a few milliseconds and a release of a hundred or so, the output follows each hit. With a long release of half a second or more, the hits merge into a smooth line that rises with the busy parts of the groove and falls in the quiet parts. A short attack and a longer release is the usual starting point, because many sounds, drums especially, start quickly and fade more slowly.
Uses
Sent to a filter's cutoff, a follower makes an auto-wah: each note raises the cutoff in proportion to how loud it is. Sent inverted to the level of another sound, it lowers that sound whenever the first one plays. A bass line or a pad ducked by the kick drum drops in level on each kick, so it does not mask the kick in the same low frequencies. Sent to a comparator, it turns a drum part into gates and triggers that can start envelopes or advance a sequencer. Envelope followers are also the measuring stage inside compressors, noise gates and limiters, which use the level they measure to turn the signal itself up or down.
Common questions
What is an envelope follower?
It is a circuit or process that turns the level of an audio signal into a control signal. It rectifies the audio, so that both halves of each cycle count as positive, and smooths the result, so its output rises when the input gets louder and falls when it gets quieter.
What do attack and release do on an envelope follower?
They set how quickly the output responds. Attack is how fast it rises when the input gets louder, and release is how fast it falls when the input gets quieter. With short times the output follows quick changes such as individual drum hits. With long times those changes are smoothed out, and the output follows the input's level averaged over a longer stretch.
What is sidechain ducking?
It is a way of lowering the level of one sound whenever another plays, by using an envelope follower on the second sound to control the level of the first. A pad ducked by a kick drum drops on every kick and swells back between them, a pumping effect common in dance music. Ducking is usually done with a compressor whose envelope follower is fed from its sidechain input rather than from the signal it turns down.
What is an auto-wah?
It is a filter whose cutoff is moved by an envelope follower on the input, so each note raises the cutoff in proportion to how loud it is. A wah-wah pedal moves a similar filter by the position of the player's foot.
Published Glossary