Introduction to Sound Synthesis Effects
Delay
A delay plays a signal back after a set time. With feedback, part of the output is sent back into the input, so each echo produces another, quieter one, and a single note becomes a series of repeats that fade away.
An echo effect needs a way to store a sound and play it back later. Early units recorded the signal onto a loop of magnetic tape and played it back from a head further along the tape, so the delay time was set by the distance between the heads and the speed of the tape; Roland's Space Echo of 1974 is a well-known example. Analogue delays later stored the signal in a chain of capacitors called a bucket-brigade device, and digital delays store it as numbers in memory, which is how the tool below works.
The tool sends plucks or drums through a delay with feedback. The plot shows the echoes a single note produces: their timing, their levels, and, with ping-pong on, which side each comes from.
A delay with feedback
Plucks or drums pass through a delay whose time is set in milliseconds or as a note length at the source's tempo. Feedback sets how many repeats there are, Tone sets a lowpass filter that darkens each repeat, and ping-pong alternates the repeats between left and right.
Listen for With a short time and no feedback, each note is followed by a single close echo, called a slapback. Raise the feedback for a trail of repeats. Set the time to a dotted eighth and the echoes fall between the plucks and add a second rhythm in the gaps. Lower the tone and each repeat is darker than the last, as on a tape echo.
Time
How an echo is heard depends on the delay time. Below about 30 milliseconds the echo is too close to the original to be heard as a separate event, and the two combine into a comb filter that changes the tone, as described in comb filters. Between about 50 and 150 milliseconds a single repeat gives a slapback, a doubling heard on many early rock and roll vocals. Longer than that, each echo is heard as a separate event. Setting the time as a note length keeps the echoes in time with the music: at the source's 100 BPM an eighth note is 300 milliseconds and a dotted eighth, three quarters of a beat, is 450. Dotted times place the echoes off the beat, in the silences of the part's rhythm, which is a common way of building a busier rhythm out of a simple part.
Feedback and tone
Feedback sends part of the delayed signal back into the delay, so each repeat is the previous one again, quieter by the feedback amount. At 50% each repeat is half the level of the one before, about 6 dB quieter, and the trail fades over several seconds. Close to 100% the repeats barely fade, and on hardware delays turning feedback past that point makes the repeats grow until they distort, an effect players use deliberately. The tone control here is a lowpass filter inside the loop, so each pass removes a little more treble, as each pass through a tape machine did.
Stereo
A ping-pong delay feeds each repeat into the opposite channel, so the echoes alternate between left and right. Delays with slightly different times on the two sides, or a short delay on one side only, are also used to widen a sound across the stereo field. A room produces many closely spaced reflections arriving from all directions, which is the subject of reverb.
Common questions
What does feedback do on a delay?
It sends some of the delay's output back into its input, so each echo is delayed again and produces another, quieter echo. With low feedback there are one or two repeats; with high feedback the repeats continue for many seconds, and above 100% they would grow louder each time.
What is a tempo-synced delay?
A tempo-synced delay is a delay whose time is set as a note length, such as an eighth note or a dotted eighth, and calculated from the tempo. At 100 BPM an eighth note is 300 milliseconds. The echoes then fall in time with the music.
What is a ping-pong delay?
A ping-pong delay is a stereo delay whose echoes alternate between the left and right speakers. Each repeat is fed into the opposite channel's delay, so the echoes bounce from side to side.
Why do tape and analogue delays sound darker?
Each pass through a tape machine or an analogue delay chip loses some high frequencies, and with feedback every repeat passes through again, so later echoes are progressively duller. A lowpass filter inside the feedback loop of a digital delay imitates this.
Related modules
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