Delay is one of the most important effects. This post is a clear explanation and helpful overview of the topic from Mark Garrison's book Encyclopedia of Home Recording.
Delay is an effect that repeats a signal that is fed into it. A delay may repeat only once, multiple times, or it may repeat indefinitely. Delay differs from echo in that an echo gets progressively quieter with each repeat. The repeats on delay can be quieter than the original sound, but they may also be the same volume or louder than the original.
Some delay processors allow for special delay effects such as ping-pong delay (a stereo effect where delays alternate back and forth between left and right channels), reverse delay (where the delay is backwards), or even phrase looping (the delay will replay a specified segment on a continuous loop). A variety of rhythmic options for repeats may also be included, such as triplets or dotted eighth notes.
Most modern delay processors are digital; however, analog delays are not uncommon and are often prized for their tone. One obsolete, though coveted, form of analog delay is the tape delay. A tape delay works by feeding a continuous loop of magnetic tape past a series of tape heads. A sound is recorded onto the tape by the first head and then replayed by the following heads. Parameters such as tape speed allow control over the delayed sounds.
Delay With The Beat
When using delay, the effect will be more subtle when it occurs in time with the music. Some delay units, called multi-tap delays, allow the performer to tap a pad in time to the music, setting the tempo of the delay.
When multi-tap functionality is not available, the same effect can be achieved by using a simple equation. Take the number 60,000 (which is the number of milliseconds in a minute) and divide it by the tempo of the song (in beats per minute). If the song is in 3/4 or 4/4 time, the resulting number is the number of milliseconds that make up a quarter note (if the song is in 6/8 time the result will be an eighth note, etc.). If you divide that number in two, you then have the delay required for an eighth note. You can dividing the number by two to calculate the duration of the next smaller note value (see Fig. 34).
If a song feels like it is dragging, placing the delay slightly before the beat will make it feel more upbeat. If a song is feeling rushed, placing the delay slightly after the beat will help alleviate the rushed feeling.
Doubling With Delay
Delay is often used to fatten up a sound trough a technique called doubling. If a very short delay time is used, in the range of about 15–40 ms, the brain cannot perceive the two as discreet sounds, and instead we hear a fuller version of the original. Care should be used, however, as this technique can cause phase problems. Careful listening (with the original and the delay both panned to center) will reveal these problems, and they can be alleviated by shifting the delay by a few milliseconds in either direction until the phase problem goes away or is reduced to an acceptable level.