Delay is an audio effect which records an input signal and then plays it back some time later.  The delayed signal may either be played back multiple times or played back into the recording again to create the sound of a repeating, decaying echo.
You may be wondering what the difference is between delay and echo. These two effects are actually the same, it is really their origins that differ. Delay tends to be purely digital in nature and often refers to digital delay lines. Echo, on the other hand, is a term that's been used when referring to analog tape based delays. 
- Dry/Wet - The balance between the processed and dry signals.
- Delay Time - The time between when the original signal is played and when it is repeated. This parameter usually comes in two varieties: Sync and Time.
- Sync - In sync mode, the delay unit calculates the delay time for beat divisions (such as an 1/8th or 1/16th note). This allows the user to easily sync the delay to the song's master tempo.
- Time - In time mode, the delay unit measures the delay time in ms. This gives the user more precise control over the delay time and allows for delay effects that aren't synced with the master tempo.
- Feedback - Sets the amount of the delay's output that is sent back to its input.
Types of Delay
Rather than using digital sampling to create individual echoes, tape delay units used a physical, repeating tape loop. The saturation combined with the wow and flutter created by the tape resulted in a very unique sound. Many digital delay plugins try to emulate this vintage analog sound. Look out for a 'tape delay' setting in your delay plugin, or for specific delay plugins that emulate tape.
Slapback delay is achieved by playing back the wet signal between 70 – 120 ms after the dry signal is played. The result is a quick doubling effects that fills out an arrangement with quick subtle delays with a fast decay.  Normally slapback delay includes little to no feedback.
More pronounced uses of delay were popularized by dub and reggae throughout the 70s and 80s. The effects of delay are a lot more prevalent in these examples as sounds are repeated, echoed, and fed back to achieve richer layering to create a rhythmic symphony of psychedelia. 
Ping Pong Delay
Ping-pong delay is a type of dual delay where the first echo appears in the 'ping' channel (usually the left), delayed by the ping amount, and the second appears in the opposite 'pong' channel, delayed by the ping time plus the pong time. For example, if you set the ping to 200ms and the pong to 400ms, you'd first hear the ping 200ms after the dry signal out of the left channel, and the Pong 600ms after the dry signal out of the right channel. This process will then repeat, assuming the feedback values are higher than zero. 
The German scientist Helmut Haas wrote that when two identical signals, each played through a separate speaker, are delayed by anything from 1-30ms, a sense of a broadening of the primary sound source is heard, but without there being a perceptible echo. This effect, often referred to as the 'Haas effect', can be created using delay plug-ins or track offsets, and can be used to add stereo width to a sound. 
The simplest way to use the Haas Effect is to add a delay plug-in to a track and increase the right output channel's delay time to 10ms more than the left. You should notice that rather than creating an echo, this short delay offset will make the track feel 'wider', thereby introducing a stereo spatialization effect.
Another approach is to duplicate a track, hard pan the original left and the duplicate right, then apply a delay to one of the tracks. If your DAW has the capability to set a track delay, you may use it to accomplish this. Otherwise, use a simple delay plugin that has a small (e.g. 10ms) delay, a 100% wet signal, and 0 feedback.
Unfortunately, this approach can create problematic side-effects. First, simply delaying one side in relation to the other will result in unpleasant comb filtering when the left and right channels are summed to mono. Therefore, this sort of processing can be risky if you want to ensure mono compatibility. 
A second problem with this treatment is that you can perceive that the undelayed side as louder than the delayed side, causing potential balance problems. 
To take advantage of the widening effect the Haas delays offer while avoiding the mono-compatibility trap, try adding a small amount of pitch-shifting to the left and right channels. To achieve this effect:
1. Set a mono delay to 11ms and with zero feedback, then pan it hard right.
2. Then, add a pitch-shift plug-in to take it up seven cents. Repeat this with a second delay, but this time pan it hard left, and pitch-shift it down by seven cents.
Alternative: Ping-Pong Delay
Very short ping-pong delays (e.g. 30ms) can also make excellent wideners. The output of the ping delay can also be polarity inverted before going into the pong delay. This technique is great for transparent widening and has excellent mono compatibility. 
- https://ask.audio/articles/music-production-techniques-part-1-delay-and-echo Cite error: Invalid
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