Delay is an audio effect and an effects unit which records an input signal to an audio storage medium, and then plays it back after a period of time. 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.

Delay effects range from a subtle echo effect to a pronounced blending of previous sounds with new sounds. Delay effects can be created using tape loops, an approach developed in the 1940s and 1950s; analog effects units, which were introduced in the 1970s; digital effects pedals, introduced in 1984; and audio software plugins, developed in the 2000s.

Early systems

The first delay effects were achieved using tape loops improvised on reel-to-reel audio tape recording systems. By shortening or lengthening the loop of tape and adjusting the read and write heads, the nature of the delayed echo could be controlled. This technique was most common among early composers of Musique concrète (Pierre Schaeffer), and composers such as Karlheinz Stockhausen, who had sometimes devised elaborate systems involving long tapes and multiple recorders and playback systems, collectively processing the input of a live performer or ensemble. Audio engineers working in popular music quickly adapted similar techniques, to augment their use of reverberation and other studio technologies designed to simulate natural echo. In the late 1940s and early 1950s, several sound engineers began making devices for use in recording studios and later more compact machines for live purposes. Guitarist and instrument designer Les Paul was an early pioneer in delay devices. A landmark device was the EchoSonic made by American Ray Butts. It is a portable guitar amplifier with a built-in tape echo, which became used widely in country music (Chet Atkins) and especially in rock and roll (Scotty Moore).

Tape echoes became commercially available in the 1950s. An echo machine is the early name for a sound processing device used with electronic instruments to repeat the sound and produce a simulated echo. One example is the Echoplex which used a tape loop. The length of delay was adjusted by changing the distance between the tape record and playback heads. Another example is the Roland Space Echo with a record and multiple playback tape heads and a variable tape speed. The time between echo repeats was adjusted by varying the tape speed. The length or intensity of the echo effect was adjusted by changing the amount of echo signal was fed back into the pre-echo signal. Different effects could be created by combining the different playback heads.

Analog delay

Before the invention of audio delay technology, music employing a delayed echo had to be recorded in a naturally reverberant space, often an inconvenience for musicians and engineers. The popularity of an easy-to-implement real-time echo effect led to the production of systems offering an all-in-one effects unit that could be adjusted to produce echoes of any interval or amplitude. The presence of multiple taps (playback heads) made it possible to have delays at varying rhythmic intervals; this allowed musicians an additional means of expression over natural periodic echoes.

Tape based

Many delay processors based on analog tape recording, used magnetic tape as their recording and playback medium. Electric motors guided a tape loop through a device with a variety of mechanisms allowing modification of the effect's parameters.

Popular models included Ray Butts' EchoSonic (1952), the 1959 Echoplex by Mike Battle, "whose sounds are still being experimented with today.", and the Roland Space Echo (1973),

In the Echoplex EP-2, the play head was fixed, while a combination record and erase head was mounted on a slide, thus the delay time of the echo was adjusted by changing the distance between the record and play heads. In the Space Echo, all of the heads are fixed, but the speed of the tape could be adjusted, changing the delay time. The 1959 Ecco-Fonic had a spinning head.

Thin magnetic tape was not entirely suited for continuous operation, however, so the tape loop had to be replaced from time to time to maintain the audio fidelity of the processed sounds. The Binson Echorec used a rotating magnetic drum or disc (not entirely unlike those used in modern hard disk drives) as its storage medium. This provided an advantage over tape, as the durable drums were able to last for many years with little deterioration in the audio quality.

Often incorporating vacuum tube-based electronics, surviving tape-based delay units are sought by modern musicians who wish to employ some of the timbres achievable with this technology.

Digital delay

Digital delay systems function by sampling the input signal through an analog-to-digital converter, after which the signal is passed through a digital signal processor that records it into a storage buffer, and then plays back the stored audio based on parameters set by the user. The delayed ("wet") output may be mixed with the unmodified ("dry") signal after, or before, it is sent to a digital-to-analog converter for output.

The availability of inexpensive digital signal processing electronics in the late 1970s and 1980s led to the development of the first digital delay effects. Initially digital delay effects they were only available in expensive rack-mounted units, such as the AMS DMX 15-80 of 1978. As digital memory became cheaper in the 1980s, units like Lexicon PCM42, Roland SDE-3000, TC Electronic 2290 offered more than three seconds of delay time, enough to create background loops, rhythms and phrases. The 2290 was upgradeable to 32 seconds and Electro-Harmonix offered a 16-second delay and looping machine. Eventually, as costs came down further and the electronics grew smaller, they became available in the form of foot pedals. The first digital delay offered in a pedal was the Boss DD-2 in 1984. Rack-mounted delay units evolved into digital reverb units and on to digital multi-effects units capable of more sophisticated effects than pure delay, such as reverb and audio time stretching and pitch scaling effects.

Digital delays present an extensive array of options, including a control over the time before playback of the delayed signal. Most also allow the user to select the overall level of the processed signal in relation to the unmodified one, or the level at which the delayed signal is fed back into the buffer, to be repeated again. Some systems allow more exotic controls, such as the ability to add an audio filter, or to play back the buffer's contents in reverse.