Delay is one of the most widely used effects in music and audio today. Simply stated, a delay is one or more repeats of a sound, where the repeat happens some time after the original sound.
Delays are ubiquitous in nature, but are not usually heard as such. The classic example of an echo is shouted at a distant canyon wall, and hearing the sound repeated back to you. However, delays occur wherever there is a surface that is reflective enough to bounce a sound back to the listener, which describes pretty much everything except absorbent surfaces such as foam rubber and snow. For the most part, echoes are either heard as coloration to the original sound (comb filters), or as reverberation, which consists of many short delays that blend into each other so that they aren’t heard as discrete echoes.
The earliest use of delay in recordings was realized using reel-to-reel tape recorders. In a tape recorder, the playback head is located a few inches away from the record head. If the playback head is used to monitor the recording in real time, the input signal to the tape deck will be delayed by a factor that depends on the tape speed, as well as the distance between the record and playback head. This technique can be heard on Les Paul’s guitar and Mary Ford’s vocals in their 1951 recording of “How High The Moon”:
https://www.youtube.com/watch?v=ihIR81n9I0Y
Starting in the late 1950s, dedicated tape echoes such as the Echoplex were used to create delay effects. These tape echoes used a loop of tape, that was continually being recorded onto (and later erased). The tape loops would quickly become worn and stretched, producing all sorts of distortion and warbling pitch artifacts. At the time, these artifacts were considered a drawback, but today tape echoes are sought out precisely for their warm, slightly pitch modulated sound, as well as for the unique oscillating sound produced when the feedback is turned up.
In the early 1970s, bucket brigade delays (BBD) were developed. These used a specialized integrated circuit that samples the input voltage, and moves the voltage along a series of capacitors, with the voltage being transferred once per clock cycle. Bucket brigade delays are known for their warm sound, which is mainly due to the filtering and noise reduction techniques used to bring the fidelity up to passable standards.
BBD pedals such as the Electro-Harmonix Memory Man added LFOs to the basic circuit. By modulating the delay time via an LFO, the pitch of the output could be slightly varied in time, producing a vibrato sound. BBD chips were also used in chorus and flanger pedals such as the Boss CE-1 and Electro-Harmonix Electric Mistress, usually using BBD chips that had much shorter delay times.
The 1970s also saw the introduction of digital delays. A digital delay samples the input signal, and stores the results in digital memory. In theory, this can produce a delay that is an exact duplicate of the original signal, just shifted in time. In practice, the limitations of early digital circuitry (such as fairly low sampling rates and bit resolutions) meant that early digital delays had to use some of the same filtering and noise reduction tricks as BBD delays. Older digital delays often had a fairly warm and gritty sound; it wasn’t until the later 80s that advances in digital technology brought the fidelity up to the level that was promised by the theory.
Today, digital delays are ubiquitous, both in hardware and in software form. Many of the current digital delay offerings embrace the high fidelity that is inherent to modern technology. Other digital delays emulate the tape and BBD delays of the 60s and 70s.