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Digital Delay Tactics

Tips & Techniques By Paul White
Published April 1995

Digital delay tends to be taken very much for granted, but the majority of today's studio effects would be impossible without it. Paul White describes how to set up your own DDL effects from scratch.

Though we have come to accept digital reverberation as perhaps the most important of all studio effects, it's probably still true to say that the simple digital delay, or DDL, is the most versatile. The majority of multi‑effects units offer digital delay as part of their repertoire, but all the most common delay‑based effects can be produced using any digital delay line, manual or programmable, that has modulation facilities and control over delay time and feedback. The range of effects that can be coaxed out of a DDL includes: delay, repeat echo, slap‑back delay, chorus, vibrato, flanging, phasing and resonant 'tunnel' echo.

In the early days of digital delay, the units were all controlled by knobs and were non‑programmable. This meant that you had to learn to use the machine, because there were no presets — but because there was a dedicated knob for each function, it was easy to fiddle around until you got what you wanted. With a modern programmable unit, setting the parameters is less intuitive, so it helps if you know how the effects are created in the first place.

The DDL is fairly simple in principle; a sound is digitised, stored in RAM memory, then, after a short time, it is read out, converted back to an analogue signal and sent to the output socket. The result is a copy of the original sound but slightly delayed. For those of you who remember tape‑loop echo machines, the DDL does the same job but with no moving parts. But, because the DDL has a much wider range of delay times than a typical tape echo unit, and because it has delay modulation facilities, it can be used to create a far wider range of effects.

As well as having the option to vary delay time from a few milliseconds to around one second or more on a typical DDL, we can vary the feedback to produce more than one repeat from a single sound. All the feedback control does is to send some of the delayed output back to the input so it gets delayed again; the more feedback, the more repeats. But if you set the feedback too high, the level of the repeats builds up rather than dying away, until the result is an uncontrollable howl. This feedback effect was often used with tape echo machines to create sci‑fi effects, but with a DDL, the distortion caused by level overload when feedback occurs usually results in a pretty unpleasant noise.


The modulation section is based around a low‑frequency oscillator, rather like the LFO on a synth, which modulates the delay time in much the same way as rapidly turning the delay knob up and down would do. The available controls are speed and depth, which determine how the sound is modulated; modulating the delay time causes the pitch to waver both sharp and flat at the rate set by the speed control. The depth control sets how far sharp or flat the sound goes, but it should be noted that the longer the delay time you have set, the more pitch variation for the same depth setting.

With the old tape echo units, pitch modulation could be achieved manually by turning the tape speed knob back and forth, but to my knowledge, none of the units had a device for automatically modulating the capstan motor speed. Even so, many were so badly built that the inherent wow and flutter, usually caused by less than perfectly round capstans, or squashed pinch rollers, served to add a little natural pitch variation to the echo — an effect that was quite pleasing in moderation. Of course, you could simulate this using a digital delay unit, simply by adding a very mild modulation (at between 4 and 8Hz) to the basic delay setting.

Every DDL has a mix control allowing you to balance the original and delayed sounds, and some may have a feedback phase invert switch, which I'll look at in more detail a little later.

Creating Delay Effects

  • The simplest effect is a straight, single delay. For this you set the modulation depth to zero, the feedback to zero, and the mix control to around halfway. What you get is a single repeat that occurs after whatever delay you have set on the delay time control. Short delays of between 30 and 100mS are used to create slap‑back echo effects, which are quite effective on vocals and rock guitar, while longer delays can be tweaked to fall in with the tempo of your song, to achieve rhythmic effects, particularly when applied to percussion instruments. A timed delay with a percussive synth sound can also help produce a mechanically sequenced or arpeggiated effect.
  • Multiple delays are easily obtained by increasing the feedback control, but the subjective result is very different depending on whether a long or short delay time is being used. At delay times in excess of 100mS, you'll get the familiar tape echo type of sound, and this is a valuable effect for warming up vocals and guitar. On clean electric guitar, this type of echo helps recreate the old (and once again popular) Shadows guitar sound. To some extent, digital reverb has replaced echo as a vocal treatment, but repeat echo is still worth exploring, especially if you're after vintage sounds. Most of the early rock singers used liberal amounts of vocal echo, and the famous John Lennon 'trippy' vocal sound relied heavily on short, 'slap‑back' delay.
  • At very short delay times, (1 to 50mS) increasing feedback will give a resonant cardboard tube or tunnel echo sound, the pitch of the resonance being set by the delay time. This effect is useful in creating new sounds or modifying existing ones beyond recognition; used with a synth, it can create the illusion of ring modulation or phase sync.

Modulating The Delay

    The modulation section is what makes the DDL so versatile. Probably the simplest modulation effect is pitch vibrato, and for that we need to set the mix control (or parameter) fully up, so that only the delayed sound is heard, with none of the original added. Obviously the output will be delayed slightly relative to the input, but if the delay time is set to less than 10mS, the delay will be too short to notice. If you then set a modulation rate of 4 or 5Hz and turn the depth control up slowly, you should end up with a pleasing pitch vibrato. Turn the depth up too far and the result will cease to become musically useful, though it might still work as a special effect.
    Early phasing on records was done with two tape machines playing almost in sync. Nowadays, we tend to refer to this effect as tape flanging, but in the days when it was actually used, everyone knew it as phasing.

It's a simple matter to convert the earlier vibrato effect to phasing by setting the mix control to exactly half way; the best effects are obtained with delay times of between 1 and 10mS. With such a short delay time, the individual harmonics that make up your sound are moved in and out of phase with each other so that some cancel while others add. This has the effect of filtering the sound in a very dynamic and complex way — you'll recognise the effect as being similar to that produced by guitar phaser pedals. Try changing the rate control from a slow sweep to a fast modulation and you'll see that the range of effects is quite wide. A very slow modulation rate generates a nice, evolving sweep which can sound great on pad keyboard sounds, while faster rates can sound similar to a rotary speaker cabinet.

You can also 'tune' which harmonics are affected by changing the basic delay time; the shorter the delay time, the higher the frequencies that are affected, and vice versa. A delay time of between 1 and 3mS gives a thin, whining phase sound, whereas longer times sound fatter or smoother.

    Electronic phasing never achieved the depth of tape phasing, but when electronic delay chips were invented, Flangers soon followed, in a bid to recreate this most spectacular effect (well, we thought it was spectacular in 1972!). The original phasing effect was created by running two tape machines carrying copies of the same music, side by side. When the two machines were in time with each other, the two signals added up to a more or less normal sound, but if one or other of the machines was slowed down slightly by touching the tape supply reel, the resulting delay would create a phasing effect. By slowing one machine and then the other, the phasing effect could be maintained without the machines getting too far out of time with each other.

Electronic flanging involves the use of feedback to make the effect even stronger. Essentially flanging is similar to phasing, though it may use slightly longer delay times — say up to 50mS — and the feedback control is advanced to give a dramatic, swirling effect. In general terms, the slower the modulation rate, the more depth you can get away with using. The higher the feedback setting, the more 'whooshy' the sound, and on digital units, you may have to turn the input level down a touch to prevent overload when high feedback settings are used. Instead of using the modulation controls, you can try changing the delay time manually, which produces a closer approximation to tape phasing/flanging.

    And now that mysterious phase invert button or parameter that your unit may or may not have; the effect of this is most noticeable on effects that use a very short delay time, particularly flanging. By inverting the phase of the signal fed back to the input, it allows different harmonics to be accentuated by the filtering process, and so gives a choice of two types of tonal coloration, one usually sounding thinner than the other; which one you use is entirely a matter of personal choice. On longer delay times, the invert facility will produce little or no subjective change to the sound.

Effective though the above processes are, they don't exactly duplicate the tape flanging effect, because the delay can never pass through the zero‑delay point as it did when one tape machine overtook the other. Some people have attempted to get closer to the original sound by adding a short delay of 5mS or so to the dry part of the signal (which allows the relative delay between the dry and modulated signals to pass through zero), but this doesn't seem to be entirely convincing either.

    Chorus is so‑called because it goes some way towards imitating the sound of two or more instruments or voices playing the same part together. No two singers or players will have exactly the same pitch or timing, so by adding a delayed, modulated version of the original, you can create the illusion of multiple performers. By setting a delay time of between 30 and 100mS and adding a little gentle modulation with no feedback, you get the classic chorus effect which works so well on electric guitars and string synth patches, and normally, the mix control should be set centrally so that you hear an equal amount of dry and modulated sound. The modulation speed is normally set in the range 2Hz to 6Hz and the depth set by ear so as not to sound too out‑of‑tune. Fast chorus can also be used in place of rotary speaker effects.

Because of the regular modulation, the electric chorus effect isn't entirely authentic compared with real life, but has gained acceptance as an effect in its own right. You can also use very gentle modulation on longer delays to create a combined chorus and echo effect.

Chorus effects can be made to sound more dramatic in stereo by panning the original, untreated sound to one side and the modulated delay to the other. Flanging and phasing, on the other hand, only work properly when the original and delayed sounds are heard from the same speaker and mixed in roughly equal proportions.


Versatile though DDLs are, they can't be used to create natural sounding reverberation effects. In nature, reverberation is created when sound bounces around inside a confined space such as a hall or cavern, and the complexity of these echo patterns defies mathematical analyses. In order to fool the human ear into believing electronic reverberation is real, it is necessary to generate several thousand, randomly‑spaced echoes every second which, if done digitally, requires a great deal of complicated processing.

DDL‑based effects are usually connected into the effects send system of a mixer so that more than one tape track or mixer channel can be treated with the same effect. In this case, the mix control on the effects unit would normally be set to give the 'effect only' signal at the output and the mixer controls would be used to set the balance of direct and processed sound. However, for phasing and flanging effects, where the dry/effect balance is fairly critical, it may be better to patch in the effect via channel or group insert points and use the unit's own mix control to set the required balance.

All the techniques described in this article can be explored using either a manual or programmable delay unit, though the manual ones are far quicker to set up. The box shows a table of DDL settings that may be used as starting points to set up your own delay‑based effects, and by evaluating the effects on different material, you'll soon get an idea of what can be achieved and may find yourself looking at your DDL in a new light.

Useful Initial Settings

Slap‑Back Delay: 20‑80mS delay, no feedback.

Tunnel Echo/Resonator: 2‑10mS, feedback as high as possible without going unstable.

Tempo Delay: 60/bpm gives you the delay time of 1 bar in seconds. Divide this delay time by four to give you the delay time per quarter note.

Echo: 100‑300mS with feedback. The higher the feedback level, the more repeat echoes.

Vibrato: 3‑10mS, dry signal off, modulation rate 3‑8Hz, depth set by ear.

Phasing: 3‑10mS, mix control 50/50, modulation rate 3‑8Hz, depth set by ear. Try Feedback Invert if you have it.

Flanging: 5‑50mS, mix control 50/50, modulation rate 3‑8Hz, depth set by ear. Increase feedback to make effect more dramatic. Try Feedback Invert if you have it.

Chorus: 30‑100mS, mix control 50/50, modulation 3‑8Hz, depth set by ear. Little or no feedback. Increasing feedback creates a rotary speaker effect.