Curtis Schwartz explores some of the creative applications of digital delays.

It is not unusual for someone to take a look at a rack full of effects in a studio, and wonder "what is the point of having all those 'echo' units?" A typical 24-track studio can have as many as seven or eight DDLs (Digital Delay Lines), a couple of which may cost well over five thousand pounds, and have a delay time of up to five or six seconds. If thought of as simply being devices for providing good quality repeat effects, then having seven or eight DDLs might well seem extravagant. However, DDLs are much more than simply 'echo units', and there are different DDLs for different purposes. The extent of uses to which a DDL can be put can extend a lot further than the established Chorus, ADT Flanging, Repeat Echo and Slap-Back Echos.

Another way to enrich a sound with a DDL, is by setting it on a long delay time — say 700mS, and mix this with the direct sound of long-sustained strings, for example. On the second passage I use this effect on the Korg 3000 to build up incoherent modulation in the sound, which simulates the effect of many instruments playing together. It is additionally possible to, very subtly, modulate the delayed signal with the VCO on the Digital Delay. This will add a chorus effect to the sound, to thicken the sustained sounds even more.

In this article I'll outline the different uses, other than the 'norm', to which a DDL can be put, in conjunction with some practical demonstrations on the accompanying cassette.

What, How and Why is a DDL...

A Digital Delay Line is basically a device which uses digital technology to store sounds and then play them back after a certain length of time.

It stores the sounds by taking 'samples' at regular intervals and 'measuring' what the sound is doing at that moment. On their own, these measurements don't mean much, they have to be combined with other samples in order that a sound may be produced. As audible sound can change at up to 20,000 times a second, the sampling frequency must be very high in order that high fidelity be maintained. If the unit does not take samples often enough, it is impossible to get a good frequency response. This is because the high frequencies change the most quickly. Thus the sampling frequency must be two or (better) three times faster than the highest frequency to be reproduced, for the sound to be reproduced faithfully. Thus to reproduce sounds faithfully up to 20kHz, it is necessary to have a sampling frequency of 40kHz or more.

Having a good frequency response is not the only consideration. It is equally important that the sounds be reproduced with the minimum of noise and distortion, and this is where 'bits' come into it. If, for example, a sample measured 9.2 and the circuit could only handle values like 7, 8 and 9; then 9.2 would come out of the digital circuit with a value of 9.0. This would be called 'Quantizing Distortion', which has a characteristic 'gritty' tone. This can be only be avoided by making sure that the gaps between numbers (resolution) are small enough for the distortion to no longer be audible. The more 'bits', the higher the resolution and therefore the less distortion.

Having lots of bits also directly increases the basic dynamic range and signal-to-noise ratio of the unit because it increases the range of numbers that can be handled.

It is in this way that a DDL can store sounds, and after the selected delay time, 'de-codes' them and can then play them back either straight, or with modulation, and phase inversion etc.

Creative Delays

I have selected four DDLs for varying prices under £1,000 to demonstrate the kind of effects that can be produced which are a little more 'creative' than a straightforward flange or repeat echo:

Boss DE-200 Digital Delay RRP: £375.00 (Typically £299.00)

This is Boss's first rackmounted effects unit, and has a maximum delay time of up to 1,280mSec. The delay times are defined into two modes — x1 and x2.The maximum delay time is the x1 mode is 640mS and in this mode the frequency response extends from 10Hz to 10kHz. If the unit is then switched to the x2 mode, the maximum delay time extends to 1,280mS, although the frequency response then drops down to 4.5kHz in the top end. Even so, the sound quality is still very reasonable for most amateur and semi-professional uses (at which level the price is aimed).

It has a full complement of modulation, feedback and mixing controls; however, what is of special interest here, are the two sockets on the back panel under the header "Rhythm Sync". Labelled 'Foot Switch' and 'Trig In', these sockets make it possible to trigger whatever is in the DE-200's memory from an external source (say a Drum Machine, Foot Pedal or Sequencer etc), more of which later...

Korg SDD-1000 RRP: £375.00

Korg have been producing rack mounted effects for some time now, and this is their latest offering. A delay time of 1,024mS is available with a frequency response up to 10kHz, which can be doubled up to a 'whopping' 2,048mS, however this then reduces the top end down to 5kHz. All the usual modulation, mixing and feedback controls are available as well as an extremely useful section labelled 'Rec Sync'. This has three settings — Trig Overdub, Sampling and Sequencer. These facilities make it possible to set the unit into sampling mode standby, and as soon as there is a signal input into its circuitry, the sampling will automatically start, and can hold up to 2,048mS of information (depending on the delay setting). This can then be remotely triggered from an external source as on the Boss DDL.

Peavey 1300 'Digital Effects Processor RRP: £553.72

This is the first quality effects processor to come from the Peavey camp. It is capable of delay times of up to 1,310mS, yet its price difference is justified by its exceptional sound quality of the delayed signal — a frequency response up to 20kHz at all delay settings (with only ±1dB)! It is able to do this by using 12 bit sampling (as does the Boss) yet with a 50kHz sampling rate. Another 'plus' is a VCO waveshape that is variable between Triangle and Sine waves which are 180 degrees out-of-phase. One omission, however, is that it has only two LEDs for input metering, which seems odd for a studio-quality piece of equipment; however, as in all four of these DDLs it has all the 'compulsory' controls for modulation, feedback, phase inversion, hold and bypass.

Korg SDD-3000 Programmable DDL RRP: £999.95

You may well wonder at the reasoning behind a comparison between DDLs priced between three and five hundred pounds with one costing twice that amount — well (other than 'it seemed like a good idea at the time'...) the fact is that I am not really comparing these DDLs; rather, I am hoping to demonstrate what can be achieved with affordable-ish Digital Delays...

This item, again from Korg, has some additional signal processing features, as well as the ability to memorise most of the front panel control settings into nine individual memory locations. A maximum delay time of 1,023mS with a high end response of 17kHz is available with only 0.03% distortion and a massive 94dB dynamic range and 88dB signal-to-noise ratio. In effect (excuse the unintentional pun) this means that the delayed signal's sound quality is quite superb, and all this is done without any noise reduction or 'compansion'. On top of this, it has versatile input and output level attenuation, six step ladder metering, LED read-out of delay and memory settings, four stage high and low filtering for the feedback circuitry and sine, square, random and envelope following waveforms for the modulation section.

The 3000 is, not surprisingly, a joy to work with, however the omission of external triggering is a little sad, especially as its little brother, the SDD-1000, is so versatile in this region. However, it is an excellent unit, offering sophisticated control over its sound processing for live and studio work. It also has to be the only contender for the live band with 'mega-bucks' for PA and stage use.

The Fun Stuff

All four of these digital delay lines sport a hold function on their longer delay times. By first building up a repeating rhythmic phrase (on a high-ish feedback and delay time setting) and then holding it into the memory, it is possible to repeat a phrase indefinitely without a degradation of the sound quality. This can then be 'played over', live with either the same sound (on a synth in this case) or an entirely different sound. I demonstrate this effect on the Peavey 1300 in the first passage from the Digital Delay Demonstration section of the accompanying cassette.

As both the Boss DE-200 and the Korg SDD-1000 have sockets for external triggering input, it's possible with these DDLs to synchronise the repeats to an external device, such as a drum machine or sequencer. For example, on the third passage, I synchronise two sampled notes in the SDD-1000 to a Drumulator. This was done by programming a cowbell into a rhythm pattern, and connecting the cowbell's output to the 1000's Trig In. In this way, the length of the sample can be altered by re-triggering it before it has ended. This, you might recall, is how the Duran Duran lyric 'Reflec-flec-flex' came about.

On all four units there are two outputs for the delayed signal. These are out-of-phase with one another, and if individually assigned to the right and left of the stereo mix, they can provide a 'quasi-stereo' effect that actually disappears when played back in mono! Therefore, if you had the DDL set up for a hard ADT effect, this would add avery thick texture to the mix, and yet it would not clutter the mono playback of the sound, which might otherwise become too obvious.

On the intro of the cassette, I have echoed the words "digital delay" at the end of the narration, and have used the phasing of the signals to cancel out the echo when played back in mono, yet remaining audible in stereo.

On the final piece of music I used both the SDD100 and Boss DE200 to sample various household sounds such as a banging door, dog(!) footstamping etc to show some of the more 'unusual' delaying tactics.

Each of these Digital Delay Lines has its own nuances, and its own 'niche' in the type of use to which it may be put — the Peavey 1300 for impeccable sound quality, the Boss DE-200 for sequencing and external triggering, the SDD3000 for the 'Action Men' who need versatility and fast changes at the press of a button, and Korg's SDD1000 for most things... especially for giving the easiest and fastest sampling in the West!

As delay times become longer and longer, the division between Digital Delay Lines and Digital Mastering could become increasingly blurred. The company 'Advanced Music Systems' (AMS) are producing the ultimate (and most expensive at £4000+) DDLs at the moment with delay times exceeding 30 seconds! In an AMS Digital Delay, it is possible to edit a sound after it has been locked into the memory; and the uses of these high quality and longer delay times can be in storing vocal passages — as in the Reflex, or a particular vocal or instrumental phrase with the best 'feel', which could then be inserted into any place within a song. The possibilities are limited by imagination and bank balance.