• Digitech DSP 128 Plus
  • Digitech DSP 128 Plus
  • Digitech DSP 128 Plus

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Digitech DSP 128 Plus

Multi-Simultaneous Effects Processor

Hot on the heels of the Multiverb and Quadraverb comes Digitech's contender for the title of best 'four effects at once' signal processor. Paul Ireson checks out the form.

Hot on the heels of the Multiverb and Quadraverb comes Digitech's contender for the title of best 'four effects at once' signal processor. Paul Ireson checks out the form.

The standards by which we judge the worth of new technology change at an alarming rate: a product which would have seemed quite remarkable, and might not even have been contemplated two years ago, could easily come and go with relatively little attention being paid today. Examples to fit this scenario can be found in the fields of synthesizers, samplers and effects, and on a shorter time scale, it often seems that a few months can make all the difference between something being perceived as 'excitingly new and different' or just one of 'a recent trend' for units of a certain kind.

So it is, to some extent, with the DSP128 Plus, a new Multi-Simultaneous Effects Processor (MSEP) from Digitech. The DSP128 Plus is based on Digitech's DSP128, which itself hasn't been around for all that long. The original version is capable of producing three high quality effects at the same time, applied in series to the input signal. At the time this generated a good deal of excitement, but since other manufacturers quickly went one better with signal processors that can produce four effects simultaneously, the DSP128 didn't look quite so hot anymore. The updated Plus version therefore has the improved features necessary to make it a contender again: four effects at once, and the ability to assign its effects parameters to MIDI Continuous Controllers.

Good though this is, the fact that it comes after the release of the ART Multiverb and Alesis Quadraverb somewhat takes the edge off the excitement. However, such reactions can be misleading in that it is the true strengths and weaknesses of equipment that really count: what product came first really doesn't matter - whether this new box will do just what you need does.


Moving on to first impressions of a different kind, the DSP128 Plus resembles many other effects units in that it comes in a 1U rackmount case and is relatively shallow, projecting back only about eight inches or so into a rack. Unlike so many other units, however, the case is brushed aluminium not matt black - a refreshing change. The principal front panel controls are a power switch and three rotary pots which set Input, Output and Mix levels. Four cursor and parameter buttons, located next to a four-digit LED display, are provided for editing and selecting effects programs from the front panel.

Other features of the front panel are a 4x3 grid of LEDs, up to four of which are illuminated at any one time to show which of the unit's effects are in use. MIDI and Bypass buttons complete the roster.

Audio inputs and outputs are via two sets of left and right jack sockets, which share the rear panel with two footswitch jacks and MIDI In and Out/Thru ports. The (optional) footswitches can initiate program shift, bypass and delay hold functions. A small switch adjacent to the input jacks allows the input sensitivity to be switched for optimum performance with both +4dB line and -20dB instrument level signals. Inside the DSP128 Plus, D/A and A/D signal conversion is 16-bit linear as you'd expect on any well-bred signal processor these days.


The effects produced by the DSP128 Plus emanate from four processing sections: EQ, Delay, Chorus/Flange and Reverb. How these sections are used is not under the total control of the user, rather they are configured by algorithms. Any of 14 algorithms (see 'Effect Algorithms' box) can be chosen as the basis for an effects program, and each of the algorithms specifies which of the effects sections are utilised, exactly what type of effect is produced by each, and how audio signals are routed between them.

One aspect of these algorithms that is common to other MSEPs is that there is a trade-off between the number and complexity of effects used. The first six algorithms are the only ones that use all four effects sections, providing various combinations of EQ and mono delay (up to 750 milliseconds) with chorus or flange, and small, medium or large room reverb treatments. The next two algorithms offer stereo multi-tap delay combined with EQ and either chorus or flange, and the rest offer such delights as reverse and gated reverb, parametric and graphic EQ, and the immodestly titled Ultimate Reverb.

All programming functions on the unit are carried out with the cursor and parameter buttons, and the four-digit LED display provides modest visual feedback. On powering up, the display reads 'P 1', indicating that Program 1 is currently selected. When the current program number is displayed, the parameter buttons can be used to step through the 128 programs one by one, although effects programs can also be selected by MIDI Program Change messages, of course. The cursor buttons are used to access the various effects parameters: each push on the buttons cycles one way or the other through the pages representing the various parameters, and on each page the parameter buttons can be used to edit the parameter values. Each parameter is represented by a two-letter abbreviation ('rd' for reverb delay, 'Fl' for flange level, and so on) and next to each abbreviation is a number indicating that parameter's current value. Perhaps I've been spoilt by too many 32-character LCD displays recently, but the four-digit LED display of the DSP128 Plus does seem rather inadequate for satisfactory programming.


Running through the 128 factory programs, the unit sounds very good, though there's not actually a huge variety of effects on offer - my initial impressions were of an inordinate number of variations on fairly straightforward reverb and delay combinations. Interestingly, a large number of the programs seemed especially suitable for guitar - lots of slapback echo and reverb. Bearing in mind that the input sensitivity of the DSP 128 Plus is switchable to cope with both line and instrument level signals, it seems that Digitech have very much kept the guitarist in mind when designing the DSP128 Plus. A further axe-oriented feature is the facility to switch the delay into 'infinite hold' mode with a footswitch, whereby it simply repeats the current contents of its memory buffer over and over.

Some of the more interesting factory programs feature heavily flanged echoes, a combination of delay and reverse or gated reverb effects ranging from the subtle to the bizarre, and a good number of pure reverb programs. Sound quality is excellent - although I've just suggested that this unit has been designed with the guitarist in mind, it's definitely a studio quality effects processor. All effects sections process signals at full bandwidth (20-20kHz), and the result is that even at the top and bottom end of the audio spectrum the processed signal is clear and well defined.

Having checked out the presets I began to investigate the algorithms that lie behind them, and just what kind of effects are available within each. The first six algorithms offer identical delay and EQ sections, with only the reverb and chorus/flange options changing. Delay time is up to 750ms, and the EQ is a simple lowpass filter. The three reverb programs - small, medium and large rooms - offer maximum reverb decay times of 1.1, 2.8 and 20 seconds respectively. All are very smooth, authentic effects, with a lot of stereo depth. The stereo chorus effect is clean, and more subtle than many I've heard: it doesn't give much of a sweep to a sound, more of an unobtrusive thickening, which is lovely in stereo. Speed, depth and delay amounts are all variable. The flange, on the other hand, has a good deal more bite: turning the depth and feedback up adds a vicious, swirling edge to the processed signal. As always seems to be the case with this effect, when no signal is being processed, a heavy flange setting produces a good deal of background noise in the form of a kind of swept whistle/hiss. In general, however, such an effect would not be applied to any sound in a context where the noise would show up much.

There's no indication in the DSP128 Plus manual of just how the signal is routed from one effects section to another, but it sounds very much like EQ-Delay-Flange-Reverb to me. The EQ section in these six algorithms consists only of a low-pass filter, with programmable roll-off frequency. This is not a particularly powerful tonal tool, though it is at least quiet. An output level can be set for each effects section, apart from the filter (EQ), through which the input is automatically passed. It seems that at each stage of the effects chain, where a level for any section is set, what is actually being specified is a mix balance between the signal at the input and output of that section: so setting the delay level to zero does not mean that there is no signal being sent to the input of the flange/chorus section, rather that the signal being processed by this section has not previously been processed by the delay section. This caused me a moment of confusion initially, as I was expecting to find some means of controlling the routing of the effects signal, however it actually makes the unit much easier to use, albeit at the expense of a little flexibility. There are never any problems with the signal getting 'lost' anywhere because you forgot to set a particular mix level: you just set the amount of each effect that you want.


The next algorithms I tried were the two containing the multi-tap delays, which provide stereo repeat echoes. These multi-tap facilities were the first things to really impress me on the DSP128 Plus. Independent initial delay times (up to 1.25 seconds) can be set for left, right and centre repeats, and subsequent repeats at each of the three stereo locations take place at a rate determined by a common feedback delay time (also up to 1.25 secs).

With four separate delay times to play with, some intriguing effects can be obtained. If the feedback delay time is set to a greater value than all three of the initial repeat delays, the 'rhythm' of the initial three repeats is repeated over and over. If it is set to a lower value than any of the three, the impression of a simple repeating pattern is replaced by a more complex start to the series of echoes. A good trick is to set the initial delay times for the three stereo positions to almost - but not quite - the same value. A few milliseconds difference between the placement of the repeats at each position is enough to give the echoes a refreshing stereo depth and movement. Other possibilities include more obviously panned repeats, or programs with echoes ricocheting back and forth in the stereo picture.

As with the mono delay effect, the number of repeats is set by the Feedback parameter, which enables anything from a single echo to infinite repeats to be created. At 99% feedback, I lost count of the number of repeats a little beyond 200, just as they were beginning to fade away. The infinite repeat facility on the delay section can be initiated with a footswitch, or via MIDI as I'll describe later. The fact that the stereo pattern of echoes is maintained on each repeat means that the effects produced by the DSP128 Plus's multi-tap delays have a real presence in stereo.

Further possibilities include producing a pseudo panning effect, or in these two algorithms, the flange or chorus effect is placed in line before the multi-tap delay, so that echoed flanged notes are heard rather than flanged echoes (and there is a real difference). Because each effects section seems to be capable of processing only a mono input, the stereo effect of the chorus/flange is lost, but in these algorithms it is obviously preferable to lose the stereo on the chorus/flange rather than on the multi-tap delay.


Having been suitably impressed by the multi-tap delay functions, the next thing to grab me about the DSP128 Plus was its Ultimate Reverb algorithm. In this configuration, all of the unit's processing power is dedicated to producing a complex reverb effect. 10 parameters are available to create and alter the effect, which can simulate a huge range of both real and very unreal acoustic environments.

The best starting point for creating your own programs with this algorithm are the Normalised Reflectivity (0.1 -1.0) and Normalised Room Volume (1.0-99) parameters. These determine the volume of the simulated listening environment, and the reflectivity of its surfaces. So, anything from a highly reflective bathroom to a train station wallpapered with mattresses can be simulated (and haven't you always wanted to know what a train station wallpapered with mattresses would sound like? You have? Weird.) According to the manual, the reverb decay time (in seconds) is the product of the numerical values of these two parameters: I tried this out with the maximum values for both, and though it was quite hard to decide on a point where the reverb tail was actually quieter than the noise floor when it decays this slowly, there was undoubtedly around a minute and a half of beautifully decaying reverb. Whereas the Normalised Reflectivity parameter controls the reflectivity of all frequencies equally, dP (damping) allows the relative decay rates of high and low frequency components to be balanced - the simulated environment can therefore be made to sound more or less bright. I found that setting this parameter to more than 7 (from 1-10) produced a very harsh, artificial sweep, more like a filter envelope closing down sharply than a natural damping.

Having created an environment of the right size, the next aspect of the reverb effect you might choose to change is the position of the listener relative to the sound source, using the Early Reflection Level and Subsequent Reflection Level parameters. Setting the Early Reflection Level higher than the Subsequent Reflection Level will create the impression of proximity to the source; reversing the situation will create an impression of distance. Further parameters determine the characteristics of both the early and subsequent parts of the reverb, with (pre) Delay and Diffusion parameters available for both. The Delay simply delays the start of that part of the reverb sound by up to 80 milliseconds, and it is best to set the Early Delay parameter to a lower value than the Subsequent Delay parameter to produce a natural effect.

The Diffusion parameters control how dense the reverb is, and it is important to set correct values for diffusion to match the 'size' of reverb created. High values tend to produce a metallic, ringing sound, and low values produce a grainy, fluttery sound: somewhere in-between is therefore generally preferable, unless you're setting out to create intentionally nasty reverb effects. The quality and smoothness of reverb produced by a particular diffusion level varies according to the size of the simulated environment, and larger environments tend to require higher diffusion values.

The final parameter is Envelopment, which controls the stereo spread of the reverb. I cranked this up to maximum all the time, and I imagine most other users will too, to give as much depth as possible to the sound. Interestingly, low values (3 or below) seemed to create a kind of flanging or phasing effect.

The Ultimate Reverb algorithm allows huge scope for creating your own reverb effects, but it does take a little time and experimentation to get the best out of it. As is always the way with these things, giving the user control over more aspects of sound creation/processing allows more unpleasant and useless sounds to be generated along with all the good ones. I found that a good deal of tweaking was often required with this algorithm to remove or minimise unexpected background ringing, and simply to find the best balance of Diffusion and Delay parameters. Overall, the reverb did seem to be slightly 'dirtier' sounding than that available in the first six algorithms, though I suspect that's partly because there's an awful lot more of it.

The gated and reverse reverb effects provided by two dedicated algorithms are both good, and allow for considerable control to be exercised over effects that can all too easily sound a little cliched. With distinctive effects like this, it's very easy to grow tired of them quickly if they sound identical every time you use them. Further variation can be introduced by the delay section, which is placed in series after the reverb. The length of time that the reverb gate is held open is variable up to 600 milliseconds, and the reverb is very dense and strong - an excellent effect. On the reverse reverb algorithm, the strength and placement of the accent at the end of the reverb build-up can be varied to produce a good range of effects: it's probably best described as a 'whooosh' of reverb with more or less snap on the 'sh', and it's a great way to produce pseudo-backwards guitar effects.

Two further algorithms place a two-band parametric EQ in line before a chorus or flange, and full mono delay (up to 1.25 seconds).

I have spent a good deal of time and space describing the Ultimate Reverb algorithm, but I make no apologies for this as it's one of the DSP128 Plus's most interesting features. That and the delay section in multi-effects configurations - both with normal and multitap delays - are the strongest of the DSP128 Plus's effects capabilities, and in these specific areas give the unit a distinct edge over its direct rivals. On the other hand, it sacrifices a little of the breadth of effects offered by other units: the best form of equalisation that the DSP offers is a 9-band graphic EQ with 12dB cut/boost (algorithm 14); by way of comparison, the Alesis Quadraverb can provide an 11-band graphic with 14dB cut/boost, as well as a delay section on top of that. Nevertheless, the Digitech unit is still capable of producing all the important basic effects that people buy multieffects units for: vocal doubling; thickening programs for strings/pads; delays; and normal, gated and reverse reverb.


One of the significant enhancements to the DSP128's facilities that has been made with the Plus version is that any of its effects parameters can now be controlled in real time with MIDI Continuous Controllers. Thus, the level or decay time of reverb (say) can be modulated with a synthesizer's mod wheel, or anything else that can generate Continuous Controller information. With a Roland A50 master keyboard conveniently to hand, I set up its four sliders to transmit MIDI Continuous Controller numbers 11-14. I then allocated these controllers to reverb and delay levels, and delay and flange feedback parameters, giving myself real-time remote control over these effects parameters from my master keyboard. With the delay feedback parameter, infinite hold is enabled when the parameter is raised to its maximum value by a remote controller. Although any Continuous Controllers (apart from Polyphonic Aftertouch) can be used to control parameters, Note Velocity cannot, which is a bit of a shame as it's the means of expression that is most easily available and controllable for the average keyboard player. A slight limitation to the usefulness of this realtime control over effects parameters is that changes in parameter values do produce slight 'zipper noise' (a series of clicks generated as the values increment), but the pure effect level has to be turned up very high in a mix for this to become obtrusive.


I began this review by outlining the way in which our perceptions of new products are radically affected by what has come before. To some extent this is justifiable - any new piece of gear must be judged in comparison to its nearest equivalents, but the problem starts when one regards a product as being less interesting because it is not as original and different as it would have been had it appeared a few months earlier.


  • 01 EQ/Delay/Chorus/Reverb (large room)
  • 02 EQ/Delay/Flange/Reverb (large room)
  • 03 EQ/Delay/Chorus/Reverb (medium room)
  • 04 EQ/Delay/Flange/Reverb (medium room)
  • 05 EQ/Delay/Chorus/Reverb (small room)
  • 06 EQ/Delay/Flange/Reverb (small room)
  • 07 EQ/Chorus/Multi-tap Delay
  • 08 Flange/Multi-tap Delay
  • 09 EQ/Gated Reverb/Delay
  • 10 EQ/Reverse Reverb/Delay
  • 11 Parametric EQ/Delay/Chorus
  • 12 Parametric EQ/Delay/Flange
  • 13 Ultimate Reverb
  • 14 9-band Graphic EQ

My gut reaction on first encountering the DSP128 Plus was pretty much along the above lines, that here was another in a recent series of improved multi-effects processors. However, after delving deep into the facilities of the machine, its own character and unique features became apparent, demonstrating that it is without doubt a worthy addition to the growing range of affordable professional effects processors.

Comparisons to both the ART Multiverb and Alesis Quadraverb are inevitable, and like both of those units the DSP128 Plus has its particular areas of strength and weakness. Its strengths are good multi-tap delays, an excellent dedicated reverb algorithm, and realtime MIDI control over effects parameters (a feature shared with the Quadraverb). Its relative weaknesses are that it does not offer quite the flexibility or programming ease of either of its main rivals. If you're in the market for a high quality, powerful and versatile effects processor, details like these will determine which of the competing units is right for you, and should you opt for the DSP128 Plus, you'd be hard pushed to find fault with it.


£439 inc VAT.

John Hornby Skewes & Co. Ltd, (Contact Details).

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Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Sound On Sound - Apr 1989


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Review by Paul Ireson

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