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Digitech DSP 128 Plus

Digital Multi-effects Processor

Article from Music Technology, May 1989

In the multi-effects processor wars, Digitech's DSP 128 Plus distinguishes itself with comprehensive facilities and little sound colouration. Ian Waugh treats himself.

Continuing the search for the ideal digital multi-effects processor brings us to Digitech 's DSP128 Plus - how does it measure up?

THE D5P128 PLUS is a digital, multiple effects signal processor. Its predecessor, the DSP128 (without the Plus), is capable of producing three effects simultaneously. The Plus is capable of producing four. The effects are applied in series and their parameters can be assigned to any MIDI continuous controller. Both the DSP128 and the Plus will store 128 programs - so no prizes for guessing the derivation of their names.

One thing to be said about all multi-effects processors is that even a couple of years ago, finding such processing power in one unit and at such a price was all but unthinkable. Long live progress, I say.

Yamaha's SPX90 was the first affordable multi-effects processor (closely followed by Roland's DEP5). Since then, musicians have realised that being able to apply reverb and delay and EQ -and something else - to a sound is a good thing. Consequently we've seen quite a spate of multieffects units appearing in recent months. The Plus is currently vying for your money with the ART MultiVerb II and the Alesis QuadraVerb (reviewed MI, April '89).

In the light of these rapid advances in technology, what have the boys at DigiTech come up with? Like other units in this price bracket, the Plus has a 16-bit analogue-to-digital converter. Its quoted frequency response is 20Hz to 20kHz with a signal-to-noise ratio of around 85dB.

The 128 Plus has both balanced and unbalanced audio inputs and outputs, and a switch at the back of the unit lets you choose between +4dB (line) and -20dB (instrument) input levels. It will take quite a hefty signal before peaking. At +4dB, however, you can hear the effects swirling away in the background, enough to be a nuisance and a hindrance during recording, but at -20dB the noise level is extremely low - certainly low enough for studio recording. If that was the greatest noise source in my setup, I'd be a very happy man.

With the unit patched into a mixer and instruments DI'd into it, there were no problems operating the 28 Plus at the -20dB level. However, the +4dB option is sure to be invaluable in a live situation where a little noise is unlikely to worry anyone.

Rack 'Em Up

THE DSP128 PLUS is a 1U-high rackmount unit which will stand out from the matt black finish which seems to be standard on most rack units. The power supply is built in.

It has three rotary dials on the front to set input and output levels and the mix between the dry signal and the processed signal. An Effect button kills the processed signal but the volume of dry signal it allows through is dependent upon the setting of the Output Mix knob (the QuadraVerb Bypass button works in a similar way). If it is set to Wet you won't hear a thing. What's your objection to letting through the full dry signal, guys? In practice, to kill the effect I found it far easier to turn the Mix control to Dry.

In the centre of the unit is a four-digit LED display and I've got to say that this is just not as friendly as an LCD. The programs are displayed as numbers, not names, and the parameters are shown as cryptic two-letter combinations. LEDs may be cheaper than LCDs and use of the unit does breed familiarity with it, but I'd rather pay the extra and get some info up front.

The LED display can be varied so that as you change parameters they scroll into the display area or pop into it. Scrolling is neat but it takes time and after the novelty wears off you'll probably plumb for pop up display. The display will also show changes made to a a parameter's values from a MIDI controller - a useful function.

Next door to the display are four cursor keys which are used to access the programs and change the parameters. The up/down keys select the programs, the left/right keys scroll through the parameters and, when in this mode, the up/down keys alter the parameter values. After holding down a key for a few seconds the speed at which the parameters scroll increases, minimising sore-pinky time.

Further left along the panel is a four by three grid which shows the range of effects allowed by the current algorithm (coming up). In the absence of program names, this is really useful.

I Got Algorithms

THE UNITS EFFECTS are created from 14 algorithms, each of which has between nine and 11 parameters. As the range of effects is dependant entirely upon the algorithms, I'll list them here:

1. Large room, Delay, Chorus, Filter
2. Large room, Delay, Flange, Filter
3. Medium room, Delay, Chorus, Filter
4. Medium room, Delay, Flange Filter
5. Small room, Delay, Chorus, Filter
6. Small room, Delay, Flange, Filter
7. Filter, Chorus, Multi-tap Delay
8. Flange, Multi-tap Delay
9. Gated Reverb, Delay, Filter
10. Reverse Reverb, Delay, Filter
11. Parametric EQ, Chorus, Delay
12. Parametric EQ, Flange, Delay
13. The Ultimate Reverb
14. Nine-band Graphic EQ

The parameters are represented by one- and two-character abbreviations. Some are fairly obvious - LP (Low Pass), rL (reverb level) and FF (Flange Feedback) - but others are rather more obscure. Take SP; this controls speed and is applicable to both Chorus and Flanging, and rF (Normalised Reflectivity), which is used with the Ultimate Reverb algorithm. In true LED style, "v"s look like "u"s, and "t" looks like a capital "E" with the top line missing. Not particularly friendly, to my way of thinking.

The manual claims the abbreviations are printed on the top of the unit - but they weren't on the review model. I suspect the casing was manufactured before the list of parameters had been finalised - but then what if you wanted to place the unit directly below another in a rack? Perhaps a pull-out card similar to the Operation Guide on the Yamaha TX81Z is the answer.

But don't get the idea that all this makes the 128 Plus' parameters difficult to manage; it's a simple matter to recall the factory presets either individually or for all the programs. I do, however, wish there had been a Store function (and perhaps a Compare function, too) to allow you to make an alteration to a program without committing it to memory. As it is, any edit you make takes effect immediately. With all those abbreviations floating around it's quite easy to alter a parameter and move on without intending to (or perhaps I'm just clumsy).

As there are so many parameters it helps if you know what they all do. The manual gives some details, although a few more pages and a little tutorial section would not go amiss. Many parameters will be self-explanatory to most musicians but some, particularly those used with reverb, may not be so obvious.


THERE ARE 15 parameters which are used throughout the range of reverb effects (including reverse and gated reverb). These include Decay Time (the amount of time it takes for the reverb to disappear), Pre-Delay Time (the length of time between the original sound and the beginning of the reverb), Reverb Level (intensity), Early Reflection Diffusion (controls the amount of diffusion of the early reflections), Early Reflection Delay (the amount of Pre-Delay for the early reflections) and Envelopment (determines the width of the stereo image).

As you can see, reverb isn't just a question of bouncing the sound around a large tiled bathroom. By carefully choosing the parameters you can determine the size and absorbency of the reverb environment as well as the position of the listener in relation to the sound.

The modestly-named Ultimate Reverb algorithm uses ten parameters to produce some extremely effective and impressive reverb effects. A smattering of acoustics (and psychoacoustics?) would not go amiss when designing environments but you should be able to pick up enough from the manual to get started.

Delays On Tap

THE MULTI-TAP ALGORITHMS allow you to set different delay times for left, centre and right repeats. These are under control of an overall feedback rate and amount. Put the lot together - in any one of a number of permutations - and you can produce some very clever effects indeed. You can make the sounds do more than just bounce from left to right - they can circulate around the stereo image. If you use a high feedback rate you can build up little pieces of music, a la Terry Riley, for example.

The maximum delay time varies according to the algorithm used, as this determines how much processing power is available for each effect. The first six algorithms (the only ones to allow four effects at once) have a maximum delay time of 0.75 seconds. Each delay in the Multi-tap effects may run to .25 seconds, which is the maximum offered by the other algorithms, too.

"The multi-tap algorithms allow you to produce some clever effects - at a high feedback rate you can build up little pieces of music, a la Terry Riley."

The Chorus and Flanging effects range from the sublime to the ridiculous. At maximum settings you get out of tune instruments. Tone them down a little for a honky tonk piano and use just a touch for a warm chorus or a subtle flange. For fattening sounds, add some reverb and a little delay. I found the first six algorithms ideal for thickening sounds.

Most of the EQ functions on the DSP are simple lowpass filters although two of the algorithms have two-band parametric EQ. One algorithm is dedicated to a nine-band graphic but with no effects. Well, EQ is handy to have around but the DSP is primarily an effects unit and I personally wouldn't worry too much about EQ facilities (feel free to worry if you wish). I'd go so far as to say that

I'd rather have another effect than EQ on a multi-effects unit, but EQ seems to go with the territory.

All 128 programs contain factory presets which use the above algorithms. They are an assorted bunch, although many are simply variations on a theme and I don't think they show off the unit to the best of its ability; odd, but then some synths come with poor factory presets. However, it's quite easy to create your own effects once you get the hang of the parameter abbreviations.

If the perfect signal processor should do its job without adding any colouration to the sound, the DSP comes pretty close to perfection. You can obviously colour a sound with chorusing, flanging and so on, but the quality of the output vis-a-vis the input is pretty pure.

The inevitable consequence of this cleanness is that the output lacks character. Some effects units sound thin, others warm. Is one better or more desirable than the other? Time to dust your ears off and put them to work.


THE DSP CAN be set to receive on any MIDI channel, on all of them (Omni) or none at all.

Any of the algorithm parameters can be assigned to MIDI continuous controllers. These are global assignments allowing you to control pre-delay, reverb time or any of the other parameters - there are 44 in all - in real time from a master keyboard or from a sequencer, for example, by recording the controller data onto a spare track. You may need to exercise a some caution here, as running through a range of values on the fly can produce glitching, but this is not unique to the DSP. This control is limited to continuous controllers so note velocity, aftertouch and the like are not catered for.

The 128 Plus responds to incoming patch-change messages by selecting the corresponding program number. What is lacking, however, is a table to let you assign a program number to any incoming patch change message. This is a sad omission as it means you have to configure your other equipment to the DSP and not the other way around, as it should surely be.

As the programs aren't named you'll need to keep careful track of the numbers when organising the programs. The Program Copy function transfers the contents of a program to a new location and that will help.

You can dump the programs to a MIDI storage device (many sequencers can now handle this) so if you create more than 128 programs you can save them to make room for some more. You can't save or load just one program, however, so if you wanted to combine programs from two separate banks you'll have to note the values of the parameters of one set and enter them manually, but that shouldn't take long. The manual even contains blank sheets on which to write the parameters of new programs you create.

Fiddling around with my MIDI switching unit caused a MIDI error on the 128 Plus - and on other equipment, too - but the only way to escape from the error was to switch off and on again. I found this rather odd as my other equipment can be reset by changing patch. Some even simply flag the error and continue working. In a live situation, it's possible that the DSP would hang up. When you switch on you go back to program one, not the last program used, so you'd have to reselect the desired program again.

The 128 Plus boasts two footswitch jacks on its rear panel. The default configurations for these are bypass and to switch in infinite repeat, but they can be reconfigured to step backwards and forwards through the programs and they can be used with DigiTech's three-button footswitches (FS3000 and FS303) to step through assignable program numbers. Extremely useful for live work.

Other software features include a self-test mode (what happens when self-test procedures go wrong?) and a scrolling "Hi, I'm the DigiTech DSP128 Plus". You can also see the software version number (the review unit contained version v2.OO). Perhaps software upgrades are in Digitech's corporate mind - a software upgrade has got to be a better prospect than realising your unit has depreciated to less than half its cost and shelling out all over again.

The manual supplied with the review unit was a temporary manual. It admits as much in the introduction and says that the DSP software was still being developed while the manual was being written. Many of the parameters on the unit were new and not listed in the manual which resulted in most of the "Programming the DSP" section being inaccurate.

Also, the pages which should have contained details of the factory presets were totally blank so the uses to which some of the presets could be put were not always obvious. Even the algorithm section was not completely correct, although the meaning of the parameters could be gleaned from the algorithm list. The operating procedures, however, were basically correct. Hopefully, by the time you read this, a current manual will be available.


I WAS IMPRESSED with the clarity of the DSP's output and the low noise level when using the -20dB input (I wouldn't consider using +4dB unless the situation demanded it). I do feel that a few areas of the 128 Plus' implementation could be tidied up, however - although what you lose on the swings you gain on the roundabouts (I hate that expression). Other, perhaps, than the lack of a MIDI program reference table, the pros and cons balance each other reasonably well.

How does the DSP stack up against the QuadraVerb and the MultiVerb? I knew you were going to ask that - go back and read the rest of the review. The DSP has a different sound - it's clean, it has an excellent range of effects - some are superb, others are, perhaps, average and programming is relatively easy taking the LED display into account.

Now, I'm afraid, it's down to you and yer ears. But if you want to know whether it's worth looking at and listening to, the answer is a definite yes.

Price £439 including VAT

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Source Of Inspiration

Publisher: Music Technology - Music Maker Publications (UK), Future Publishing.

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Music Technology - May 1989

Gear in this article:

Studio FX > Digitech > DSP 128 Plus

Gear Tags:

Digital FX

Review by Ian Waugh

Previous article in this issue:

> Source Of Inspiration

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