Design Labs Prologue Plus

Once simple one‑channel units, MIDI‑CV converters have become multi‑output devices capable of handling various kinds of pre‑MIDI control protocols. Design Labs' latest offering even sports a built‑in resonant filter! Gordon Reid bleeps and oscillates with delight...

The Design Laboratories is a one‑man company within a company (the Synthesizer Service Centre), and has just four products in its range. The first, launched in 1992, was a RAM cartridge that offered the equivalent of four Roland M64Cs in a single package, and cost little more than one of the originals. The DX version arrived a year later, and this offered a similar price advantage over the Yamaha cartridge. The same year also saw the preview of a far more exciting product: an 8‑channel MIDI‑to‑CV converter incorporating a 24dB/octave filter for processing audio signals. The projected price was just £299, and delivery was expected in mid‑1994. Now, in mid‑1995, the production models have only just appeared. During the wait, the original specification has been re‑written, the hardware and software enhanced, and two models now replace the planned one — the Prologue and Prologue Plus. It's the Prologue Plus which is under review here, although those of you considering the purchase of a Prologue shouldn't be put off reading this, as the only difference between the two is that the Prologue lacks the Retro‑Synth and the connections to it, of which more directly. The question is: was it worth the extra year's development?

Connections

The Prologue Plus packs a great deal into a small case. The back panel is busy, with no fewer than 16 3.5mm CVs and Gates, plus sockets for MIDI In, Out and Thru. There are also two quarter‑inch jack sockets that provide audio In and Out for the Retro‑Synth (Prologue Plus only). The fairly basic controls (a simple numeric keypad) are situated on the top panel, but as neither Prologue is rackmountable, there are no access problems. Also on the top panel is the decidedly retro LED display, and a schematic of the Retro‑Synth menus. The unit is controlled using the keypad, four keys of which emulate cursor keys, while the LEDs display the parameter names and values. This arrangement is far from ideal, but cost‑effective. The cursors take you through five master menus (four on the basic Prologue, which lacks the menu controlling the Retro‑Synth) and these govern every aspect of the unit's operation.

The Prologue Plus has four MIDI‑to‑CV processors, each of which can handle four notes simultaneously. The first menu, General Control, allows the following to be assigned to each processor: a MIDI channel, one of five keying modes, LFO, clock and portamento rates, four MIDI controllers, and pitch‑bend amount. The processors can be treated individually, or stacked to give 8‑note and 16‑note polyphony (although you'll need a second Prologue Plus if you want to play 16 notes simultaneously).

The Gate Output Table allows any of 11 Gate‑type signals, derived from each of the four processors, to be assigned to any of the eight output Gate sockets. Similarly, 24 continuous‑voltage signals can be assigned from any of the processors to any of the eight CV outputs. The pitch CVs can adopt the linear Hertz/Volt standard, as sometimes used by Korg and Yamaha, or the logarithmic Volt/Octave standard, favoured by Moog, ARP, Sequential, and Roland, and each may be scaled or offset as required.

In use, the flexibility offered by eight pairs of CVs and Gates is staggering. The gates offer just two states, 0V and 5V, but each of the eight CV outputs is a source of continuous voltage, and the range of mapping options means that each of these can carry a pitch CV, or any one of hundreds of controller CVs. This means that in addition to driving up to eight monosynths with basic pitch and gate information, you can play a synth such as an Oberheim 4‑voice or 8‑voice with 4‑note polyphony and four continuous controllers (such as filter, amplitude, PWM, and so on). You can also use spare CVs to add vibrato, tremolo, filter LFOs, or other modulators to patchable synths. Indeed, just two CVs will free up Oscillator 3 on your Minimoog, and three will turn an ARP2600 with a monophonic 3604 keyboard into one with a duophonic 3620 with an LFO.

If there is a problem (and there always seems to be), it's that the Prologue Plus's trigger and gate pulses are only five volts high. While five volts will drive most analogue synths, there are some insensitive beasties that require more. I had no problems during the review period, but be warned — it's a good idea to check before you hand over cash. Of course, you can route a Gate to a CV output set up to offer a higher voltage — but you then lose that CV for other duties. Why is life never simple?

MIDI Facilities

The Prologue Plus features a simple MIDI analyser that displays the last six bytes received, but its channel filter and re‑channeliser is far more interesting. This allows you to select a single MIDI channel, and filter the other 15 from any data stream presented to the MIDI In; the information on the one you've selected can then be re‑transmitted on any desired channel. The Prologue Plus is, therefore, a solution to the permanent Omni On and/or 'MIDI Channel 1 only' problem of early MIDI synths, such as the Roland JX3P and Prophet 600. Anatek offered a MIDI Filter, but it had no power supply, and had to be re‑configured every time you turned your synths on.

The Retro‑Synth

The basic idea behind units like the Retro‑Synth is simple; they imbue the signal produced by a relatively sterile source (such as an FM synthesizer) with some of the qualities and warmth of a good analogue synthesizer, by passing the sound through an analogue filter with envelopes that can be triggered and dynamically controlled using CVs or MIDI data. The result is equivalent to having a single Moog‑ or ARP‑style resonant filter within your digital synthesizer. Dedicated filter banks from Analogue Systems and Peavey accomplish some of this, as do a number of analogue synths with audio inputs, but probably the best example is the Prologue Plus's Retro‑Synth.

The Retro‑Synth comprises a multi‑mode resonant 24dB/octave analogue filter and two envelopes, but no oscillators, which means it is slightly misnamed — lacking any sound generating circuits of its own, it can't really be described as a synth [Design Labs' David Croft has explained that the name refers to the unit's rightful position behind a synth, hence Retro‑Synth, which some may feel is taking poetic licence too far — Ed]. The dedicated ADSRs (one for the VCA, and the other for the VCF) may be triggered over MIDI, but, unlike most monosynths, the VCF, VCA, and envelopes are also sensitive to velocity and aftertouch.

Using the Retro‑Synth couldn't be more intuitive. The Prologue Plus's fifth master menu selects five sub‑menus that replace the knobs and switches of a true '70s instrument. Menus 1 and 4 are the VCF and VCA envelopes, Menu 2 adjusts the filter's initial frequency, resonance, ADSR amount, and keyboard tracking, Menu 5 controls the VCA level, velocity, and aftertouch responses, and Menu 3 gives access to the filter mode selector, and the filter's velocity and aftertouch responses. Hold on a sec... filter modes? There are 14 of these, offering low‑pass, hi‑pass, notch‑ and band‑pass filters, each offering a range of Q‑factors and roll‑offs (see the 'Filter Modes' box for a full run down). Some include equalisation, some do not. All are fully resonant.

So how does it sound? To find out, simply connect the Audio Out from your sound source to the Retro‑Synth In, the MIDI Out to MIDI In, set the parameters (as you would on a JX10P, for example), and play. The Retro‑Synth envelopes are triggered each time a Note On is received, and all the velocity and aftertouch information is interpreted and directed to the appropriate parameters. The result is similar, but superior, to that obtained when you play other polyphonic, but single‑filter, synths, such as the Polymoog, ARP Quadra, and Korg Poly800 — it's magic. Taking one of the most soulless synths in existence (an Akai AX80), I created resonant voices, and punchy lead and bass patches. OK, so the warmth of a Minimoog or Pro One eluded me, but the Akai's DCOs are just about Fourth Division in the waveform department. Nevertheless, the flexibility of the envelopes (10mS to 10S), and the almost ridiculous range of resonant filter effects, will keep even the most demanding knob‑twiddler happy for a long, long time.

Conclusion

At just under £300, the basic Prologue demands attention, with its 16 assignable CVs and Gates, its MIDI filter and re‑channeliser, and its 24ppqn DIN Sync (which shares the MIDI Out, and drives fashionable items such as the TR808 and TR909). Let's face it, it's excellent value for money. Another £120 also buys the Prologue Plus with its Retro‑Synth, which is (in this analogue enthusiast's opinion) a 'must‑have'. There are, of course, compromises, and the Prologue's low costs are clearly reflected in Design Laboratories' choice of hardware. The software also shows a number of short‑cuts, though deficiencies, such as the lack of configuration memories, and SysEx, could be rectified in a software update. Provided that Design Laboratories achieve the right compromise between low cost, build quality, and reliability, the features of the Prologue and Prologue Plus make them winners.