THE SEQUENTIAL CIRCUITS PRO-ONE IS ONE OF THE UNSUNG HEROES OF ANALOGUE MONOSYNTHS - USED AND LOVED BY MANY; IT OFFERED FACILITIES BEYOND THE SCOPE OF EVEN THE MINIMOOG.

IN THE WORLD of analogue monosynths, two instruments stand out as the machines to own: the Minimoog and the ARP Odyssey. The SCI Pro-One never really acquired the same "legendary" status, but it came closer than almost anything else. It also has the distinction of coming from the same stable as the Prophet 5 - and no more distinguished analogue polysynth exists than that. The Pro-One inherited the quality of its filter, its modulation possibilities, its punchy sound and its classy good looks from the Prophet 5 and Prophet 10.

Features-wise the Pro-One has all you would expect from a classic monosynth. Two oscillators with sawtooth and square waveforms plus triangle on oscillator B, two ADSR envelopes, one for the VCA and one for the filter, which is a 24dB/octave affair with variable cutoff, resonance, envelope modulation amount, and keyboard tracking. The LFO has three waveforms (sawtooth, triangle and square), which, like the oscillator waveforms, can be mixed.

Beyond this, the Pro-One has what must be one of the most versatile modulation systems on any analogue monosynth other than the modular systems themselves. Based on principles inherited from the Prophet 5, this innocent-looking section of the control panel sports eight switches and three knobs, and governs the routing of three modulation sources through two routes to five modulation destinations. The modulation sources are the filter envelope, oscillator B, and the LFO; the modulation destinations are oscillator A frequency and pulse width, oscillator B frequency and pulse width, and the filter. Two modulation paths exist to control these signal flows, one being direct, the other being controlled by the modulation wheel. It sounds rather simple - deceptively so, as many more possibilities exist than spring instantly to mind.

The modulation possibilities extend beyond the modulation panel itself. Oscillator A can be synchronised to oscillator B, and oscillator B can have keyboard tracking disabled and/or be put into low-frequency mode for use as a second LFO. The Pro-One also has a number of playing modes, including low-note priority (as standard), new-note priority, drone, repeat, arpeggiate, glide and fingered (auto) glide, and external triggering. You should be able to find a combination to meet your playing style requirements somewhere in that lot.

To get a feel for the Pro-One's modulation facilities, it's a worthwhile exercise to explore one effect in detail. Oscillator syncing is one of the all-time classic analogue sounds. Contemporary instruments like the Ensoniq VFX, the Waldorf Microwave and, more recently, the Yamaha SY22 represent a revival of interest in the depth of timbral movement possible with synchronisation - although, to their credit, the Ensoniq ESQ1, Cheetah MS6 and Oberheim synths feature traditional oscillator sync. In case you're unfamiliar with how oscillator syncing works, one oscillator is slaved to the other, such that the slave oscillator will be forced to restart its wavecycle every time the master oscillator restarts its own wavecycle. Oscillator A is the slave, and oscillator B the master on the Pro-One. The pitch of the slaved oscillator is controlled by the pitch of the master oscillator. If any attempt is made to modulate the slaved oscillator without applying the same modulation to the master oscillator, then the waveform of the slave oscillator is modulated, giving rise to a change in timbre. For example, if the frequency of the slaved oscillator is raised through modulation, then it will restart its wavecycle at a frequency determined by the master oscillator - so its pitch will not modulate - but it will also restart its wavecycle a number of extra times per master cycle which will be determined by the ratio of the theoretical modulated frequency to the master oscillator frequency. The higher this ratio, the more wavecycles will appear within the master wavecycle. The behaviour of a sync'd oscillator is in fact rather similar to that of one of the three resonant waveforms (5-8) on a Casio CZ synthesiser, where the DCW value controls the number of sinewaves within the wave's "window".

The most popular sync effects fall into two categories: envelope modulated, and wheel modulated. The latter is more of a performance effect - the famous "sync-bend" (which still raises the hairs on the back of my neck). The former created hard "wow" effects. You get these on the Pro-One by routing the filter envelope directly to oscillator A frequency. Since the filter envelope doesn't have to be used to modulate the filter, you can set the ADSR controls however you need, although sadly there is no envelope polarity inversion. Oddly, versatile though the Pro-One's modulation system is, there's no means by which the modulation wheel can be used directly as a modulation source itself; consequently, in order to obtain a simple sync-bend you have to use wheel-controlled filter envelope modulation of oscillator A frequency, with the filter ADSR set to the following envelope; zero attack and and maximum sustain (obviously enough), but also maximum release, or at least a longer release than the VCA envelope, otherwise the sync effect will be cut off before the note decays, regardless of the position of the wheel. Of course, you can use a combination of wheel control for sync-bend with envelope control, by routing the filter envelope set to a fast attack, slow decay, lowish sustain, routed via the wheel.

But there's little point in having the most versatile modulation system in the world if the basic sound sucks. The Pro-One, with its ancestry and all, definitely doesn't. Probably the most significant contribution to the character of any analogue synth is made by its filter, and the Pro-One's filter is marvellous. At open settings, it has an upfront, fizzy sound. Lower settings are pure and mellow without being thin, while some resonance feeds the filtered signal back through the filter, adding punch and presence through a little distortion at the cutoff frequency.

"THE PRO-ONE NEVER ACQUIRED THE SAME 'LEGENDARY' STATUS AS THE MINIMOOG, BUT IT CAME CLOSER THAN ALMOST ANYTHING ELSE - IT ALSO CAME FROM THE SAME STABLE AS THE PROPHET 5"

The Pro-One is also capable of a weird and wide variety of sound effects, courtesy of oscillator B's function as a modulator. Using oscillator B to modulate oscillator A gives a range of FM-type bells and screeches, while using oscillator B as a low frequency oscillator with filter envelope control of oscillator B's pitch can create variable frequency low frequency oscillator (VFLFO) effects with ADSR controlled LFO frequency - the sort of effects much used by Jean-Michel Jarre on Oxygene. Don't forget to turn oscillator B down at the oscillator mixer, though.

Many of the sounds and textures created through the more, shall we say, esoteric possibilities that the Pro-One offers, will not be the sorts of sounds that have a musical purpose which is immediately obvious. Some may be simply too weird. Of course, you could try sampling them and using them as an element in a more complex sound (see The Analogue Sampler, MT Dec '89). If you are using a MIDI-to-CV convertor and sequencing your Pro-One, then you can experiment with adding the Pro-One's texture to other instruments (as long as your sequencer will let you split a polyphonic part into discrete monophonic lines, otherwise the Pro-One will have an awful time trying to figure out which notes to play). Some atonal component exists in many of the "evolving" sounds that characterise synths like Roland's D50 and Korg's M1 - you might be surprised how well a little weirdness can work. Another surprise is how "natural" many of the Pro-One's textures can be - the exponential envelope stages undeniably help here.

External signal processing with the Pro-One is possible, courtesy of a useful selection of interfacing possibilities found on the back panel. Among the standard 1V/octave CV in/out and 5V gate in/out, can also be found an audio input and filter CV in. When a signal is connected to the external audio input, the noise generator is disabled and its level control now controls the level of the external audio signal. The filter CV on the other hand accepts 0-10Vdc while adhering to the 1V/oct standard; the filter keyboard scaling amount is overridden by this CV in, which is, in turn, attenuated by the filter keyboard amount control. Those of you who read the retrospective on the Korg MS20 a few months back will have some idea of how useful these two inputs can be.

As a self-contained analogue polyrhythmic processor, the Pro-One is unrivalled. Forget your performance MIDI; this is real-time processing - with no glitches unless you put them there. Suppose you have a drum pattern somewhere: multitrack tape, a drum machine, or a multitimbral sampler or synth expander with a drum section and a sequencer. Take something spiky and downbeat from this (like a kick drum), and subgroup it on your mixer along with some chordal instrument like rhythm guitar. Route the mix of the two signals to the audio in of the Pro-One, and set the Noise/Ext control to trigger the envelope detector whenever the kick gets loud enough; Repeat/Ext Mode should be switched on to enable the gate generator. The filter and envelopes on the Pro-One can now be set so as to modulate the guitar signal in a triggered fashion. This process will, incidentally, also effectively quantise the processed guitar to match the kick drum.

With me so far? Now program a sequence into the Pro-One's onboard sequencer - no more than eight or nine notes. (The pitches themselves aren't important unless you intend to have the Pro-One's oscillators make some contribution.) For the present, turn them down at the oscillator mixer, and/or turn off all oscillator wave-forms. Make sure that the pitches you record into the sequencer are widely scattered about the three octaves available to you. These will now be used to control the timbre of the processed guitar chords by modulating the filter cutoff. Setting the sequencer to play the sequence and rolling the tape, the intensity of timbral modulation can now be controlled from the filter keyboard amount control. Each time the kick drum triggers the gate generator, the sequencer advances to the next step, and a new filter frequency will modulate the guitar chords. If the sequence were a simple up-and-down arpeggio pattern, then the effects of filter frequency modulation would not be unlike a wah-wah pedal or slow phaser - with resonance, of course. If the sequence is unlike a simple up-and-down arpeggio pattern, then the modulation effect will be quite unlike anything else.

Unless the kick pattern is very predictable, there's unlikely to be any relationship between the number of kicks in a measure and the number of steps in the sequence. Consequently, although the sequence of timbres will repeat itself, its rhythm should change at each pass. You have instant, self-contained, polyrhythmic signal processing - with knobs on. The possibilities extend beyond the simple ones just described, as the Pro-One also sports external control of gate, oscillator CV and filter CV; add something like a Philip Rees MCV MIDI-CV converter, with which either MIDI note number or MIDI velocity could be used to control filter CV, and possibilities for linking the Pro-One to a MIDI sequencer as a MIDI-controlled signal processor start to open up. Using MIDI to trigger the Pro-One, there's no longer any need for the external signal being processed to be percussive enough to control the gate generator. This means that string pads, sampled choirs, even backing vocals could all be subjected to the Pro-One's filter and envelopes.

I confess that I find it difficult to summarise the Pro-One. In the preceding text I've only covered a couple of applications of this classic machine. It goes without saying that the Pro-One is also capable of a remarkable range of powerful leadline, solo instruments, basses, and even the odd passable imitation of a monophonic acoustic instrument. What I can't convey is just how good something like the Pro-One sounds after years of progressively more digital and less expensive synthesisers. Make no mistake, I am as big a fan of FM, LA, PD and ROM as you could hope to find, but there's still something special about the classic analogue sound. It's something quite impossible to replace, something I can't imagine doing without.