We are often asked which fully variable monophonic synthesiser we can recommend. Generally, I advise the person enquiring to take a look at either the Moog Prodigy or the Sequential Circuits Pro-One (though I know that our editor isn't too keen on the Prodigy because it doesn't have a white noise source). Anyway, in my, and most other peoples' book, the Pro-One takes a hell of a lot of beating - i.e. it's good, not that you can start laying into it with a piece of lead piping; so we thought, as we haven't previously taken time to examine said instrument, that now was a good time to give it the proverbial 'once over'.

Sequential Circuits, as you should all be aware, are the people who developed the Prophet 5 synthesiser - the first commercially successful polyphonic programmable voice-assignable. They are headed by a guy called Dave Smith, who started the company some eight years ago building sequencers in his bedroom. He is now President of a very big synthesiser manufacturing and design company based in San Jose, California - so take heart all you closet designers, you too could 'come-out' in a big way.

The Pro-One was launched officially in February '81, as a monophonic non-programmable version of the Prophet 5. SCI (Sequential Circuits Inc.) had wisely decided not to have all their eggs in one basket, and by bringing out a monophonic version of their prime line, they could take advantage of the quantity orders they had for components, thus keeping costs down as far as possible. And this was very important because at the time, it appeared that the last thing the musical instrument market wanted was another monophonic synthesiser. But, as you will see from this review, the Pro-One has some very nice design features, and for the money it represents excellent value.

The styling of the Pro-One is much in keeping with the Prophet 5 - similar control fascias, same performance wheels, wooden end cheeks, however the main body of the instrument is basically an ABS moulding, which could be a bit stronger. The forming is accurate, and there are no nasty bits of unwanted plastic, but the casework could have been a bit more substantial, without a drastic increase in price, couldn't it? Having said that, I have popped into Rod Argent's Keyboards (the U.K. importers) repair shop on several occasions, and never have I seen a Pro-One with any form of damaged casework. In fact, I'm informed that, save for some initial problems with the mains on/off switch and oscillator scaling, the Pro-One is a very reliable little unit. I have, however, made some comments regarding the casework, such that I would strongly recommend the use of a flight case for anyone taking a Pro-One on the road. Ignore this advice at your future Pro-One's peril!

The keyboard is a three octave C to C Pratt Reed job, and very nice it is too; a good firm positive action, although the contact system employs silver J-wires, and as such there is some physical noise to be heard as the contacts make - though there is no noise introduced to the key voltage and it's derived digitally anyway, so this is not too important. The actual layout of the main control panel is remarkably similar to that of the Prophet 5, with the synthesiser voice controls almost corresponding one for one. The most obvious difference, however, is that the Pro-One uses basic slide switches as opposed to the Prophet's LED momentaries - but then you can't have everything on a low cost machine.

This is a dual oscillator synth, with Oscillator A providing sawtooth and pulse (width variable) waveforms that can be pitched in octave steps, over four octaves, and varied continuously between the octaves. Oscillator B offers the same frequency control along with a LO Frequency option, for sub-audio modulation work. Osc. B generates ramp, pulse (width variable) and, in addition, triangle waveforms, and can also be disconnected from the keyboard control voltage if necessary. Syncing is also available, and operates such that Oscillator A can be latched onto a harmonic of Oscillator B. So a pretty versatile pair of oscillators.

The low frequency oscillator, which will only work between 0.1 and 30 Hz, cannot be used as an audio oscillator, nor is it voltage controllable, but then you seldom find LFOs that are. This LFO will give you ramp up, triangle, and pulse modulation waveforms, and you can also combine these shapes together for some interesting, if not particularly useful composite waveforms. We will come onto the modulation section itself, a little later, but I can warn you that it is one of the most complex, and hence versatile, modulation sections that I've ever seen on a non-modular monophonic.

The Oscillators' output level is balanced via a three channel audio mixer before being fed into the filter. The third channel introduces a noise source, but it can also be used to control an external audio input. There's an audio input jack on the rear panel, and when this is connected to an external signal, the mixer's Noise/Ext knob acts as a threshold set control such that when this signal passes the threshold level, a gate signal is produced which will open the two envelope generators for as long as the amplitude is above the threshold - thus the Pro-One can effectively be used to process other instruments. Those of you who are home constructors might find this feature a worthwhile mod on your own instruments.

The mixed audio signal is fed onto the Pro-One's filter - a voltage controlled low pass 24dB/octave type, which gives the Pro-One rather a nice clear crisp sound, not coloured like the sound of many other synths. I generally consider that if you can recognise what make of synth you are listening to, then (with the exception of the Minimoog) this is a bad thing. It's a bit like the old school of thought that reckons that if you notice the incidental music to a film or TV drama, then the music isn't doing its proper job. The Pro-One's filter, as it causes no residue colouration is then a useful tool for synth work. The filter offers all the usual control parameters, variable keyboard tracking, resonance that can be advanced so that the filter breaks into oscillation, and its own envelope generator (ADSR) and amount control. So the filter section scores full marks.

The final stage of the voice module is the VCA, which has a particularly silent background 'noise'. It too is controlled by an ADSR envelope generator; there is no control voltage bias facility for manually opening up the VCA - though this isn't particularly vital.

We must now look at the modulation section - which is quite something. It can be best described by considering the three modulation sources - the filter envelope; Oscillator B; and the LFO. Each of these three sources has an amount control, and each signal can either be sent directly, via a summing node, to its destination, or it can be summed and fed through the modulation wheel before being passed to its destination. There are five modulation destinations: Osc A frequency; Osc A Pulse width; Osc B frequency; Osc B Pulse width; and Filter Frequency. At each of these points there is a three-position slide switch that will accept either the direct modulation signal, the signal from the wheel, or no modulation at all... so it's as simple as that! In fact it really is quite simple after you've got used to it, and it certainly is versatile - you can do things here that just aren't possible on any other monophonic - save for a fairly comprehensive modular system.

We've mentioned the modulation wheel, which is situated to the left of the keyboard, and next to it is the pitchbend wheel, which will raise and lower the oscillators up to a fifth. This control isn't sprung, but there is a centre-dente, so that it can be returned simply to its initial position. If you look inside, this is the most simple of mechanisms, relying on nothing more than half a cable grip!

There are three areas which are still to be looked at: The glide, the sequencer, and the arpeggiator, and all of these contribute to the excellent performance characteristics of the Pro-One. The glide circuitry (or lag processor) has two modules, Auto and Normal. In Normal mode, the glide operates in the traditional manner, i.e. the control voltage continually slews between notes at a rate determined by the rotary control - if the rate knob is at zero, then the slew is so fast that it becomes indiscernible from normal stepping between notes. The Auto mode will only introduce glide between notes when the first note is still held whilst the second one is played; so if you play legato you will have continual glide between notes, whereas, you won't cause any glide by playing staccato in Auto mode (you hum it, I'll play it).

Believe it or not the Pro-One also incorporates a 40 note sequencer - which can be split into two banks: Sequence 1 and Sequence 2. The sequencer is programmed via the keyboard, but every note is assigned an equal duration so it doesn't function as a true digital sequencer. Nevertheless, it is a handy tool to have on board. The sequencer's replay rate is determined by the LFO/clock rate control, and the sequence(s) can be transposed using the keyboard control voltage.

Finally we have the arpeggiator, which I found to be particularly useful, though its operation is very simple - you just play two or more notes on the keyboard, and the Pro-One plays them in turn at a rate again determined by the LFO/clock control. There are two arpeggio pattern options UP or UP/DOWN. A nice feature about the arpeggiator is that it can be latched by playing a chord and moving the sequencer Play/Record switch to Record - to unlatch it simply return the lever to Play.

As you will have guessed, the Pro-One is processor controlled, using an 8021. Inside the unit there is very little discrete circuitry, as SCI have opted to use the Curtis range of custom synth chips. However, even though there is little circuitry, I would have liked to have seen the PCB and the PCB pots better mounted - this does look as if it could be an area of weakness. Otherwise, the Pro-One is a marvellous synth, and very very versatile - I don't think that there will be a non-programmable monophonic that is going to beat it for some time. The Owner's Manual is also very well prepared and designed, and I would have no hesitation in recommending this instrument to a beginner. On the rear of the unit there are CV and Gate inputs and outputs, (the Gate in will trigger the Sequencer/Arpeggiator) so this unit can be linked up to all sorts of external devices. I found that it was particularly useful when used with the Roland TR-808 rhythm unit, as you can use the Accent trigger out (say) to trigger the sequencer, so as the TR-808 is fully programmable, the combination of the two units can provide a great bass/rhythm section.

Full marks to SCI here, but a possible word of advice. At present the Pro-One retails at £416.00 with Rod Argent's Keyboards acting as the importers directly from the Californian factory. Over the next few months SCI are setting up their own European distribution network in Rotterdam, and will be supplying the products through there - which is an extra step in the chain. This could cause the price of the Pro-One, and other SCI products to rise, so if you are thinking about a Pro-One then I'd advise you to get in quick. I don't say that the prices will definitely rise, but there is a good chance.