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OSC Advanced Sound Generator

Computer Music System

Gerry Queen brings us an exclusive preview of what will be Britain's first custom-designed computer music system - from the people who brought you the OSCar monosynth. It's capable of combining analogue, digital and sampled sound sources, and it's cheap.

Shown for the first time at Frankfurt, the ASG is Britain's first attempt at a custom-designed synthesiser and sampling system. What follows is an exclusive preview of what it is now and what plans OSC have for developing it in the future.

Ever since the OSCar was first released back in 1983, the principal criticism levelled at it is that it's monophonic. At the time of its release, this simple adjective was rapidly becoming pejorative. That was the era during which the blinkered mentality of a few synth manufacturers led to a sacrifice of sound quality, programmability and versatility to the great ideal of polyphony. In the mad rush to pander to a particular group of keyboard players who judged a synthesiser by the number of notes that could be played on it, first oscillators (down to one VCO), then envelopes (down to one ADSR) and finally filters (down to one VCF between eight voices) gradually disappeared from the control panels, unnoticed except by those who understood that a synthesiser could be more than a glorified 16-voice organ.

As this process continued unabated, the OSCar remained aloof from all of it, partly because its designer, Chris Huggett, saw little logic in the way things were going, and partly because his company were in no position to finance the development of a polyphonic variant, though he himself was well aware of the limitations his synth's one-note capability imposed on the musician.

Figure 1. How the advanced Sound Generator's unique liquid crystal display shows parameters and their programmed values.


So, logically enough, work was started on a new, upmarket machine that would be fully polyphonic but which would sacrifice none of the OSCar's programming versatility. In fact, it was Huggett's intention from the word go to build even more in the way of synthesiser functions into the polysynth. For example, there would be three oscillators for each voice, three envelopes (more complex than the conventional ADSR format) to go with them, and a more versatile filter section. And advances in sampling quality didn't go unnoticed, either, as OSC decided to add a sampling capability to the analogue waveform and additive harmonic sound-generation methods already present on the OSCar.

As things turned out, advances in music technology gradually made Huggett's originally awesome task easier and easier. The advent of MIDI meant that his company could get away with designing and building a stand-alone sound module, leaving the Japanese and the Americans to come up with controlling keyboards of sufficient quality to do the system justice. Huggett admits that MIDI leaves something to be desired and that his company is looking into the possibility of designing its own controlling keyboard complete with a faster, parallel interface, but had it been undertaken before now, that work would have held the poly project up by months if not years. Which is why OSC are perfectly content to leave the manufacture of weighted wooden-key controllers to the Big Boys, and concentrate instead on doing what they do best - designing versatile sound generators and flexible control systems to go with them.

A further aid to the development of the Advanced Sound Generator, as it had by this time been named, was the reduction in cost of large, custom-built liquid crystal displays. The availability of just such a display enabled Huggett to surround the ASG's main information source with an array of knobs and switches that relate directly to what's visible. In other words, a system that's both more useful and more user-friendly than either the mass of pots and LEDs favoured by the big synth makers, or the monitor-and-CWERTY-keyboard approach so beloved of the designers of custom computer music systems.

And so it was that the ASG took pride of place on OSC's Frankfurt stand (which they shared with forward-looking Italian software company LEMI) as a 5U-high 19" rack-mounted box, the centrepiece of which was an LCD programming screen with a high resolution of 600 x 240 dots. It's on this that parameters, waveforms and music displays appear.

To the right of the screen are the eight main Function buttons. These select principle modes of operation such as Synthesiser, Sequencing, Waveform Building and so on. When one of these buttons is pushed, it calls up the main screen for that mode, though this may have one or more subscreens. This means that at any time you can 'jump' between, say, Synthesiser and Sequencing modes without having to enter and exit different routines. And as all current ASG software is ROM-based (as opposed to being on disk), all command actions can be instantly accessed without any delay caused by different lots of software having to be loaded or overwritten.

To the left of the screen are a further eight buttons whose functions aren't labelled for the simple reason that they change according to the screen display selected: they work in conjunction with 16 continuously-rotating knobs located along the bottom of the screen. To alter any one of the parameters that are shown on the screen (and there can be as many as 128 of these at any one time), all you have to do is hold down the button level with the parameter you wish to alter and turn the knob directly below it. To give you some idea of how flexible yet economical this system is in practice, let's say you want to alter the Filter Decay time. In this particular mode, you can see the settings for Filter Mode, Filter Cutoff Frequency and Filter Envelope Amount as well as the other components of the envelope, merely by glancing at the appropriate areas of the LCD.

Figure 2. A few ASG display examples.

"The idea behind the display is to develop a better relationship in the user's mind between the sound and the way it's made up."


As is the case with the OSCar, the waveforms offered by the ASG comprise both standard analogue ones (triangle, sawtooth, square, variable pulse), while the extra-beefy PWM (Pulse Width Modulation automatically set up with independent LFOs for each oscillator) has extra control over speed and depth of effect, and there's a digital construction facility. This also has similarities with the set-up on the OSCar, in that there is a range of preset digital waveforms which offer things like basic organ, plucked and bell-like waveforms as well as provision for you to construct and define your own waveforms. The three improvements to this area of performance are first, a greater range of preset waveforms, secondly, visualisation of the waveshape as it's being built up, and thirdly, the fact that 64 harmonics are now definable, their levels being more easily programmed by use of the knobs as individual level controls for groups of 16. This means you can set the basic sound of the waveform with the first 16 harmonics, then alter the most obvious overtones (17-32) and then proceed to the more subtle ones (33-48). The last set are really only available on very low bass notes (elsewhere, such harmonics can only be heard by dogs and bats), but it's in the context of these low fundamental notes that they give a degree of high harmonic control unequalled by any other commercially available system.

Figure 2. A few ASG display examples.

Visualisation of the waveform takes place in one of two ways. On the left-hand side of the screen is a graphical display of the waveshape as it is built up (preset waveforms can also be represented in this manner), while to the right lies a bar chart display showing the level of each of the 64 component harmonics. You can of course listen to the waveform as you're changing its constituent parts, and these changes are shown on-screen in real time (ie. as they happen sonically). The idea behind this is to help develop a better relationship in the user's mind between the sound and how it's made up - and there's little doubt in my mind that it works.

It's also possible to change waveforms while a note is actually sounding. By using an envelope or an LFO to sweep behind two or more pre-defined waveforms, an effect such as pulse width modulation can be applied to any basic waveshape, not just that of a pulse wave. Using an envelope means that simple one-way transitions can be achieved, whilst the LFO will, of course, give a cyclic effect at both low and high speeds. The inclusion of this facility means that precise control of harmonic content is possible in real time, and the effects of that aren't just confined to the crude (if rather satisfying) effect obtainable from a conventional analogue filter.

Figure 2. A few ASG display examples.

What we've looked at thus far is merely the framework within which each ASG oscillator operates. Beyond that framework, the OSC design makes it possible for you to mix the actions of each of the three oscillators, either statically or in real time, using one of the three envelopes for each oscillator. Each envelope can be used either in an expanded ADSR mode (with peak and two sustain levels programmable) or in an eight-stage string of modes. The resultant mix of both overall and individual envelope volumes can then be fed into the unique OSC filter design at either low (clear) or high (overdrive) level.


Each of these comprises two 12dB/octave filters which can be used in a multitude of combinations, including lowpass (24dB), highpass (24dB) and bandpass (12dB) as well as for more recherché purposes such as band reject (notch) and comb filtering. It's also possible to split the two filters so that each can work separately on different oscillators. And you don't need a Mensa IQ to realise that this feature, in conjunction with the different oscillator envelopes, enables you to undertake a myriad of weird and wonderful synthetic tasks, such as the layering of totally different patches within one sound program. Good stuff. It almost goes without saying that the filters can also be controlled by expanded ADSR or Rate/Level envelopes, separately if required.

All voice channel components that can be affected by an LFO (ie. pulse width, oscillator pitch and filter frequency) can have different rate and depth settings to achieve real independence between vibrato, tremolo and PWM effects. Again, the sort of programming potential rarely afforded by today's dedicated polysynth designs.

"The OSC design makes it possible for you to mix the actions of the three oscillators, either statically or in real time."

Future Updates

So much for the Advanced Sound Generator (it may or may not eventually be marketed under that name) as it was demonstrated at the Frankfurt Musik Messe. That's far from being the end of the story, however, because as I intimated earlier on, it's OSC's intention to produce a complete computer music system equal in capacity and versatility to the best that current technology can offer. As the system evolves, developments on both the software and hardware sides of the musical fence will be fitted as options to all production ASGs, with retrofit packages being made available to those already in possession of the OSC flagship.

Let's look at the software first. Shortly after the release of the basic unit, a comprehensive update will be available to cope with what OSC envisage as being just about every conceivable sequencing requirement. The program will be able to control not only the ASG's 16 internal voices multi-timbrally but also those of up to 16 external synths (via MIDI) to the full extent of their individual polyphony and multi-timbral capability. It's also more than likely that, thanks to the speed with which the 16-bit master processor controls the system, it'll be possible to incorporate a real-time music display of both recording and playback into the sequencing package. In effect, this means that notes will appear on the screen as you play them, their time value in relation to the audible metronome click being filled in when you release the notes. Input to the software will be possible in real time using the controlling keyboard) or step time via the controls on the ASG's front panel.

Turning now to the subject of hardware updates, these are all designed to fit within the current unit, the idea being to avoid the problems inherent in systems in which several modules have to be plugged together correctly in order for anything to work as it should do.

The first - and for many people, most significant - of these introductions will be a polyphonic sampling facility. This will allow storage of up to 10 seconds of sampled sound within the machine at 50kHz, with faster playback possible. Plans are also afoot to keep the samples backed up by a battery-powered memory, so that they don't need to be reloaded whenever the machine is switched off or suffers a momentary power drop. Sample libraries can then be built up using the onboard 3.5" disk drive.

The second hardware update will locate a Winchester drive within the rack unit, and this will allow monophonic samples of rather longer duration (perhaps four or five minutes) to be stored and accessed with something approaching lightning speed. This should find a lot of favour with the 12" single production fraternity, who are currently stretching present-day technology to its limits in an effort to record the most creative 20-minute dance mix in history. Well, you know what I mean.

Finally, work is already in hand on the development of a printer interface to the ASG, so you'll be able to dedicate programming screen displays, voice data, harmonic-structure graphs and music scores to the printed page courtesy of a dot-matrix printer, should your interest demand it.


One thing that becomes pretty clear just from a quick glance at the ASG's specification is that it's an immensely ambitious project. As a small company endeavouring to take on the might of multi-million pound organisations single-handed, OSC are obviously going to have the dice loaded against them in the battle to capture a sizeable slice of this end of the hi-tech music market.

Yet the Advanced Sound Generator is good enough to enable them to do just that. Its own particular collection of facilities and the way they've been implemented have been given careful consideration by the OSC design team, and the result is an instrument of great logic whose inherent versatility is more instantly usable than that of almost any other competing product.

The company themselves would be the first to admit that all is not presently as it should be: the chosen LCD isn't the clearest of its kind in the marketplace (as well as being tricky to read, it's also well-nigh impossible to photograph, hence the proliferation of artist's impressions illustrating this feature), the front panel aesthetics leave a little to be desired, and the separation of keyboard from sound generator means that there are still a couple of connecting cables involved in setting the whole thing up - that's a couple too many in my opinion, because the DIN plug isn't the most reliable form of hardware interface currently available.

You're probably wondering by now (if you aren't, there's something seriously wrong with your powers of discrimination) how much the ASG system - in any of its forms - is eventually going to retail for. Well, the answer is that final asking prices have yet to be decided upon, but suffice to say for the moment that nobody expects them to be anything but ultra-competitive, which, when you consider the circumstances under which the machine is to be produced, is a mighty creditable achievement.

Further information on the Advanced Sound Generator system can be had from the Oxford Synthesiser Company, (Contact Details).

Previous Article in this issue

Simmons SDS1 Drum Pad

Next article in this issue

Sycologic MI4

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Mar 1985

Scanned by: Stewart Lawler

Gear in this article:

Synthesizer Module > OSC > ASG

Gear Tags:

Analog/Digital Hybrid Synth

Review by Gerry Queen

Previous article in this issue:

> Simmons SDS1 Drum Pad

Next article in this issue:

> Sycologic MI4

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