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Korg STR1 & OASYS v1.1

Plucked String Synthesizer Expansion & OS Upgrade
Published October 2006
By Gordon Reid

An amazing new string-modelling synth is added to Korg's flagship workstation.

I'm sure that everybody in SOS-land knows that Korg's OASYS workstation keyboard is basically a PC running Korg's own streamlined Linux-based operating environment. An advantage of this approach is that one can update the synthesizer's OS with no more than an installation CD (or four). Right from the start, Korg made it clear that this was their intent, and it seemed reasonable to assume that technology from the Z1 and OASYS PCI would eventually appear. No surprise then that the new v1.1 upgrade to the OASYS includes a new STR1 synthesizer modelling plucked strings. Installation of the upgrade from the supplied four CDs is not entirely straightforward, but for me the procedure went by without a hitch, and my OASYS soon contained the new OS, the STR1 synthesizer, and 128 EXi Programs that use the new synthesizer.

Korg STR1 & OASYS v1.1Photo: Richard Ecclestone

Before moving on to STR1 itself, it's worth noting the other improvements in v1.1. Foremost amongst these is support for 2GB of RAM, and installation of an extra 1GB memory module took me less than five minutes with a screwdriver. In my view, all owners should add the extra RAM, because it allows you to run both expansion sample libraries (EXS1 and EXS2) simultaneously, with almost 700MB still available for sampling. Alternatively, you could can leave EXS1 and/or EXS2 unloaded, retain the use of the ROM-based HD1 synthesizer, and treat the OASYS as a sampler with up to 1.5GB of RAM. Of less immediate impact, but still welcome, are minor improvements such as the new Gate options in the AMS Mixers, and the ability to step through Programs and Combinations using a footpedal.

Plucked String Synthesis

STR1 is a fully integrated EXi (Expansion Instrument) within the OASYS architecture. This means that its sounds can be integrated within Combis, used with KARMA, sequenced, resampled, and much more. I discovered very quickly that you don't need to program STR1 deterministically to achieve remarkable results. Although I normally frown on the practice of aimless twiddling, serendipity never seems far away. It takes 50 pages of the OASYS's expanded manual to describe the architecture, so, rather than trying to explain it in full, I'm going to take one of the factory sounds and investigate the key parameters to see how one can transform it into a range of remarkable new timbres, at the same time using this as a good excuse to remind myself how the physical modelling of plucked strings actually works.

Korg STR1 & OASYS v1.1Photo: Richard EcclestoneI'll use one of the instruments in the dual Nylon STR1 Acoustic Program, and it seems reasonable to start experimenting by changing the nature of the pluck, which I can do by selecting EXi1 and navigating to the Pluck/Noise page. However, before doing this, I need to select the Excitation Mixer page to ensure that the string is being excited only by the pluck, not by the noise generator and/or PCM oscillator (of which more in a moment).

As loaded, the Program uses Ac Guitar 1 as the pluck, and this excites the virtual string in a way reminiscent of a real string being plucked by something moderately pliant — such as fingertips. Stepping through the other options allows you to apply excitations analogous to plectrums, the pluckers of a harpsichord or clavinet, and four mathematical waveforms, each of which excite different spectra in the resulting sound. Selecting each in turn allowed me to create new 'instruments' and, changing nothing else in the Program, I liked applying the Dark Jazz Guitar plucker (which produces a richer, deeper timbre) as well as the Harpsichord, which adds noise to the excitation and which — in this patch — creates a hybrid that lies somewhere between a guitar and a harpsichord.

There are numerous parameters associated with the plucker, and you can select from dozens of modulation sources to adjust every aspect of its action. By and large, you don't obtain radical changes here, but subtle modifications that allow you to manipulate the sound in ways that would be impossible on something fashioned from dead trees, dead horses, and dead cats.

Having investigated the 16 pre-programmed plucks, I returned to the Excitation Mixer, reduced the Pluck Level to zero, and increased the Noise Level. The noise generator allows you to create your own impulses, tailoring them with an in-built filter, the Excitation Filter in the Excitation Mixer page, and numerous modulation sources. I shaped a simple impulse using one of the assignable EGs (envelope generators), and found that a brief AD curve results in something harp-like, while long curves created all manner of effects, depending upon other parameters. I found overdriven noise to be superb for generating aggressive (nay, violent) plucked effects and, later on, I was stunned when I accidentally created an incredible imitation of a piano's hammer action.

The STR1 synthesizer's architecture.The STR1 synthesizer's architecture.Photo: Richard Ecclestone

PCM Excitement

Nevertheless, I suspect that it's STR1's ability to use samples as excitation sources that will garner the most interest. In short, STR1 allows you to excite the string using many of the samples in the OASYS's ROM, the EXS sample libraries, and even your own samples when loaded into memory.

With the EXS1 sample set loaded, the PCM Oscillator in the patch I'm using defaults to the Nylon Guitar 1 Mf multisample. You might expect that this would give similar results to selecting the Ac Guitar 1 pluck but, whereas Ac Guitar 1 excites the virtual string with an impulse, the PCM excites it with a much longer sample, and the resulting sound is much like a bowed string. Applying other sustained PCMs led to similar results, but I obtained innovative sounds by selecting percussive and special-effects PCMs. For example, the Marc Tree creates an unearthly sound reminiscent of a guitar being plucked by small tubular bells, and just a few steps away are other previously unheard sounds generated by using the likes of the Chinese Gong, Cymbal Reverse, and Door Creak as exciters.

As before, there are buckets full of parameters that you can use to modify the action of the PCM oscillator — or oscillators, because you can assign up to four of them in four non-overlapping velocity zones. Particularly useful is the ease with which you can alter the pitch of the oscillator, and how it tracks the keyboard. Try moving the PCM oscillator's pitch down to 32' and changing the keyboard tracking. Depending upon the PCM used, the results can include different timbres under different notes, and unexpected changes in pitch as you play up or down the keyboard.

Even more interesting, perhaps, is what you can achieve by shifting the excitation frequency relative to the string, which you can do by transposing, retuning, and offsetting. This has the effect of making the excitation enharmonic with respect to the string, and it results in all manner of strange timbres. Better yet, you can modulate the pitch relationship, so that the resulting 'vibration' in the string changes from moment to moment in all manner of unpredictable ways. For example, if you use an LFO to modulate the excitation pitch, the result is the physical modelling equivalent of a flanger. If you use an envelope, you can obtain sounds of a similar nature to oscillator sync, but with quite different timbres. Sure, there are limitations, but to belabour these would be churlish, because the number of excitation parameters (and, therefore, ways to manipulate and use the PCMs) is mind-boggling. Furthermore, you can mix the excitation types together, with plucks, noise generator, and PCMs all contributing to the excitation.

The Reality Of Reality?

Some people on Korg's OASYS forum have suggested that STR1 does not sound as realistic as, say, the guitar patches based on Korg's sample-based HI and HD sound engines. For a single note, this may indeed be true, because the sample is, after all, a carefully recorded sample of the real thing. But when you start to consider the ways in which STR1 can modify a sound to take account of playing techniques, or even changes in the way that the instrument is set up and used, it's clear that the 'snapshot' approach of PCM-based synthesis is the poor relation.

String Characteristics

It's time to move on to defining the string itself, determining how the energy in the pluck is dissipated. To investigate this, I returned to the original patch and removed EXi2 as before, leaving Korg's programmed pluck untouched. I then selected the String Main page. As every guitarist knows, plucking a string close to the bridge or the nut accentuates the high harmonics, while moving the plucking point toward the middle of the string makes the sound rounder and fuller. The OASYS can imitate this perfectly, and also allows you to modulate the Position parameter using velocity or other controllers. (Of course, the effect of changing the Position doesn't only apply to imitations of guitars, and you can move the plucking position on any virtual string.)

The largest box of parameters on this page allows you to determine how the string responds to light fingering to create 'harmonics', but the most radical is the Nonlinearity. This models anything from a rigid bridge (the ideal for a western guitar) to a floating bridge (which stops the virtual string from being an ideal harmonic oscillator, and introduces all manner of detuned and buzzy artifacts, just like on a sitar and some other Eastern stringed instruments). If you're into the weird and wonderful, modulating the Harmonics and Nonlinearity parameters can create all manner of effects, and is well worth investigating.

As with a real stretched string, the energy in an STR1 model is dissipated most quickly from high frequencies, so a low Damping value not only lets the note ring for longer, it retains the high-frequency content for longer. In contrast, high Damping ensures that all the high frequencies are contained in the attack, and that the sustained portion of the note (if any) is rounder and more mellow. Reducing Damping to zero lets the strings ring for much longer, and the high frequencies are retained, so the sound is brighter and more sustained, a bit like one of the more esoteric keyboard instruments. Increasing the value to 100 reduces the sound to a pizzicato thunk. Mind you, this doesn't sound like a violin; again, it's a hybrid sound that is probably impossible to obtain by physical means. Amazingly, you can set the Damping individually for six keyboard zones. This lets you model the individual Damping of (for example) the unwound and wound strings of an electric guitar, or even the nylon and steel strings of a classical guitar. Astounding!

To understand the action of the Dispersion parameters, consider the difference in sound obtained by hammering a thin string and by hammering a thicker metallic rod such as found on a glockenspiel. The string is close to being a perfect harmonic oscillator; the rod is not and, although you can discern the note it produces, its sound is enharmonic, much like a chime or a bell. Reducing Dispersion from its default 27.6 to zero has a subtle effect, eliminating a little of the je ne sais quoi of the patch. Raising it toward 100 has a far more radical effect, converting it into the cheapo door chimes that infested the middle-class homes of the 1960s. There's even a String/Bell parameter that accentuates this further, which means that you can use STR1 to create all manner of chimes, bells, and metallic instruments.

The third screen dedicated to the string itself contains the parameters relating to frequency, and this is another place where you can alter the relationship between the pitch of the exciter and that of the string. Sticking with the nylon guitar patch, I selected the Sawtooth PCM wave as the sole excitation and changed the Pitch Slope of the string rather than that of the Pluck. This created a set of timbres that sounded aliased. Offsetting the pitch by a few cents then moved the sound deep into Asian territory as the overtones became enharmonic. Experimenting with other PCMs demonstrated that the potential for sound design is enormous, although you might be best served by sampling the output of individual notes and using them to build consistent patches across the keyboard rather than trying to play the Program itself!

The Mixer Page

When it comes to deciding what you hear, STR1 is extremely flexible. If you navigate to the Mixer page, you'll find that you can mix the sounds generated by the virtual string, the PCM oscillators, and the noise used (or not) to generate the excitation, plus the outputs from two virtual pickups.

Here is just a selection of the modelling parameters available to the STR1 synth.Here is just a selection of the modelling parameters available to the STR1 synth.Photo: Richard Ecclestone

As on a real stringed instrument, the positions of the pickups allow you to determine the harmonic content of a sound. If you want to test this, you can demonstrate how effective it is by generating a Harmonic at the same position as the pickup. Just as on an electric guitar, the result is near silence, because the string is not moving above the pickup. I found that choosing appropriate pickup positions helped me to create some remarkably accurate guitar models, as well as all manner of effects when I modulated the positions to create chorusing and comb-filtering effects. However, I soon learned not to place the pickups outside the range in which the string is vibrating (in other words closer to the virtual nut than your fingers would be if you fretted a real guitar), because silence ensued.

The Pickups/Feedback page is also where you'll find the Feedback parameters, which allow you to take the output from an STR1 patch, pass it through effects such as overdrive and amplifier models, and then feed the results back into the patch. Unfortunately, setting this up is not intuitive, because you select the audio input in the EXi Audio Input page in the Basic/Vector section, not in the STR1 patch itself. No matter; once you've got to grips with this, it all works, and no doubt someone has already created an authentic 'Hendrix Plucking The Strings With His Teeth And Waving The Guitar In Front Of The Amplifier' patch. I should also mention that you can use this architecture to route external audio into STR1. The opportunities afforded by this are obvious, so I won't go on about them here.

There's one additional page of controls present in STR1. It's called String Track and you'll find it alongside the AMS Mixer pages (see screens on the previous page). Like the equivalent sets of controls in the Damping/Dispersion page, this lets you divide the keyboard into six zones and then enter offset values into two rows of parameters that you can use as AMS sources anywhere within STR1. This approach isn't perfect, and it does nothing to allow you to model the differences between a note played on one string and the same note played five frets lower on the next higher string (and so on). So, just to show how fiendishly complex modelling technology can be, there's another parameter, Fret Number, that determines whether a note is being played on an open string, or how far up the neck it is. One has to be impressed.

Once you've finished designing your pluck/string/pickup sound, STR1 routes it through the powerful filter, multi-filter, amplifier, and inserts effects sections first seen on the AL1 synthesizer, with all the LFOs, envelope generators, and modulation possibilities that this implies, as well as taking advantage of the OASYS's sequencer, KARMA, the master effects sections, and so on. However, if I were to try to suggest the ways in which you might wish to use STR1 with KARMA, or how you might use it to create realistic imitations of performances of stringed instruments, I would fill this whole issue of Sound On Sound. I'll just have to leave that part of it up to you.

CPU Limitations

When you insert an STR1 Program into a Combi or Sequencer setup, the OASYS dedicates some CPU time to it, whether it's producing notes or not. This is known as allocating 'EXi Fixed Resources' and it imposes two limitations, although neither should prove to be an insurmountable problem in the real world. Firstly, you can only use 16 STR1 sounds simultaneously before the OASYS runs out of CPU resources to allocate. Secondly, this means that, for each STR1 sound requested, you use approximately 6 percent of the OASYS's processing power, reducing the power to do other things accordingly. Bear in mind, however, that there can be two STR1 sounds in a Program, so keep an eye on things if you're invoking lots of Programs with dual sounds.

Plucking Marvellous

If you design an STR1 sound sticking to the traditional concepts of impulsive excitation and string, all manner of 'real' instruments pour forth, including acoustic guitars and basses, harps, harpsichords, electric pianos, and sitars. But excite the string using filtered noise and/or sustained samples and you obtain very different types of sounds. Then, when you start to experiment with parameters such as Nonlinearity plus the multitude of filters, feedback, modulators, envelope generators, and the mixers, what you obtain can bear no relation to the concept of a plucked string. Indeed, if STR1 were a hardware synthesizer in its own right, it would be a remarkable instrument, and I doubt that many players would ever fully plumb its depths.

Given this complexity, you might think that an STR1 patch would be difficult to control, but it isn't. Korg have created a default STR1 control surface for the top panel of the OASYS, and most factory sounds conform to this. You can customise this, but I found it to be well chosen, allowing me to modify sounds in interesting ways even before delving into the full programming system. For live performance, it will be a boon.

The question is, therefore, should you rush out to buy it? Well sorry, you can't. Although OASYS v1.1 (and, therefore, STR1) has a nominal price tag of £199 in the UK, you'll just have to let Korg supply it to you free of charge, which is the company's way of saying 'thank you' to people who adopted the OASYS early in its life, and who trusted them to make good their promise of new software synthesizers and added facilities. This makes it superb value for money (if you see what I mean) so it seems almost ungrateful to criticise it in any way.

But I'm going to anyway, just a tiny bit. The factory sound set, while impressive, is rather too vanilla for my taste, and I think that the sound designers could have added more pistachios, chocolate chips, black cherries, and other goodies to demonstrate its radical capabilities. As I've stated throughout this review, STR1 lends itself to sound design, and it would be good to see this side of it explored more fully. But to balance this, I would like to compliment the company on its manual. Not just a list of functions, it makes a positive effort to explain why a parameter exists, and what you might do with it. Well done Korg!

In summary, STR1 is a brilliant addition to the OASYS. It allows you to create myriad sounds (string-like and otherwise) and its potential for performance control is unsurpassed. And it's free. What more do you want? 

Published October 2006

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