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The Korg Sigma

Korg SigmaIn 1976, three years before launching their first true pressure-sensitive instrument, Korg experimented with one of the strangest little synths ever to see the light of day. The 900PS featured an unusual contact sensitive rail which, when touched, acted as an on/off switch for vibrato and other 'touch' effects. This was, of course, equivalent to the LFO button that Roland incorporated in a number of their early '80s instruments, but had the advantage that the rail ran underneath the keys for the whole length of the keyboard. Consequently, you could activate the effects with the hand you were using to play the synth, leaving the other free for different duties. Curious, certainly innovative, the 900PS lacked the appeal of its 700, 700S and 800DV siblings, and was not a great success.

Nevertheless, the subsequent two years saw Korg climb from the lower divisions into the keyboard premier league with a new family of monosynths and polysynths. The MS-series of semi-modular monosynths, released in 1978, are now classics, and no less an authority than Keith Emerson promoted their large polysynths - the PS3100, PS3200 and PS3300 - as expensive dream machines.

Nineteen seventy-nine saw Korg launch three instruments that comprised yet another family. A beautifully designed piece of polished wooden furniture, the Lambda was a preset polysynth offering string and piano-type voices. Alongside it, the cheap and cheerful Delta combined a string ensemble with a very basic polysynth section. The third instrument was a semi-preset monosynth - the Sigma - which combined a few of the type of voices found on the Roland SH2000 and APR ProSoloist with a quirky synthesiser section.

Based around a three octave pressure- (but not velocity-) sensitive keyboard, the Sigma was beautifully built. It retained the wooden end-pieces and metal chassis from earlier Korg designs, yet the control panel bristled with knobs, rockers and joysticks. Unlike the Lambda and Sigma, the look was distinctly hi-tech, if in a Star Trek ("It's a synth, Jim, but not as we know it") sort of way. Likewise, the back of the Sigma was busy with inputs for filter modulation, VCO modulation, VCO pitch and trigger IN, and outputs for trigger OUT, and keyboard CV OUT. There were even independent outputs for the two distinct sound creation sections (termed 'Instrument' and 'Synthe') and a headphone socket.


Instrument corresponded to other pressure-sensitive monosynths of the time but offered just eleven voices instead of the more usual thirty or so. These were grouped in footages rather than instrumental families, and each voice had a single variable parameter for added flexibility. For example, you could alter the filter cut-off frequency of the tuba, change the pulse width of the clavi, and modify the attack of the strings. As for performance controls, there were switches for octave up/down, portamento (which could be permanently selected, or activated by a button alongside the keyboard) and keyboard pressure sensitivity. Surprisingly, and quite unlike the ProSoloist with its six simultaneous pressure sensitive effects, the pressure sensitivity of the Sigma could only be used to bend the pitch up, or bend the pitch down, or apply vibrato. Similarly, although one of the Sigma's two joysticks offered vibrato and pitch shift, only pitch shift applied to the Instruments. On the plus side, the Sigma also featured delayed vibrato, multiple triggering, and key hold. There was also 'quarter tone' which, when a note was held, re-scaled the keyboard in (approximately) quarter tones rather than semitones. A curious feature indeed.

But, all things considered, Instrument was a disappointment. Neither gutsy like the ARP, nor clean and precise like the Roland, its patches were bland imitations of their orchestral inspirations. Of the eleven voices, only the Electric Bass, Tuba, and Oboe deserved any real credit. The Horn and Fuzz Guitar were just about passable but others, in particular the Clavi and String, were - as solo sounds - to be avoided at all costs.

So what about Synthe? This comprised eight voices offering square, PWM, and sawtooth waveforms of various footages ranging from 32' to 4', as well as Sample & Hold, and Noise. Five had variable attack & release times, while two had variable decay. Only the S&H had a different structure - its variable was Clock Rate. These offered much more than their 'Instrument'-al counterparts. The 32' sawtooth was deep and brassy, while the 16' Pulse Width Modulated wave was rich and thick. (No jokes about politicians, please.) The 8' pulse and 4' sawtooth were useful for adding aggressive attacks to other voices, but the real star was the 8' squarewave: warm, vibrant, and a genuine canditate for some classic lead-synth voices.

There were also several performance controls that acted exclusively upon Synthe. Primary among these was the filter joystick that controlled both the low-pass and high-pass filters. The second was the vibrato joystick which, when applied to a Synthe voice, combined vibrato, noise depth, and pitch bend. This offered some interesting sonic possibilities because the noise was not added to the signal, but modulated it to give some dramatic distorted effects. And finally came portamento. But the ace up the Sigma's sleeve was Synthe's ring modulator. This modulated the total output from Synthe against the sum of the Instruments and, since Synthe could be detuned against the Instruments, made the Sigma capable of some huge analogue sounds.

Whereas most monosynths could only produce one voice at a time, the Sigma possessed the ability to play more than one Instrument or Synthe sound Sigma-taneously. (Oops, sorry.) This made it possible to stray far beyond the preset patches by constructing hybrids from what were, in effect, partials. And, since Synthe and the Instruments could be tuned independently, you could create patches that gave, for example, an attack at one pitch fading into the sustain of the note at another. Detuned chorusing and playing in thirds and fifths were also just a knob twiddle away. In addition, you could sweep the filter joystick to fade Synthe voices in and out of a mix, or craft the type of filter modulation previously associated only with large modular synthesisers. With 'key hold' on, vibrato applied to one or both sections, pitch bend applied to just Synthe, portamento added when desired from the momentary button... these facilities could make the Sigma sound like a very big instrument indeed. If, with reckless abandon, you selected every voice - 19 different sounds produced simultaneously every time you pressed a key - and messed around with the other facilities, the resulting noise was monstrous. Never was there a more curious mixture of limitations and powerful synthesis than this.


When Korg released the Sigma its design and a handful of its facilities put older preset monosynths to shame. It was heavily endorsed by Rick Wakeman (who at one time replaced his Minimoogs with four of them) and also used by Keith Emerson. Yet it never caught on, and within a few years had vanished. And that's a pity because the range of possibilities contained within its weird architecture was huge. Nowadays, its resale value can be very low, but who knows... if a bit more fuss had been made of the original rather than focusing on its limitations, there might have been a Mark II, and that could have been a very weird and wonderful synthesiser indeed.

The sounds

The following lists all the Sigma's voices. Those with Decay controls have 'AD' envelopes with no sustain, and these decay to silence no matter how high the decay value that is chosen.The Tone control is a band-pass filter that progressively eliminates low frequencies when turned clockwise, and high ones when turned anti-clockwise.

Instrument Footage Variable
Electric Bass 32' LP filter cut off
Tuba 32' LP filter cut off
Clavi 16' Pulse width
Fuzz Guitar 16' Tone
Horn 16' LP filter cut off
Trumpet 8' LP filter cut off
Clarinet 8' Tone
Double Reed 8' Tone
String 8' Attack
Flute 4' Tone
Hammered Percussion 4' Decay
Synthe Footage Variable
Ring Mod Synthe pitch
Noise Attack and Release
Sawtooth 32' Attack and Release
Sample and Hold 16' Clock rate
Pulse Width Modulation 16' Attack and Release
Sawtooth 8' Attack and Release
Pulse 8' Decay
Square wave 8' Attack and Release
Sawtooth 4' Decay

Copyright ©1996, Gordon Reid.

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