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SIEL Opera 6

Touch-Sensitive Polyphonic Synthesiser

In our British Music Fair Report (E&MM September '83) we noted the fact that touch sensitivity was a major feature of new keyboards being introduced, and singled out the SIEL Opera 6 as being one of the products created to meet the demand from musicians for more keyboard control of their instrument. The first keyboards to feature this facility tended to be outside the average keyboard player's price range, but at about £1,300, the Opera 6 is one of the cheapest touch-sensitive keyboards on the market, helping make this added control available to many more players.

As its name implies, the SIEL is a six-voice poly (that is, each voice has separate oscillators, filter and envelope generator) with 100 memory locations for voice programs. The look of the instrument has changed greatly from previous SIEL products. The tough, all-metal case has a dark blue panel with individual sections of the programming divided into light blue boxes thus keeping the separate elements of the process distinct from each other. The keyboard spans five octaves (C-C) and has a nice, positive feel which is, of course, quite important on a dynamic keyboard.


Each voice on the Opera 6 has two oscillators which are of the analogue type. Both VCO A and VCO B have a choice of pulse or ramp wave and these can be mixed on either oscillator. They are selected by a unique single switch system which occurs repeatedly on the instrument. Starting from the off position, one push lights the LED above the switch (in this case indicating the ramp wave has been selected) a second push changes this to the lower LED (pulse wave), a third push lights both LEDs (signifying both functions are on) and a final push returns it to the off status.

There are three footages available on each oscillator — 16', 8', 4' (a convention which has been continued from the organ and refers to the length of organ pipe required to create a note at these pitches) and there is also a Pulse Width control on each, allowing the harmonic content of the pulse wave to be altered. The pulse width of both oscillators can also be modulated by LFO III as we shall see later.

A principle criticism of this section would be that little subtlety in the programming of respective volumes is possible. VCO A is either 'on' or 'off' (depending on whether a waveform has been selected or not) and VCO B is only slightly better with a half volume option. However, where the volume knob for VCO A might have been there is a fully variable volume control for Noise and this is infinitely preferable to those six-voice polyphonics in a similar price range which simply replace the second oscillator with noise. This configuration with an independent noise signal is much more flexible.

However, this criticism of lack of subtlety could not be applied to the Detune controls between the oscillators as there are two of them provided. Coarse detune allows intervals of up to a fifth to be set up quickly and easily whilst larger amounts can be programmed in conjunction with the footage switches. Fine detune, on the other hand, gives access to those rich ensemble sounds which can be created using a very small tuning difference between the oscillators. These two prime uses of two oscillator instruments are therefore well catered for on the SIEL and the problems of accuracy which can often result from use of a single knob are avoided.


The filter on each voice is of the 24dB/oct low pass variety (in other words it attenuates the higher harmonics of your sound first, acting last of all on the fundamental). This is the essential configuration necessary for most sounds and gives the machine the same quality and flexibility as other similarly priced six-voice polysynths, with the added bonus that the keyboard velocity can also be routed to control the filter cut-off frequency. This is manually controlled (ie. the initial level set) by the Cut-off knob but can also be automatically controlled by the LFO and the dynamic ADSR. There is a knob marked ADSR Amount which controls the subtlety of this latter effect.

There are two further controls in the filter section, Track and Resonance. Track allows the frequency of the filter cut-off to be adjusted by the note played on the keyboard, from 0 to 100%. On the latter setting each note played will have an identical timbre, whereas lesser amounts will cause higher notes to be less bright. The Resonance control amplifies the frequencies around the cut-off point and once past two-thirds of its travel, pushes the filter into sine wave oscillation. This is very useful for effects programs and in combination with Track allows you to tune and 'play' the filter without using the oscillators (particularly as the tracking on the Opera 6 is pretty accurate).

Dynamic ADSR

Now we come to the principle feature of this instrument. Besides the conventional Attack, Decay, Sustain and Release functions, the envelope of each voice is controllable from the way the keyboard is struck. The velocity at which each individual key is depressed can be used to control the overall ADSR level or to override the Attack time, or both (using the cyclic switch function we first saw on the oscillators). The ADSR can be routed to the VCA (determining the shape of the sound) or the VCF (controlling filter cutoff) or to both using the same switch function again. This gives you quite a few routing possibilities and it is here that the cyclic switches really come into their own, allowing you to quickly step through the different routings and compare the effects produced.

If the ADSR is routed solely to the filter (VCF) then the VCA (amplifier level) resorts to an organ envelope ie. full volume when a key is depressed and-off as soon as the key is released. This is where the limitations of the single envelope per voice begin to show, but it must be said that in this respect, polyphonic synthesisers with two envelope generators per voice cost considerably more than the Opera 6 and it more than matches conventional machines in its price bracket with the single dynamic envelope.


Above the left hand end of the keyboard are two wheel controllers. The first allows pitch bend of either VCO A or B, or both giving an interval of a fifth above or below. The second controls the introduction of low frequency modulation from the LFOs and again the actual routing is controlled by the cyclic switch function.


According to the front panel there are no less than three of these, but this is somewhat misleading. Presumably LFO I is reckoned to be controlling pitch and LFO II the filter (or vice-versa) but as they share the same Depth and Speed controls they can hardly be seen as two independent LFOs, rather one with two possible routing destinations.

LFO III is another matter. With its own separate Depth and Speed controls it is genuinely independent and is used to control the pulse width modulation of either or both oscillators. This is vital as the most satisfying pulse width modulations tend to occur at slow rates whereas vibrato (pitch modulation) and other LFO effects need to be much faster. This means that on single LFO machines you can only have one or the other in any single program but combinations are possible on the SIEL Opera 6. Both LFOs (it is fair to say there are two) also have a separate LED which flashes to show the speed at which the LFO is set.


This consists of a numeric keypad and two seven-segment LEDs which indicate the program number selected. An Enter button actually changes the program when you have put new numbers on the display and a Free button allows you to use the panel settings as they stand. The Write button enables newly created sounds to be stored in memory.

The two knobs in the right-hand section of the panel marked 'Masters' are the only two controls which are continuously live and not stored in the programs. These are overall Tune and Volume, both of which need to be adjusted to suit the other instruments with which you are playing. All other controls are always instantly available for editing sounds and edited programs can be stored in different memory locations.

Back Panel

Besides the standard jack output, there are several interesting features here. Firstly, there are two 5 pin DIN sockets which allow the pedal which SIEL supply to control either the VCA (volume) or the VCF (filter cut-off). I would have preferred to see just one socket and a switch on the front panel to enable the user to change pedal control during a performance and store this in programs, but it is still good that both can be controlled from a pedal.

Next to these we have the cassette interface sockets (mini jacks) which allow a library of sounds to be built up and reloaded at a moments notice. Last of all we have the most interesting interface — MIDI in and out connections. SIEL have certainly come up to date with a bang as it is still early days for this universal interface system, and this is one of the few machines from less well known companies to feature it.


Clearly SIEL's recent co-operation with Sequential Circuits has led them to be very forward-looking. The Opera 6 has several features which are as yet unavailable from some of the traditional 'big name' synthesiser manufacturers: velocity sensing, MIDI interface, independent LFOs etc. This clearly means that this keyboard is not about to become obsolete and makes it excellent value for money.

The clear panel layout and unique cyclic switch function makes programming quick and easy and the sound is clear, rich and flexible. Whilst the casing is a little high it does push the front panel forward to an easily visible angle and the LEDs enable you to keep a check on what is going on, even on a darkened stage.

This instrument would make an ideal first polyphonic especially for the piano player interested in 'going electronic' and fits well into a growing keyboard set up, thanks to the MIDI interface.

SIEL are now distributing their own products in the UK so for further details contact SIEL (UK) Ltd., (Contact Details). The recommended retail price of the Opera 6 is £1,299 including VAT.

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Previous Article in this issue

MXR 185 Drum Computer

Next article in this issue

Calrec 600 Series Microphones

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


Electronics & Music Maker - Oct 1983

Gear in this article:

Synthesizer > Siel > Opera 6

Gear Tags:

Analog Synth


Previous article in this issue:

> MXR 185 Drum Computer

Next article in this issue:

> Calrec 600 Series Microphone...

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