The Expander is a modular add-on unit for MIDI synthesisers, containing the same internal features as the same company's Opera 6 polysynth. Geoff Twigg broadens his horizons.

Basically, it's another Opera 6, without keyboard or parameter controls. The Expander is housed in a smart grey box incorporating the now customary Siel livery of pale blue outlines and red and white switches on the control panel. As you switch the power on, a set of LED indicators signal that the unit is ready to listen, via MIDI, to whatever the governing Opera 6 tells it to do - or what you indicate using its own calculator-style panel.

The Expander is the same internally as the Opera 6, and its voices are created and edited using the same controls. Each of the six voices has two digitally controlled oscillators, which in turn may be modulated by three LFOs. Each voice has a separate filter and VCA, with standard ADSR envelope shaping. All in all, then, nothing particularly unusual in the synthesiser department: like the Opera, the Expander omits many of the most recent analogue synth innovations such as reverse or inverted envelopes, arpeggiators, sequencers and so on.

The oscillators can produce either ramp or pulse waves (or both), and the pulse width is fully variable, being modulated by LFO 3 - different degrees of modulation for each oscillator give an unusual phase sweep effect when you play chords, for instance. The filters (low-pass 24dB/octave) are impressively clean, with an even effect over their entire range, and what's more, they're refreshingly easy to adjust accurately. There's also a keyboard tracking option that opens the filters progressively the higher you play up the keyboard, so that the timbre of your playing remains relatively constant.

Velocity Sensing

Perhaps the most impressive feature of the Opera 6 is its velocity sensing keyboard, and the ability to direct that control parameter to override the attack time or control the overall ADSR level. This dynamic control applies to the Expander as well, of course: if the envelope shaper is not directed to the VCA, it reverts to an 'organ' envelope, ie. full volume as soon as a key is depressed and no release time.

The sounds from both the Opera 6 and the Expander are of uniformly high quality, and in general they follow the Mediterranean fashion of string and brass-like textures. The ramp and pulse waves provided enable the user to create a considerable variety of different accompaniment colours, and with filters of this quality, you can alter the sound drastically without blurring the underlying texture. However, the Siel isn't quite so good at punchy, lead-line sounds. The only way you can get close to them is by hitting the Opera keys really hard to open up the VCA and VCF.

Following on from that, one thing I did miss was a monophonic option to lend strength to lead sounds. Since this is such a notable omission, I assume Siel must have considered it and subsequently decided against including it, making the Opera and its accompanying Expander module chordal accompaniment machines only, albeit very good ones.

As well as the 95 presets available, it's possible to direct the Expander's operating system by selecting numbers 95 to 98. 95 enables you to record the preset voice; 96 selects the MIDI channel; 97 is the split-keyboard option; and 98 enables you to change the velocity sensing response, four level options being available.

In Action

The Expander is connected via a single standard specification MIDI cable, and will respond to key information from the controlling MIDI keyboard. The Expander enables you to change preset voices remotely from the controlling synth, or change the synth preset from the Expander, should you wish. It works like this. On each of the Siel units there's a control panel which doubles as an interface controller for tape or MIDI. Selecting Internal on this panel means that any numbers you key into the preset selector refer to the unit you're working on - selecting External means that the selection of presets is sent down the MIDI link, and the host machine is unaffected. This process can be controlled from either the Expander or the Opera 6, while it's also possible - if a little expensive - to stack several Expanders on top of one another and address them individually by giving them separate channel numbers (preset 96).

In order to alter the Expander's presets, you have to download the voice onto the Opera 6, edit and re-record the preset, and then reverse the process to get it back into the Expander. The ability to edit voices within the Expander direct from the Opera keyboard is a provision Siel have promised but which is not yet available - it would certainly make the system an awful lot more usable.

Another slight design deficiency lies in the fact that performance controls - mod wheels and pedals - do not affect sounds from the Expander. This is despite specific provision for a control code dedicated to pitch bend in the MIDI specification, but then again...

I understand Siel are about to introduce a split-keyboard facility (as preset 97) in the new EPROMs soon to become available. Position of this split is fully programmable, and is directed so that one half operates the host instrument, while the other half is directed to another synth on the MIDI bus.

MIDI Connection

I connected the Expander to a Roland Jupiter 6 via MIDI, and, as you might expect from an Italian, it behaved impeccably. Sounds that existed previously as extensions of the Opera 6 were transformed into a flowing accompaniment for the Roland's bright, percussive voices - it followed the controlling Jupiter's keyboard without so much as a hiccup. The sonic worlds of these two machines are so utterly different - yet contrast each other as beautifully - it's almost tempting to surmise that they were designed for each other.

Not all in the MIDI garden was rosy, of course. There was no response to the Roland's arpeggiator or mod wheels - the Expander not being capable of receiving this information - and only a change in patch preset number (voices A1 to D8 on the Jupiter) brought about a corresponding change in the Expander's presets.

In practical terms, this brings us back to the fundamental problem mentioned earlier: if you want to alter the factory presets on the Expander in any way, you have to use an Opera 6, which means that buying an Expander simply as an add-on for a JP6, DX7 or whatever isn't really on. This is a great shame, though on a slightly brighter note, I undersand that Siel are considering marketing a software equivalent to something like the Roland PG200 programmer, which would work in conjunction with a MIDI interface (such as Siel's own) and allow you to alter the Expander's internal parameters using the keys on a microcomputer, without having to pay out for an Opera 6.

Conclusions

After an entire afternoon spent playing the Expander, Opera 6 and Jupiter 6 in conjunction, I can honestly say that my confidence in MIDI has been restored considerably: I was still investigating new sound possibilities when the time came to write this review, and that was without any form of sequencing facility. Quite what I would have got up to had I access to a MIDI sequencer heaven knows. Let me just say that this Expander really does live up to its name, and should do so even more in the future.

The Siel Expander retails at [price missing] including VAT, and further details are obtainable from Siel (UK), (Contact Details).