David Fox discovers how Oberheim's Xpander increases the potential of their Performance System.

In the old days of small monophonic synths and larger modular systems, an expander was a unit which perhaps had little to offer by itself, but which served to increase the capabilities of an existing synth. Examples included the Korg MS50 and Roland 102, and typical facilities on an expander module could include an extra oscillator, spare LFO's, effects such as a ring modulator or phaser and perhaps a sequencer or mixer.

Now that you've got that definition of an expander into your head, forget it. The modern use of the term refers to something very different, and has come about through the introduction of MIDI.

Once you've conceived the idea of a MIDI synth without a keyboard, it begins to show more and more applications. If you're composing multi-channel music with a computer you may only need one keyboard, although the final piece may require many different voices. If you're using music peripherals such as a guitar synth interface you don't need a keyboard at all, and if you're short of studio space the idea of controlling several instruments from a single versatile keyboard can be very attractive.

This, then, is what we mean by an expander nowadays — a polyphonic synth with no keyboard and very few if any external controls, the sound changes being controlled by your single keyboard or computer. Examples already on the market include the excellent SIEL Expander, the Roland Planet-S and Planet-P modules, and a forthcoming Korg expander version of the Poly 800. The Oberheim Xpander, like all those mentioned, is perfectly compatible with all MIDI equipment, but naturally has an affinity with the existing Oberheim Performance System.

Oberheim Xpander

As we've already seen, the Xpander is one of a new breed of musical instruments, a compact keyboardless synthesiser owing its existence to the MIDI specification.

In fact the Xpander breaks this new tradition in a couple of ways. Firstly, it's not particularly compact and secondly, it's not a keyboardless version of an existing synth — it's a unique design with quite a few innovative features.

The basic spec alone is quite frightening — the Xpander boasts six voices each with two oscillators, 15 VCA's, five LFO's, five EG's, four Ramp Generators, three Tracking Generators, Lag Processor, 15 Mode filter and FM Synthesis. To help you get this little lot under control there are three 40-character LED displays which are labelled Programmer and Page Modifiers.

The Page Modifier displays refer to six infinitely rotatable knobs which alter different parameters according to what's been called up on the large right-hand Patch Display. This comprises a diagram of a single voice, with pushbuttons at the vital points — the VCO's, LFO's, VCA's and so on.

Once you've called one of these sections onto the Page Modifier display, it's possible to adjust all the relevant parameters such as the Mode, Cutoff Point, Resonance and so on of the Filter. Additionally a row of buttons at the top of the Xpander allows you to select modulation sources, which are, to say the least, comprehensive. In fact there are 14 Modulation Source switches and you can control almost anything with almost anything else — for instance, a delayed vibrato on the oscillators with the vibrato depth controlled by one of the envelope generators and the pitch of the oscillators controlled by another. Sounds complicated! In fact you can produce enormously complex and subtle sounds on the Xpander, and the preset sounds don't do it full justice, although there are some impressive effects.

It's important to realise that the Xpander, like the Sequential Circuits Six Trax, is in fact Multi-Timbral, that is, it can produce different sounds simultaneously with each of its six voices. For this reason there are two types of sound patches — single and Multi Patches, of which there are 100 each. The single patches are conventional polyphonic sounds, and the multi patches are programmed combinations of these with the control source (CV or MIDI channel), keyboard split points, volume and stereo Pan position of every voice specified. Multi Patches can be chained for fast access and all patch information can be dumped to tape.

As you can imagine, the potential for imaginative sound programming on the Xpander is enormous. Apart from the five envelopes and five LFO's available on every voice there are several special functions such as a Zoning facility, which alters the scaling of keyboard tracking functions according to keyboard position. In other words, the Xpander can sound quite different over different portions of the controlling keyboard.

In addition there's the FM section, but this has little to do with the Yamaha system seen on the DX7 and DX9. To be fair, it is an example of Frequency Modulation in that the pitch of LFO1 modulates LFO2, but this is more or less the same as the Poly Mod on the Sequential Circuits Prophets. The system's capable of some very impressive metallic and harmonic sounds, but not to be confused with anything the DX7 has to offer.

There's no doubt that the Xpander is another totally professional Oberheim product, and it has a set of hidden testing and engineering functions to match. Apart from tuning all its oscillators and filters it can individually test every segment of the LED displays, check Pulse Width and monitor the performance of the VCA's. It's good to see an instrument which is so universally interfaceable, but of course the expense of the unit is going to limit its applications a little. In fact the Xpander will be £3,950, which is in line with the cost of the OB8 although in many ways it has a more versatile specification. It's even capable of responding to velocity information from keyboards such as the Yamaha DX7.

The major problem of course is cost, and it would be as well to remember that some of the other expanders on the market are a lot cheaper. Of course something like the SIEL expander can't offer you 80 VCA's, 25 LFO's and so on, but you could buy five of them for the price of one Oberheim. If you're into multitrack composing this is of course a massive advantage, and although the 30 voices at your disposal wouldn't interact to give the complex modulation effects of the Xpander, you would have some very impressive layering effects. Another minor disadvantage of the Xpander is size — it's much wider than a standard 19" rack mounting and in fact makes a pretty bulky package.

Still, it exudes quality and has some powerful sounds to offer, whether in combination with the OB8 or with a cheaper keyboard controller — even something as inexpensive as Casio's CT-6000 MIDI keyboard at £700.

The Oberheim Performance system has a lot to offer the professional musician, and has seen chart action (as they say in the hipper circles) with New Order and many others. Now that the OB8 and Xpander are MIDI-equipped they'll find a much wider market, and if the DSX Sequencer had MIDI added it would sell in its thousands.

On the tape in combination with the Xpander and Performance System we use an MXR01 a digital delay, an updated version of the 01 which features a new reverse sound facility.