At under £350, Yamaha's new DX100 is the cheapest FM synthesiser yet, offers nearly 200 preset sounds, and looks like bringing digital synth programming to thousands for the first time.

You know, if there hadn't been the pocket calculator, the portable home keyboard and the digital watch, there would never have been the Yamaha DX100. Because like it or not, the company responsible for mass-marketing the first three innovations, Casio, are the main reason why attention in the Land of the Rising Sun has suddenly switched to small, easily portable versions of larger polyphonic synthesisers. It was Casio who realised, not much more than a year ago, that if you made polysynths cheap enough, you could persuade users of domestic portable keyboards to enter the world of the synth. How many people started with a Casio home keyboard and have since gone on to buy one of the company's CZ synths?

Now Yamaha, whose post-Geneva relationship with Casio is now worse than that between Reagan and Gorbachev, have decided that enough is enough, and launched a couple of synthesisers to tackle the CZs head-on. The mini-keyboard DX100 is pitted against the Casio CZ101, the full-size key DX27 against the CZ1000. And whereas the Casios use their company's PD (Phase Distortion) synthesis principle to produce their sound, the Yamahas use FM (Frequency Modulation), the digital technique that's made the DX7 such a compelling success story. As with the CZ pair, the insides of the DX100 and DX27 are exactly the same as each other, and as befits a portable synth, the DX100 can be powered through either mains or batteries (the latter with a claimed ten-hour lifespan), though this in no way affects the contents of the synth's memory, which has its own battery backup system.

The DX100 is a cut-down version of that previous breaker of price barriers, the DX21. Essentially, this means it can play a maximum of eight notes at any one time, has (most of) the DX21's preset sounds and (virtually) its synthesising capacity, but no facilities for splitting the keyboard or doubling up sounds on top of one another. The front panel layout should be familiar to anyone who's dealt with a DX in the past, as most of the functions are tried and tested and the front panel is finished in the instantly recognisable DX colour scheme. Unfortunately, the 100 is fitted with the same squidgy rubber switches as the RX21 drum machine, which tend to slip around under your fingers; you need to press firmly to ensure your selection is made.

Frankly, you've got to be prepared for economies like this on a digital polysynth that costs less than £350.

What you have a right not to be prepared for is the fact that this DX has more preset sounds on it than any of its predecessors: 192 next to the DX21's 128. In contrast, the editable voice positions which Yamaha call 'Internal' memory have decreased in number from 32 to 24, which means there's less space for people to change those preset sounds. This emphasis on factory voices is understandable, because many people buying the DX100 will be first-time users who'll want to get to grips with the idea of a programmable synthesiser before they actually start programming it. And in any case, the 100 goes one better than the DX21 by allowing you access to its preset (ROM) voices without having to call them into the 32-voice Internal memory (RAM) first.

The 192 preset voices are selected with the four bank selector buttons and then accessed singly by pressing one of 24 front-panel voice selectors.

But hang on a minute. Orwell was wrong. Four times 24 does not 192 make. It does make half of 192, though, and in fact the preset sounds are organised as two 96-voice groups — shame there's no indication of which bank you've currently selected.

Time for clarification of a subject there's been some confusion about. You may remember that the DX21 had 32 'performance memories' — a neat way of putting single, split and dual combinations of the 32 voices in RAM in any order you wanted. Rather than storing actual voice data, these performance memories stored pointers to the chosen voices in RAM. Now, despite what Yamaha's pre-publicity might lead you to believe, performance memories live on in the DX100 in an enhanced form. The 100's equivalent comes in the shape of a 96-position Bank memory (arranged in four banks of 24 positions), with each position capable of storing a pointer to a voice from either the internal RAM or the 192-voice ROM.

There are thus three 'levels' of voice selection which all occur from the same buttons, and which can get a bit confusing. The front panel provides precious little information here — merely a prefix to the voice number in the display (which still isn't backlit).

Anyway, the parameters that go to make up a DX100 voice will be familiar to anyone who's encountered a Yamaha FM synth before, though quite how many owners will have done this, I'm not sure. There are four operators and eight algorithms (ie. operator configurations) which form the basic structure within which you work when changing sounds. Each operator has its own envelope generator of the ADSR type to shape the way the sound changes through time, and there's one LFO governing all the operators, with key scaling available in the form found on the DX21. In fact, the only things the 21 has which the 100 doesn't are a pitch envelope generator and a built-in chorus. The former isn't too serious, but it's a shame DX100 owners will have to fork out for an external chorus pedal — some of whose facilities they may have no use for — just to beef up the synth's output.

More encouraging is the fact that the performance parameters (not to be confused with performance memories) are individually programmable for each sound, as they were on the DX21. These include poly/mono selection, pitchbend range, portamento, footswitch assignment, mod wheel pitch and amplitude range, and breath controller parameters.

Other useful features are the key set and pitchbend mode facilities, though neither of these are voice-programmable. The former allows you to transpose the keyboard up or down by any interval over a two-octave range at the press of a button, while the latter lets you define the lowest note, the highest note or all notes to be bent when you use the pitchbend wheel; the first two options will come in especially useful if you ever have to convince anyone the DX100 is actually a guitar.

Included in the 100's selection of 192 ROM voices are virtually all the DX21's presets. And I can only recap what I said about them when I reviewed the DX21: they're a very impressive selection. Most of them are usable, and very few of them could really be described as weak.

There's a healthy variety of acoustic and electric piano sounds, for a start, though no electric piano to give the DX7's famous interpretation a run for its money. 'IvoryEbony' is worth singling out, though: an acoustic piano patch that's terrific in its lower register, on both single notes and sustained chords, where the resonance of the real thing is splendidly replicated. Sadly, the sound doesn't quite cut it in the higher registers, even with judicious use of keyboard scaling.

"Facilities - The pitchbend mode options should come in useful if you ever have to convince someone the DX100 is actually a guitar."

Harpsichord sounds are less effective, mainly because they don't attain the complexity of the DX7's equivalents. However, organ sounds (Church, Hammond and beyond) are among the most convincing in the 100's arsenal, along with tuned percussion and some marvellously funky clav sounds.

The world and his wife agree that FM string sounds still aren't entirely satisfying, and the DX100 doesn't take the state of the art any further. All its string voices sound 'electronic' when sustained, and lack the warmth and expression of the analogue synth breed.

Bass sounds are much, much better-punchy, dynamic and contemporary — whilst there are the unavoidable silly sound effects like 'space talk' and 'ghosties'; I guess they're there to assert the DX100's identity as 'a synthesiser'.

Peering around the back panel reveals MIDI In, Out and Thru sockets, a single audio jack out, headphone and footswitch jacks, cassette and breath controller, sockets and an LED contrast wheel (not found on the DX27) which I could comfortably have done without.

The familiar pitch and mod wheels have been moved back to the rear edge of the synth for the DX100, and have had their size and shape suitably altered. This usefully reduces the length of the machine, and also puts the wheels in just the right place for when you've got the synth strapped around your shoulders. Yes, you can attach a strap to the DX100 à la electric guitar, and stride out to the centre of the stage to pose with the best of the axe-wielders. You can wear the synth equally well over either shoulder depending on whether you want to play left- or right-handed, and its compact size and light weight (it's probably lighter than many solid-body guitars) allow it to fit rather well into the across-the-shoulder role. What a pity most British synth players seem happier tucked away in the corner, out of the limelight...

The DX100 has MIDI, of course. To be precise, it has much the same MIDI implementation as the DX21, which means you get such features as MIDI on/off and separate transmit/receive channels (with an Omni receive option, too).

All the DX100's preset voices can become touch-sensitive, if attack velocity is transmitted to the synth via MIDI from a dynamic keyboard. This is a voice-programmable feature, with the sensitivity of each operator independently variable from each of the others. Only problem is, all the preset sounds have been set to zero response on all four operators, which means that if you're in a position to take advantage of this feature, you have to edit all the sounds each time you call them up from the ROM, or else store them as RAM voices in edited form. As neither option is particularly desirable or practical, you're inevitably going to end up with a whole bunch of voices that aren't touch-sensitive. A pity, that.

The DX100 is still well suited to being controlled from another MIDI keyboard, though, especially as its miniature keys prevent you from paying over the odds for mechanical parts you don't really need, and it's capable of receiving an eight-octave pitch range over MIDI.

Voice data (single or bulk) is compatible across DX21, DX100 and DX27, so Yamaha's DX21 Editor package (reviewed E&MM December '85) should work with both the 100 and the 27. I was able to transfer voice data successfully between a DX21 and DX100 in both directions — a bonus for any DX21 owners who might be considering adding a 100 to their soundmaking gear.

The manual gives every detail of the 100's MIDI implementation you could possibly need. Things are looking up at last.

You'll not be surprised to learn that the DX100 also has cassette storage, and again, compatibility has been maintained with the DX21, so you can swap sounds from one to the other via the tape method, too.

A small red LED (the only one on the front panel) flashes at you unnervingly whenever battery power runs low. But with all four sets of batteries that I'd used before it was time to send the 100 on its way (expensive business, this reviewing), the light insisted on flashing regardless of whether the batteries were ageing seriously.

There was also a moderate beating noise evident on both the audio and headphone outputs, which mysteriously disappeared on the audio out when the 100's volume slider was at full. This is apparently quite usual when the batteries run low, but all the time? Teething troubles, most likely, but check your 100 out on batteries before you reach for your wallet.

"Sounds - Most people agree that FM string sounds still aren't that satisfying, and the DX100 doesn't take things any further."

Incidentally, enclosed along with the owner's manual are a clever Voice Programming Guide and a Playbook. These effectively supplement the descriptions given in the manual, and provide details of several extra voices, along with invaluable commentaries on the relationship between DX parameters and the voices' sonic components. This is incredibly useful information for all newcomers to FM synthesis, whether their background is home keyboards or analogue synths.

The DX100 scores on several fronts, not least of which is its price: 192 FM voices at under £350 can't be bad, especially when those voices present a broad selection of sounds for just about every conceivable occasion.

Yamaha's designers have put some careful thought into what should and should not go on an instrument of this price; the Bank Memory system is an effective (if initially confusing) way of organising a mass of preset and programmed voices, and presenting them in an accessible way on what is really a very small musical instrument indeed.

Whilst a mini-keyboard is nobody's ideal playing tool, you can play it without having to adjust too much, partly because its shallow travel is just about right for its size.

The DX100 should attract plenty of players into the FM fold for the first time, from both the 'serious' and domestic ends of the small keyboard market. With its velocity reception and eight-octave pitch range over MIDI, it makes a fine voice expander for mounting atop either an analogue polysynth, another of Yamaha's FM digital synths, or, for that matter, one of the many MIDI-equipped home keyboards now available. And whereas a keyboardless expander prevents you from playing two sets of ivories at once, you can reach up from your master keyboard to play a quick solo on the DX100 whenever the mood takes you.

So, a synth which should appeal to a variety of people for a variety of reasons, an important step in bringing complex and versatile music synthesis to the masses, and an instrument that's unlikely to be outdated too quickly. Play it and believe it.