Just when the DX7 looked under threat from competing synths, Yamaha come up with a budget FM machine to consolidate their position. Value for money was never this good.

There can be no doubt, not even the faintest shadow of one, that the Yamaha DX7 has done enough to gain a permanent place in the history books of music technology. It's got there by virtue of offering - through the newly-harnessed principle of FM synthesis - an unparalleled range of uncannily realistic musical sounds, at a time when machines of comparable price have provided nothing of the sort. It isn't the easiest synth to program, with the result that a lot of owners have found themselves eagerly awaiting each new voicing ROM as it's been released, but in absolute terms, its sonic capability is still unequalled at its £1300 price level.

However, a good couple of years have passed since the DX7's introduction, and a lot of synths have passed under the bridge in that time. In particular, a number of companies have entered the fray with significant instruments in the sub-£1000 price bracket, and Yamaha's own budget FM poly, the DX9, has been suffering too heavily in the shadow of its more expensive brother to make much of an impact. That, surprise surprise, is where the £699 DX21 comes in.

For those of you already au fait with the DX7, DX9 or CX5 computer (which is probably a fair number, all in all), the DX21 will be familiar ground in terms of both appearance and programming. At half the price of a DX7 (that's even cheaper than the now obsolete DX9), there are obviously going to be a number of economies, but the end result is still impressive. It certainly outshines the DX9 and the sonic capability of the CX5, whilst even DX7 owners will find a number of features their instrument can't boast.

The DX21 is an eight-voice polysynth (the DX7 and DX9 both had 16) with a five-octave C-to-C keyboard. First reports a few months back suggested the DX21 would contain two DX9 FM sound chips, but a quick peek inside the new machine reveals just the one chip - of entirely new design - whose functions are multiplexed to provide a dual-output configuration, which is neat enough. Touch-sensitive the 21 is not, at least not from the keyboard, but attack velocity data can be read over MIDI from a touch-sensitive keyboard. What the DX21 won't respond to over MIDI is aftertouch data, which is a shame.

Like the DX7, the DX21 has a 32-voice RAM that's directly accessible from the front panel via two rows of 16 selectors. But the new synth also has 128 voices stored internally on ROM, and you can call these up singly or in groups of eight into the RAM. More on this later.

Thankfully, the early DX membrane switches have been replaced by slim buttons that have a pleasingly firm response - you actually feel you've pressed something. The central display is the same 2 x 16-character LCD window present on the DX7/9, which means it's just as difficult to read in subdued lighting, but what's worse is that the DX21 hasn't been given the life-saving two-digit red LED display that accompanied the earlier DXs. This wouldn't be so bad if Yamaha had gone for a backlit display, but they haven't, and peering into the thing on a dimly-lit stage isn't going to be much fun as a result.

The familiar data entry slider and +/— buttons are also present, but a nice touch - new to the DX21 - is that once you've selected a parameter for editing, you can press that parameter's button to alter its value, rather than having to return to the data entry controls every time. If a parameter is a dual-state one, you can toggle between the two states in precisely this fashion. If it's multistate, repeated pressing of the associated button increments the value - a nice touch that eases editing by no small degree. And another welcome change is that each stage (rate and level) of the amplitude and pitch envelopes has been given its own front panel selector.

But where the new synth really scores, even over the DX7, is in the inclusion of Split and Dual voice-assignment modes. These were once only to be found on the outrageously expensive DX1, but more recently they've appeared at the lower end of the market on rival machines (SIEL DK80, Casio CZ5000), which makes their inclusion on the DX21 rather opportune.

Split-point is user-definable as any note on the keyboard, and four voices are allocated to each side of the split. Similarly, Dual mode reduces the DX21 to a four-voice instrument. You can determine the volume balance between the two sounds by means of a slider between the familiar volume and data entry sliders on the left of the front panel. In both modes, one voice must always be chosen from the upper row of 16 selectors and the other from the lower row, so you're going to have to apply a bit of forethought when organising your patches.

Current memory buffers are maintained for both Split and Dual sounds, and these are accessed by pressing dedicated buttons. Once called up, the contents can be readily altered by selecting voices from the front panel in the normal manner.

Turning to the DX21's synthetic capabilities, we find to our enormous surprise that it reverts to the DX9's configuration of four operators and eight algorithms. The 21's algorithms are the same as the 9's, too.

"Design: The early DX membrane switches are replaced by firm buttons — you actually feel you've pressed something."

Another plus point for the newcomer is that a number of Function parameters are now voice-specific, a feature first seen on the TX7 and TX8I6 modules. These are the 16 functions to be found along the lower row of patch/programming buttons, referred to collectively as Performance functions. Appropriately enough, most of these are concerned with the status of pedals and wheels. You can program sustain pedal on/off, pitchbend range, volume pedal amount, portamento, mod wheel range and breath control - all specific to each voice patch.

Yamaha's lovely breath control option has been greatly enhanced by the addition of a pitchbending facility, which allows the pitch of notes to be bent continuously by the controller up or down over an extremely wide pitch range.

And pitchbending in general has been enhanced by the inclusion of three extra modes on the DX21: one in which the lowest note only can be bent, another for the highest note only, or (particularly effective, this) a third in which notes played and then held by the sustain footswitch aren't affected, whilst notes played subsequently are. This isn't a voice-specific feature, though.

As I've said, the DX21 has 128 voices stored internally in ROM, or the equivalent of the DX7's two plug-in ROM cartridges, and follows the DX7 in having 32 voice positions in RAM for you to put your own sounds in.

The ROM voices are organised as 16 groups of eight voices each, and each group contains a particular family of sounds (see later for a full rundown of these). The 32-voice RAM is logically divided into four eight-voice banks, any group may be loaded into any bank, and single voices can be called up into any of the 32 positions.

As soon as you select a RAM voice for editing, it's read into an edit buffer, and as on the earlier DXs, there's a function for comparing your edited voice with the unedited version. Also carried over from the first-generation machines is the Edit Recall function, which allows you to return to an edited voice if you call up another patch in mid-flight by mistake (this is possible because the 21 keeps a backup edit buffer once changes have been made to a voice).

A new feature is the Performance memory. Not to be confused with Performance functions, it's a group of 32 patch memories that let you call up predetermined single, split and dual voicings at the touch of a button. They could come in particularly handy in a live situation, hence their name. Storable parameters are voicing mode, voice(s) selected, keysplit and voice detune data (the latter applies to Dual mode voicings), key shift data (a transposition value) and pitchbend mode.

To get the most out of this mode, it's important to realise that each performance memory contains no voice data as such, but pointers to voice positions in the single-voice RAM. Thus if you call up a new voice into a position accessed by a performance memory, that performance memory will effectively be changed as well. This obviously requires some careful planning, but the performance memory is still a useful addition that allows you to set up all the voices and voice combinations you need prior to a gig or a recording session. And you can easily align voice combinations with suitable voices on other MIDI instruments for program change selection over MIDI, or have a single voice in more than one position without having to duplicate voice data - and lose a voice position.

Sadly, the strengths of the DX21's internal storage system are matched by the weaknesses of its external one. For whereas on the DX7 you could instantly store new voices on a plugin RAM cartridge, the DX21 has no ROM/RAM cartridge facility at all. Like the DX9, it uses slow, unreliable cassette storage instead. All 32 RAM voices can be saved to cassette as a bulk dump (this takes 40 seconds), and either bulk or single-voice loaded.

But I can't help feeling cassette storage is really a false economy - it's simply too fraught with problems to become useful in professional applications. If SIEL and Casio can build cartridge slots into similarly-priced (or cheaper) synths, why can't Yamaha? Well, maybe they want people to execute patch dumps over MIDI using the DX21's System Exclusive implementation. It's certainly quick (a 32-voice dump takes just a couple of seconds), but immediately introduces further hardware into the picture in the shape of computer, disk drive and MIDI interface. It also means that the whole process becomes computer/disk-specific, so swapping and selling sounds becomes a less universal process. Mind you, using a computer has its attractions (availability of the micro for non-musical purposes, price advantage of disks over cartridges) and doubtless software for both Yamaha's CX5 and other popular micros will be available soon. You could even try writing your own...

To the sounds in detail. There's no doubt about it, a large number of the DX21's ROM-based sounds would give the DX7's factory presets a pretty good run for their money. Clearly, Yamaha's programmers (in this case Gary Leuenberger, who helped program the original DX7 sounds) are putting their two years' FM experience to good use. Because although its greater number of operators/algorithms means the DX7's ultimate potential must be greater, the DX21's factory sounds give you little clue as to the machine's electronic inferiority.

"Facilities: The DX21 has no plugin cartridge facility at all. Instead, it uses slow, unreliable cassette storage."

The ROM families are Piano, Electric Piano, Organ, Strings, Brass, Plucked, Comping, Percussion 1 and 2, Lead Synth, Other Keyboard, Wind Reed, Bass, and Sound Effect 1, 2 and 3. It's not really possible to single out any particular group for praise (or for the thumbs down treatment), but the percussion (both tuned and untuned) is as strong as it is on all DX synths, and there's a good selection of them, with tubular bell, gong, marimba, and glockenspiel sounds, among others. There's even the currently obligatory attempt at a Simmons tom sound, though this is only moderately successful. I was impressed, however, by the wonderfully delicate splash cymbal sound.

There are some very praiseworthy acoustic and electric piano sounds, though I still couldn't find an electric piano to match the DX7's best effort. And when it comes to harpsichord impressions, the DX7's increased number of operators gives it a noticeable edge in the complexity department; you can hear a lot more going on in the more expensive version. The 7's church organ sounds have an extra bit of 'oomph' over the 21's, too.

On the wind instrument front, flute and clarinet sounds come out very well, while the lower-register bassoon has a marvellously gruff tone to it.

Meanwhile, plucked sounds (especially guitar) show a great improvement on the DX7's initial efforts; they're crisper, cleaner and reward sympathetic playing technique with an excellent reproduction of the real thing.

String sounds are still an FM problem, though. Apart from lacking strength by comparison with good analogue synth versions, the 21's strings also omit the detail of the DX7's (still thin-ish) endeavours. They're OK so long as you play them staccato, but if you ask them to sustain, their lack of movement makes them appear all too synthetic.

The bass sounds still have that familiar FM noise component, which is a shame, but there's an excellent selection of them (mellow, punchy, or both at once) from which to start editing. They'd benefit further from being played from a velocity-sensitive keyboard, however.

The sound effects include racing car, helicopter, a rather nifty Doppler FX sound, and FM-derived square, pulse and sawtooth waves. Not particularly musical, but a lot of fun.

Turning to the actual voice parameters that make up the DX21's sounds, all the familiar DX elements are present, though some have been simplified or otherwise scaled down. As a general rule, the facilities fall somewhere between DX9 and DX7 in their completeness.

Gone are the separate frequency coarse and frequency fine parameters that graced both the 7 and the 9, these being replaced by a single 'frequency ratio' parameter that can be set to any of 64 different ratios; fractional ratios are thus preset instead of user-determined. The manual claims 'these frequency ratios have been carefully chosen as the most useful for voice programming', but although I'd concede there is a certain virtue in restricting people's programming options this way, I'd much rather be given the choice.

The number of envelope and pitch envelope stages has been reduced by one in each case compared with the DX7 (though remember that the DX9 didn't even have pitch envelopes), while keyboard level scaling follows the DX9 pattern.

"Sounds: Plucked sounds show a great improvement on the DX7; cleaner, crisper and an excellent reproduction of the real thing."

The DX21 has two audio outputs, Mix/A and B, with the former being capable of carrying a mono signal. There's also a stereo headphone output, so all in all, the 21 offers a worthwhile improvement over the single mono out of the DX7 and 9. The additions also mean you can make full use of the stereo chorus facility Yamaha have introduced on the new instrument.

Continuing our lurk round the back of the DX21, we find jack sockets for sustain, portamento and volume pedals - and discover that the DX7's useful modulation pedal facility is sadly absent. The good news is that Yamaha have standardised the FC7 footpedal and FC4/5 footswitches across their keyboard range, so they'll work with the DX7/9 as well as the 21. However, if you've got the old SC3A footpedal, which was around in the earlier days of the DX7, you're going to be out of luck in the compatibility stakes.

Other sockets are an eight-pin DIN cassette socket, a mini jack for the BC1 breath controller, and MIDI In, Out and Thru. Yamaha have given their new synth the ability to transmit and receive on any of the 16 available MIDI channels (which is a very useful one-up on the DX7), with an additional Omni receive option. What's more, transmit and receive channels can be defined independently.

The 'Sys Info' MIDI function (familiar from the DX7/9) acts as a safeguard against accidental transmission or reception of System Exclusive data. When enabled, single voice or full 32-voice data can be transmitted from or received into the instrument's voice RAM. Single-voice data is received into the edit buffer (whence it can be stored in any voice position), whilst 32-voice data is sent via the front panel Bulk Transmit function. In Edit mode, and with Sys Info available, any edit changes are transmitted over MIDI in real time, while any received changes act on the voice currently in the edit buffer.

The DX21 also has a 'Channel Info' function, which acts as a master on/off switch for transmission and/or reception of certain channel-based MIDI data. Thus attack velocity, mod wheel, breath controller, portamento footswitch, program change and the Data Entry slider and buttons can all be disabled by this function. Pitchbend, sustain pedal and 'all keys off' data are enabled regardless, though. Personally, I'd have found an individual on/off facility for patch change, pitchbend and mod wheel data a lot more useful, but that's life.

The above filtering options apply to selected MIDI data. However, Yamaha have also provided a MIDI on/off function which disables all MIDI transmission and reception. This is a lot handier than it sounds, as it enables you to switch out any slave instruments if you suddenly want to play the DX21 solo, for instance. It's so handy, in fact, that at first I couldn't understand why so few manufacturers haven't fitted something along similar lines. Then it occurred to me that if you can already buy a MIDI filtering unit to do the same job for several instruments, there isn't much point having duplicate switching facilities within the keyboards as well.

What really is a pity is that the DX21 has no provision for assigning separate MIDI channels to each side of a split. Not an awkward feature to implement, I'd have thought, and SIEL's DK80 has this ability, after all.

The review DX21 was part of an early shipment, and whilst the synth itself was in a finalised state, the accompanying documentation was not. So there was I prepared to lambast Yamaha for not including System Exclusive data in an otherwise excellent manual, only to find out that a subsequent version will have all the data included.

I've been putting it off for a while, but now's the time to reach a solid conclusion about the DX21. It isn't difficult: there's no doubt Yamaha have themselves another winner here. Performing the balancing act between cost and quality is never an easy task, but the company have obviously put a lot of thought into what a budget FM synth should and should not have, and almost without exception, their decisions have been the right ones.

The two main disappointments are the non-touch-sensitive keyboard and the lack of cartridge storage. The first is easily remedied by playing the DX21 from a dynamic controller keyboard (which is simple enough), but solving the latter problem involves rather more in the way of additional hardware, which is a shame.

But credit where it's due. The addition of Split and Dual modes, the Performance memory, the more accessible editing facilities, and the simplified voicing structure should encourage a lot more people to get inside FM and start programming their own sounds. They're all worthwhile features, make no mistake.

Yet most important of all are the noises the DX21 makes. If you like what you've heard of FM in the past, now's your chance to grab hold of it at a reasonable price. And getting back to programming, the fact that there are so many ROM voices means you're bound to find a reasonable starting-point for editing, no matter what sound you're after.

It's funny, but some hi-tech music companies have an unpredictable output of new gear that's patchy in quality. On some occasions they come up with a real gem, other times they make the odd technical, operational or marketing blunder. But ever since they released the first DX, Yamaha haven't put a foot wrong.