By now every magazine, periodical and flysheet with the possible exception of the Jamaican Daily Gleaner, has reviewed the Yamaha DX7 and pronounced it marginally better than sex. So why should One Two devote three pages to it this month?

Firstly, it IS one of the most important keyboard advances so far this decade. Secondly, everybody else might have told you how it works but few have managed to describe how it sounds, and why.

And forget all your algorithm/digital/FM techno-hype... the REAL secret behind the DX series is that it produces noises like no other synthesiser in its price range — startling realistic acoustic voices, genuinely bizarre electronic tones and special effects which appear to have four things happening at once.

There are certain instruments where one review, executed over a weekend, is not enough. You need to spend time coming back to this (baffling) piece of gear until it makes some sense, until you've learned about its intricacies and capabilities. Are we talking 'relationship' here, or what?

That's why every issue of One Two will carry one special review of an instrument or device which has been sitting in the hack's hands for a month or more. This time round it's the DX7, and because ears take the lion's share in this exercise we've strapped a special flexidisc on the front cover, compiled by Yamaha, that demonstrates the DX's potential.

As we work our way through the DX7's talents, we'll refer to tracks on the flexi to explain what the machine is doing and how it makes the noises being scraped out by your family Dansette.

Third track in, pretending to be a romantic Fender Rhodes, is the DX7's Electric Piano 1. Requesting algorithm, we find it's set to number five — a popular choice, as it happens. Operator one is acting on two; four on three and six on five. So one, three and five are "carriers" and the rest are "modulators".

It turns out that the 4/3, and 6/5 combinations create the majority of the full bodied, background piano sound — it's 2/1 which adds the imitative tine-bar tinkle.

The next step is to study the frequencies of those first two operators. Operator one is fixed at 1.00 indicating that it's producing the fundamental note. If it was reading 2.00 that would be an octave above (the 2nd harmonic) and 0.50 represents a sub octave. The DX will display this information as a frequency ratio — 2.00, 3.00 up to 31.00 — or as a true frequency expressed in Hz.

Everything straightforward so far, so it must be the modulating operator that's introducing the effect. Sure enough, operator two has a frequency ratio of 14.00 — a high pitched note which adds its own sharp presence in the upper registers while emphasising those same characteristics in operator one. That's where your tinkle comes from.

If you try to lift operator one itself up to 14.00, then the effect is too drastic.

Thus we learn the first lesson... subtlety. I SAID SUBTLETY, DAMMIT. Never do anything directly such as tuning one bank of oscillators to add a fifth, the way you would on an analogue polysynth. Instead, use one section of the DX to influence another.

Further into the flexi we stumble across the DX delivering a cathedral sized blast of pipe organ, applying all six operators in algorithm 19. On this occasion, operators three, two and one are stacked in a line, four and five are side by side with six modulating both of them!

Producing the flutey, middle range part of a church organ is no great problem. Realism depends on the mouthy top end from the shortest pipes and a lower register that not only shakes the floor but growls into the bargain.

Again the secret is in the frequency of each of the operators. Number five has a ratio of two — an octave up from the fundamental — but the fourth operator has a ratio of four, filling in even more of the higher tones. Used in this manner those frequency settings are not unlike pulling out the drawbars on a console organ.

But the quality of those tones — that moutheyness mentioned a few lines up — comes from the interference of operator six working on both of the carriers. Its frequency ratio is set at 10.00 and that's just enough to sharpen the higher harmonics: lower than 10.00 and the modulation becomes clanky and gong like, any higher and you're approaching the tinkle of the Electric Piano.

We still need a growling bass end. Operators one and two are each 0.50 on the scale. Isolate either of them and you're left with a low note, sure, but it's smooth and subdued sine wave.

Link the two together and they reinforce each other, lifting the volume and the presence. But you still need a growling edge, the rasp of air flashing across the openings of the organ pipes. That's why operator three has a ratio of 1.00 just enough to "aggravate" the other two operators while still contributing its own fundamental note to the overall voice. Any higher and you've got too many harmonics working their way into the sound, and the result is a mess.

Here endeth the story of the interfering operators. Well almost. If you jump forward another six tracks then the battered needle of your Garrard will be skipping its way across Orchestra and Chimes — a clever mixture of strings and tubular bells.

The interesting half for us is the bell section which is an example of fine tuned modulation where the carriers and modulators aren't connected at a simple ratio like 2.00, 3.00 or 10.00, but at a weirder interval — 3.18 in fact. Going to a vulgar fraction such as this ensures the harmonics are thoroughly scrambled. The results are detuned and not too dissimilar from a cymbal shop dropping down a lift shaft.

If you fiddle idly with a DX, these are the sort of sounds you'll invariably generate. It has an infinity of settings... approximately 99 per cent of them are useless.

Well, that's somewhat uncharitable, but the difference between, say, a frequency ratio of 21.67 and 9.21 is not that dramatic — they'll both be clanky, oddball ring modulator type noises.

Lesson number two. It will take you longer to envisage, locate and perfect a new sound on a DX than on an analogue synth. It's not a machine where one twist of a knob offers a logical increase. Push the data slider on the front panel — especially when it's selecting new algorithms — and the timbre leaps in each and every direction.

Enough of noises, what about shapes. Once the operators have formed the desired sound, it's down to the DX's envelope generators to define the attack time, sustain levels and so on. EG rate 1 is the time taken for the note to rise in volume from silence at the instant you first press the key. EG level 1 is the volume it climbs to. A similar partnership exists for the initial decay, sustain and release.

The perky Japanese ditty two thirds of the way through the demonstration is a Koto, and a squint at the readout confirms it's algorithm 2 — operator two on top of one, then six, five, four and three in a steeple.

That tower is responsible for the major substance of the sound; operators two and one pre-fix it with a percussive click. So not surprising that the attack time is as fast as it can be (perversely, Yamaha make this a value of 99 and the longest possible rate is 0). Then when we check the sustain and release levels discovering both are at zero, which means... er... zero, confusing, eh?

BUT, those envelope generators can be fixed at different levels for every operator. What if operator four had an attack time if 30, operator five was 20 and operator six was 10? They'd all fade in one after the other, but as they were all modulating each other that would mean the harmonics would change each time a new operator swung into earshot.

Used delicately, this ability can mimic the opening or closing filter sounds found on an analogue synth and most often heard in brass programmes. Applied more adventurously, and you can soon have the DX hurtling about in timbre, up, down and sideways with more deviations than a Gary Numan flight path.

Could this be lesson three? Yup. Just because it says Envelope Generator, don't dismiss it as a mere ADSR unit. The EG system on the DX plays a greater part in tone shaping and harmonic construction than on an analogue synth. It's in there at the foundation of each new voice, and is usually responsible for the dynamic impact that typifies digital machines.

Volume isn't all the EGs effect. They can also be linked to the pitch of the operators. Near the end of the flexi there's a noise appositely named Take Off. The DX descends, hovers, zips up to form a chord then climbs through the stratosphere as your fingers lift off the keys.

Pitch EG rate 1 fixes the time taken to drop from the heavens, EG rate 3 supplies the speed of the zip up to the Close Encounters theme, and four obviously specifies the final lift off. Where Pitch EG level settings are concerned, a figure of 50 is normal to the keyboard — lower and the pitch falls, higher and it increases.

The trick is that if you listen to Take Off, the spaceship is not only descending it's also approaching you — the volume is increasing as the frequency is dropping. That's where the missing envelope generator comes in. EG rate 1 controls the speed of the pitch descent as the volume is building, then EG rate 2 takes over with the SAME value for that section of the sound when the volume is at its peak. Clever, n'est-cafe?

Conclusions

After four weeks of continual tinkering, the hack thinks thus.

1) The DX is totally different from an analogue synth. If you try to operate it to the same rules your head will dribble and you'll achieve only a fraction of the machine's capability. Abandon old approaches and start again.

2) It is a superb keyboard, but it does NOT spell death for the rest of the synth industry. Analogue polys still produce fabulous sounds that the DX is incapable of recreating.

Examples — warm string parts à la Phil Collins, extreme filter voices and dirty, synced up lead lines.

3) The DX does excel at percussive acoustic sounds, voices with a metallic edge to them and special effects featuring involved variations in pitch and volume.

4) The DX has a capacity for finesse that is rare among synths. It can add breath to flute of brass sounds or a gentle burr at the end of a fretless bass run, for example.

5) The DX does NOT make a dramatic mono synth unless you're imitating wind instruments such as flute or saxophone where the plug in breath controller adds to the realism. The DX can be too clean.

6) The touch sensitivity, second touch effects and breath controller are a boon because they can control almost all elements of the sound but they don't always work well together. On a piano selection you can induce extra bite and dynamics by hitting the keys harder.

But if you get carried away you'll accidentally hit them hard enough to activate the second touch mechanism in the keys and unearth an unwanted vibrato. The DX is still physically a synthesiser even if psychologically it may seem to be a baby grand.

7) The DX system of dumping its memory onto RAM packs is one of the best yet. While the cartridge is plugged into the machine, there are 96 memories on call (32 in the internal store, 64 waiting in the cassette). Swapping cartridges for another 64 takes seconds. This is one element other manufacturers will have to mimic.

8) It contains MIDI in, out and through, boasts 16 note polyphony (hear the difference on long runs for sustained notes), is light, looks stylish and professional, gives plenty of readout indication on what it's up to and keeps the player better informed than any other synth in this price range. BUT idle fiddling is hopeless, you need to know where you're going and the very complexity means spending a long time perfecting sounds.

9) It is excellent value for money, has a unique sound but it's NOT everything... 90% perhaps.