They also didn't get why FM synthesis was so appealing, in spite of being difficult to understand for the common musician.

It's in the tone.

Acoustic instruments respond in a complex way to the variation in strength of input: when you strike the key in the piano faster, pluck a string harder, or blow air info the saxophone stronger, you don't merely get a louder sound: the harmonic content, the timbre of the sound changes as well.

Analogue synthesis struggled accomplishing this. The classic analog synth would have an envelope generator ("ADSR") controlling the loudness of the tone, and another, most commonly, controlling the filter (the thing that makes the synth do a wowowow sound on the same note), but responsive fading and evolution of the harmonics wasn't readily available.

On the Yamaha DX7, it was built into the core idea of FM synthesis.

You don't know it when you hear it, you know it when you play it: the way the keyboard responded to the touch was alive, magical.

You didn't need to rely on the modulation wheels and joysticks and knobs to vary the timbre as you play. You could simply play the keyboard.

On my Yamaha Reface DX (which overcomes the drawbacks of FM user interface), I can easily make a tone whose character (not loudness! - or not just loudness) changes when I simply play harder. It's like having several instruments at once at your disposal, blending between them on the fly.

It's that playability that makes FM make sense — and it was what other digital synthesis technologies went for, too. Roland's "linear arithmetic", vector synthesis, and M1's multisampling all explored that area — but they came after DX7.

What makes FM synthesis unique is the heavily non-linear response of the tone to the dynamics. At worst, it's unpredictable, but once you figure out where the sweet spots are in the parameter space, you get a tone like nothing else. A bell that's also a string orchestra. A guitar with a soul of the saxophone, but not mistaken for either; an identity all of its own.

Yamaha DX7 heavily leaned into this aspect in instrument's design, via providing additional parameters that controlled the sensitivity of operators to velocity depending on where on the keyboard you are, so that the lower tones would have a different character from higher ones.

The "diminished brilliance" the author writes about was likely that — i.e., the author not figuring out how FM sound design works, which they openly admitted. It was matter of taste of whoever made the presets; without programming those curves in, the higher notes can easily sound screeching.

The point, again, was that the instrument wasn't merely responsive in a way that analogue synths couldn't dream of, but that the way in which it was responsive, tone-wise, was programmable, and varied not just from patch to patch, but across the scale and velocity range.

Again, think about how plucking different strings on a guitar harder produces a different variation in tonal response. Each string has its own character.

This is the soul of the mathematical idea of FM synthesis: that the tone evolution should not merely be controlled by time passing (as it is on most analogue synths, via envelope generators and LFO's), and not by knob twiddling (modulation wheels, knobs, sliders, joysticks,...) — but by playing the instrument itself.

And on a keyboard, what you really play with is where on the keyboard you strike a key, and how fast.

Yamaha DX7 allowed the player to vary the timbre by playing the instrument, with both hands, by having all tone generators depend on these two variables in a programmable, non-linear, interesting way.

FM synthesis of Yamaha DX7 therefore can't be separated from the physical keyboard it shipped with. The way the tones felt as you played them were determined by the response curves which simply don't map in the same way to a different keyboard.

The fact that the DX7 was a digital synth obscured the fact that it was a very analog instrument in that way; that to get a truly good FM preset, you need to tune it to the keyboard response (i.e. velocity curves), and that involves the analog components.

It's also for this reason that DX7 only has membrane buttons, and no knobs or sliders. It didn't need them. The 60 keys were your knobs and sliders, the means to control the tone.

That's why the ePiano on the DX-7 was on 60% of the new releases. It didn't merely emulate the Rhodes (which, by all means, wasn't a rare instrument).

What it did was it gave keyboard players a way to play with the tone of their instrument while playing the instrument, something the Rhodes would have a more limited range for, as the variation in tone response was constrained by how similar the actual metallic forks that made the sound were to each other, and how similar the hammers are across the octaves — and the digital DX7 didn't have that limitation.

It also gave the people used to playing the synth with one hand (to be able to tweak the sound with the other) the freedom to play truly polyphonically, and use the keyboard itself to control the tone dynamics.

Playing it was a liberating experience, and it still is, because while intricate multi-sampling can also give you that effect (at no less difficulty, mind you, even if you have the samples!), FM does it differently.

The musicians didn't need to be mindful of all that; the absolute majority (Brian Eno expected) were outright oblivious to why and what made DX7 the instrument that you had to have.

You just felt it.

And yes, new FM synthesizers keep coming. Because emulating acoustic instruments is not just easy with sampling these days, it also isn't enough. You can just hire someone to play the real instrument, after all.

You need a bit more than that to craft a distinctive sound — especially a new one.

Liven XFM, Korg Opsix, Arturia Minifreak all go boldly where manmade sound didn't go before, and these are just three novel FM synthesizers from this decade.

Reface DX came out less than 10 years ago; and its FM engine is different from DX7 (as is the UX — you can finally change the tone while playing it with live controls).

And for all the talk of how FM is old, I've yet to see someone not be captivated by the ePiano patch that comes stock on the Reface DX when I let them play it when I bring the instrument around with me on trips (which I often do).

Current developments in the controllers (like what ROLI is doing) will allow all the existing sound generation techniques to shine in new ways, including FM.

But I think it's the physical package of the keyboard, the algorithm, and the presets tuned to the combination of the two is what made the DX7 such a success.

A new FM instrument could easily be a hit with these factors, particularly if they don't skimp on including built-in speakers and making the presets sound great on them. FM truly shines when all the pieces are aligned in a performer's instrument.

Reface DX comes close to that point, but the presets it ships with are more of an engine demo than sounds to make music with, the speakers are not loud, and the mini-keys (which I love!) were a turn-off for many people — because in the Internet age, people would judge a machine without actually playing it, and that's the only way to understand what's so damn special about FM synthesis.

It's the way you feel when you play it.

A sound demo simply won't get that across.