Analog FM is not so unusual, but it's usually "real" FM, e.g. a modulation of pitch or linear frequency, and it's not so easy to tame into consistent musical pitches.

"Yamaha style" FM is however more of a kind of phase modulation. The operators are sine tables driven by phase counters, and the outputs of the modulators are scaled and summed with the phase counter of the carrier before the sum goes into the carrier wavetable. So you modulate the phase irrespective of frequency, e.g.

  modulator(phase) = sin(phase * modulator_multiplier) * modulator_amplitude
  carrier(phase) = sin(phase * carrier_multiplier + modulator) * carrier_amplitude

I suppose this could be replicated in the analog domain. A phase counter could simply be a ramping sawtooth waveform. Summing modulator outputs would be as simple as mixing. The hardest part I think would be to replicate the wavetable. You'd need a waveshaper that basically does f(x) = sin(x). There is an oscillator on the market that I think does everything except the waveshaping in the analog domain: https://wmdevices.com/products/phase-displacement-oscillator...

I do wonder if DCOs (much less susceptible to drift) would be more amenable to analog FM, though they were not common for musical instruments during the DX7's early R&D phase.

That said, it seems like music manufacturers were searching for a musically useful, cost-effective digital synthesizer around this time. Early digital synths like the Synclavier and Fairlight CMI were prodigiously expensive, while analog synths in the DX7's price range were saddled with few/no polyphonic voices and/or limited single oscillator designs. The DX7, with a varied tonal palette and 16 voices, must have seemed luxurious at the time.

You can't do analog FM with DCOs because they're numerically controlled. You'd have to modulate the frequency of the clock, which displaces the problem without really solving it, and also wouldn't work polyphonically.

This Synclavier and Fairlight CMI were hybrid designs with a digitally controlled clock per voice producing a variable rate sample clock driving memory and a DAC. The hardware was more expensive but the digital part was much simpler, because you basically just clocked the sample data straight into the DAC without having to do any interpolation or resampling.

The DX7 was the first successful fixed rate polyphonic DSP system. With some reworking, the FM subsystem could have been replaced with sample RAM. But RAM was still ridiculously expensive, so FM was a workable solution for expressive acoustic-like sounds that weren't sampled.

It absolutely was a game changer. I remember playing one for the first time and it wasn't just the novelty of the FM sounds or the 16 - 16! - voices. It also looked great, was easy to carry, was less than a quarter of the price of some of the bigger polysynths, and had an extremely playable velocity-sensitive keyboard.

When you put all of those together it completely eclipsed the older designs as a playable keyboard instrument.

And, not unimportantly, was built like a tank so if you were a musician playing gigs taking it with you without damaging it was possible without being ridiculously careful with it, as you would have had to do with other synths of the day. DX7's are incredible in this respect, to the point where 4 decades later many of the original series 1's are still playing.

Owner of a DX7 here. The only thing that needs servicing is the CMOS battery. Incredible.

I have a DX7IID that has been in constant use since it was built in the late 80s and aside from a single battery replacement it's still in perfect condition. They are extremely durable.

I recently watched Trent Reznor's clip from the 'I dream of Wires' documentary, where he blames the DX7 for the decline in synthesiser culture. His hatred of the DX7 is pretty well known at this point. He complained that after its release the market for synthesisers shifted to accomodate consumers' want for 'bell', and 'electric piano' sounds, rather than those of his preferred analog synthesisers.

Would you agree that having an affordable synthesiser capable of making credible piano, and percussive sounds really opened the world of synthesisers up to a more diverse range of musicians? The DX7 first hit shelves before I was born, so I can't speak from my own experience here. I think Reznor's opinion on the matter sounds pretty ignorant.

While I don’t know much about synthesizers, I love when someone makes an interesting or new tone with the YM2612. I can understand how synthesizer sounds became standardized post-DX7, but I’m surprised Trent doesn’t appreciate the experimentation and accessibly of playing with operators.

Gatekeeping at its finest. The DX7 democratized the synth, and for that reason alone it deserves some kind of award.

The other factor is the sheer number of oscillators you'd need to get polyphony. The DX7 has 6 oscillators per voice. With 16 voices, that's 96 oscillators. Typical polyphonic analog synthesizers have two. You occasionally see three. Polyphony is often lower. 8 voices is fairly common, but you saw smaller units... Prophet 5 was 5-voice, and Oberheim's OB-X was available in 4, 6, or 8 voice versions.

The DX7 had somewhere between 9 and 12 times as many oscillators, depending on which of those lines you compare it to.

This isn't quite FM, though Jean-Claude Risset from Bell Laboratories did some contemporary experiments using additive synthesis with analog oscillators to achieve similar complex tones. I'm guessing that Yamaha probably experimented with all kinds of interesting prototypes before developing the VLSI FM chips that would go into their earliest FM synthesisers. It was a very large investment for them as a company, so they likely tried all kinds of things first with existing technology they had on hand.

True FM would have been cost prohibitive, phase modulation sounds just the same to all intents and purposes without the associated extra technical complexity (and with two massive custom ASICs this was already quite a complex machine).