Additive synthesis bears some resemblance to FM, but it is distinct and different: where FM works by modulating one wave with another, additive works by stacking up many simultaneous waveforms of different frequencies and mixing together the results.
This may sound like a subtle distinction, but it really isn’t. Where an FM synth can do its thing with six, or even just four, operators, additive synths require many more oscillators, which are ganged together in groups to create each ‘voice’ or note. So while an FM synth can create a reasonable approximation of a square wave with just two or three operators, an additive synth would need to dedicate perhaps 10 or more oscillators to the same task. Multiply this by the number of simultaneous voices and you see that additive synthesis requires a very large number of oscillators.
As a result, the first additive synths to make it to market were horrifically expensive, and before anyone had managed to work out a more cost-effective way of making one, the DX7 came along.
Still, in 1987, Kawai released an affordable additive synth, the K5, based on technology it had acquired from Sequential Circuits. The K5 was still a pricey beast compared to Yamaha’s FM range, but by comparison, it made programming an FM synth look easy! Sound-wise, the K5 was interesting, even exciting, but it struggled for attention and did not sell in great numbers. It was the same story for Kawai’s updated K5000 released in 1996, which was a big step up from the K5, but still didn’t gain much traction.
Nowadays, there are a few plug-in synths that use additive synthesis and these are fascinating and sound good, but the underlying synthesis technique remains niche.
Strictly speaking, FM synthesis should be called ‘phase modulation’, as this is the proper name for the underlying physics that are being used. Casio Electronics, more commonly associated with home-and consumer-grade instruments, decided to develop its own version of the technology for use in its products. This they dubbed ‘phase distortion’, and in many respects it is the same as FM. However, the differences in the implementation have a noticeable impact on the resulting sound.
Casio followed up with what it called ‘interactive phase distortion’, or iPD, as found on its VZ-1 from 1988, but this was quite a far cry from the original PD and was somewhat more convoluted and arcane. Nowadays, if you want a PD synth, your only option is to hunt out a used CV or VZ synth.