Although you can load your own samples into Montage, samples cannot directly interact with the FM-X engine. By "interact", I mean to either modulate or be modulated within the FM-X engine. You can mix AWM2 with FM-X (in terms of FM - as if AWM2 was a carrier only).

The envelope follower does allow for indirect FM and AWM2 interaction. Not in the sense that SY99 allowed for more direct interaction. The envelope follower is a blunt tool comparatively speaking.

There are other differences - I believe someone mentioned EGs have some nuance.

There is 25 years difference between AFM and FM-X. Both operating systems (SY99, Montage) combine a sample based engine with an FM synthesis engine, the major differences is in how they are combined and in how they can interact.

The SY99 engine featured RCM (Realtime Convolution and Modulation) a system where the output of any Operator could be fed into any other Operator, also you could feed a Sample into the FM Operator stack. What we learned from this was the simpler the input Wave the more usable result. The more complex the source Wave the quicker the result created noise (chaos).

For example, an acoustic piano Waveform fed into an FM Operator causes so many sidebands that you're into area of noise almost immediately. In the early Manuals on RCM it mentions that much of the potential in RCM was yet unexplored... but looking back, the conclusion about the usefulness of actual samples as Source was very narrow. Narrow in that samples, already rich in harmonics, tended to generate even more dense harmonic.... so filtering the source was almost mandatory. So some very strange noises could be created without much difficulty. Artful noise, to be sure, but chaos and edgy was the rule.

That said, the result of being able to route the output of any AFM Operator to the input of any other Operator, did result in some very unique and musical results. The "nested" feedback loop, for example, proved to open its own door of infinite possibilities when set to respond to velocity, etc. - the results were extremely musical. That was the unique thing about that RCM engine that was really interesting (imho).

The AWM2 engine back 25 years ago was just 16 Note of polyphony and the AFM engine was 16 Notes of polyphony and was based on the six Operator engine of the original DX.

The AWM2 engine in Montage is 128 Notes stereo polyphony based on an eight Element system and the FM-X engine is 128 Notes based on an eight Operator engine with Spectral Forms to generate a very wide range of complex Waveforms with a single Operator.

There is a fantastic history of Yamaha FM in the 03/2016 "YAMAHA Music Production Guide" (page3: The History of Yamaha FM Synthesizers) and gets into details of the significant differences. And trust me, the differences are in the details (not just the number of Operators) there are dramatic differences in the architecture of the various generations.

One listen to the FM-X you can hear that there is a quality not previously available. The faster envelopes make for far smoother sonic changes in timbre - a far more responsive FM that can be interactively involved with the AWM2 engine. If you simply fed AWM2 into FM (as did RCM) the result quickly went to strange and fascinating noises, but the FM-X has a way of applying continuous changes and this dimension makes a huge difference in the results. The ability to stretch and morph the FM-X is one of the most exciting things... movement is the key.

Enter the AWM2 engine as a modulation/modifier... its amplitude can be used to shape the results of the FM-X via the Envelope Follower... rhythmic movement always works musically, meaning a better possibility something useable will result; versus the artfully strange noises of RCM.

Explore the AWM2+FM-X Performances. The great liquidity of FM-X sound is immediately apparent. It can be shaped in realtime in ways previously unavailable. Motion Sequences can be used to define the movement and shape the results over time... it's like having a dozen hands to manipulate controls, changing the timbre, volume, pitch... while playing DX7 era FM was mainly playing the keys, the first thing you notice with FM-X is there is far more movement in FM-X sound. The Montage engine maximizes the combining of the technologies - and its the multi-dimensional changes that are the Montage signature.

The Envelope Follower can be viewed as a blunt tool, if that is your preference: you can have a Kick Drum create the Envelope for your Synth Bass sound to Follow... but you also can use it to add sparkle by assigning a quick staccato ARP patterns to "play" (vary) the harmonics of an FM-X sound.

A/D input can also be fed to the FM-X engine... you can derive tempo from an external audio source and can feed it to control parameters within the synthesizer. This means for example, any audio coming into Montage can be assigned to control a task in the engine... open a filter, pan a sound, increase a parameter, decrease a parameter, etc.

Side Chain Modulation let's one Part interact with others... ducking, vocoding, you design what goes where.

Combined with the Motion Sequencer this combination of AWM2 samples and FM-X (fourth generation FM) synthesis is far different from back in the day! Thanks for the question...

Find that article, you'll find it on point.

Great answer BM 🙂