Which "FM" synths really are FM?

[wagtunes]

PM and FM are two approaches to getting some sort of angle modulation, which essentially is what we want the synth to do. Without knowledge of the modulation function it would be hard to tell if a resulting signal is PM or FM-based. For the simple case of sinusoids, the equation for the signal is more or less the same, with the only difference being the way the modulation index is defined. For all practical purposes, PM and FM are essentially equivalent. The question should be: which synth does angle modulation? Tons of them. But if what we mean by a "true" FM synth is one that implements operators and operator diagrams, then there aren't that many.

Exactly. And that is what it means to be an FM synth, at least to me.

[Echoes in the Attic]

I have to admit that I still don't understand the difference.

Maybe this link will help? I found it while trying to wrap my head around things. Click the (1) at the bottom to go to "step 1" and then you can adjust the slider to see PM .. then click the "next" button and the slider this time adjusts FM.

Excellent. That actually reinforced how I always understood the difference. Funnily enough I confused myself today by looking into how Phase Distortion was different from Frequency Modulation, accidentally mixing up phase distortion with phase modulation as being the synthesis method used by the DX7 and therefor FM8, Dexed etc.

[Echoes in the Attic]

PM and FM are two approaches to getting some sort of angle modulation, which essentially is what we want the synth to do. Without knowledge of the modulation function it would be hard to tell if a resulting signal is PM or FM-based. For the simple case of sinusoids, the equation for the signal is more or less the same, with the only difference being the way the modulation index is defined. For all practical purposes, PM and FM are essentially equivalent. The question should be: which synth does angle modulation? Tons of them. But if what we mean by a "true" FM synth is one that implements operators and operator diagrams, then there aren't that many.

Exactly. And that is what it means to be an FM synth, at least to me.

Problem is, the OP was actually asking which synths use the FM method as opposed to the PM method. Your example of Dexed as an FM synth technically in comparison to a PM synth isn't actually correct since it's a PM synth. Practically though, in terms of getting a certain sound, the question doesn't really matter because the results are the same.

So as for which synths actually do angle modulation by modulating frequency rather than phase, I'm not sure. I would guess that perhaps the analog modeled synths that have cross-modulation between oscs would be based on true FM, like in ARP2600. Or the minimoog or Jupiter or prophet cross-mod. I've always thought of FM as the audio rate modulation of the pitch, much like if you were to modulate the pitch of an oscillator by an lfo, but really fast. Whereas the digital PM synths are modulating the phase digitally at audio rate. But I could be wrong.

[liquidsound]

Isn't Aalto a true FM synth?

[DJ Warmonger]

Blue 2 has an option to choose either frequency modulation or phase modulation and the difference in sound is significant. But I won't recall exact settings now.

[AdmiralQuality]

Poly-Ana can do both FM and PM. But only PM is musically useful. True audio-rate FM will change the apparent pitch of the oscillator instead of just the timbre as with PM.

[Ingonator]

Phase modulation you mean. Casio cz is phase distortion. Nemesis does true fm.

Indeed Nemesis does rure FM with the "NeoFM" modes but it could also do Phase modulation (PM) with the PM modes. Last but not least it also has a Phase Disturtion (PD) mode.

You could hear the differences of real FM and PM easily with complex waveforms. In Nemesis you just have to load a compex waveform and switch the modes to hear the difference.
With a simple Sine or "Sinoid" waveforms the sound is more similar but it is still possible that you could hear a difference.

[Nielzie]

BigTick Rhino

[cytospur]

DUNE2 can do real FM via the mod matrix as well as DX style via the dedicated sections in oscillators 1 and 2.

[Chris-S]

Hi,
mathematically FM and PM are equivalent.

Wikipedia: Frequency modulation is known as phase modulation when the carrier phase modulation is the time integral of the FM signal

But PM is more easy to implement.

[Rommelaar]

Image-Line synth plugin Sytrus is FM.

[Ingonator]

Hi,
mathematically FM and PM are equivalent.

Wikipedia: Frequency modulation is known as phase modulation when the carrier phase modulation is the time integral of the FM signal

But PM is more easy to implement.

Try the NeoFM and PM modes in Tone2 Nemesis using complex waveforms and you should hear the difference...

[Burillo]

i think discoDSP Phantom qualifies as an FM synth?

[Angus_FX]

I would guess that perhaps the analog modeled synths that have cross-modulation between oscs would be based on true FM, like in ARP2600.

Cypher is that. It's a VCO FM model, not a million miles from the ARP, but capable of going "through zero" - i.e. the oscillator will go in to reverse below 0Hz (most real analogue VCO circuits won't do this, but we think it sounds more musical, so we bent the laws of physics to allow it).

True audio-rate FM will change the apparent pitch of the oscillator instead of just the timbre as with PM.

That depends. Most VCO circuits clip at 0Hz, and that's what causes the perceived pitch change when using linear FM. Thru-zero oscillators will carry on below 0Hz, there are a fair few such VCOs available for modulars, but AFAIK it's never been implemented in an analogue polysynth.

[AdmiralQuality]

True audio-rate FM will change the apparent pitch of the oscillator instead of just the timbre as with PM.

That depends. Most VCO circuits clip at 0Hz, and that's what causes the perceived pitch change when using linear FM. Thru-zero oscillators will carry on below 0Hz, there are a fair few such VCOs available for modulars, but AFAIK it's never been implemented in an analogue polysynth.

You can't have negative frequency. You can have negative phase.

Try it in Poly-Ana. When FM modulated, Poly-Ana's oscillators are designed to give the same musical pitch range positive and negative around the center frequency the oscillator is set at. This is the obvious setting for balanced vibrato and trill effects. The pitch goes down just as many semitones as it goes up. But you can't ever go negative, you just keep finding lower and lower octaves -- right? So when you apply this, even a little bit (where we don't approach 0 or negative frequency at all) the oscillator seems to go out of tune, at least when modulated at an audio rate. Perhaps there's another way of scaling the frequency modulation so this doesn't happen, but then the FM would be less useful for typical LFO applications (vibratos and trills).

Poly-Ana's Phase Modulation however is linear and wraps around in both directions, forward and backward. This is by far the more musically useful mode. See the "PM" built-in presets for examples. All of those changes in timbre are done without any application of the filter(s)! The usual technique is to use a Mod Mixer to multiply a modulator oscillator by an envelope, then apply that Mod Mixer as the PM Source for another oscillator. (Sticking to sine waves on the oscillators yields the most useful results. But of course all the other waveforms are available too.)