FM on the Pro 2

[jdt9517]

I just bought a new Pro 2 and loving it!  I do have a question on how the FM synthesis works.  I apologize if this question has been covered earlier.

I have a DX-7 and learned how to program it.  I have also read the book by Dr. Chowning on programing FM digital.  So, I have some understanding already of how FM works.

In my first experiment on the Pro 2 (using oscillators 1 and 2), I set both the carrier and modulator to sine waves.  I set the frequency of both to C2 making a 1:1 ratio.  I turned up the FM and expected to ultimately hear a sawtooth wave.   I kind of expected the FM "knob" to be controlling the amplitude of the modulator.  What I heard was very different.  It sounded like I was changing both the frequency ratio and the amplitude at the same time by turning up the "FM".  I was able to adjust the frequency of the modulator so I was getting just harmonic  sidebands, but then the ratios between the carrier and modulator were not anywhere close to what I understand they should be.

So, I am totally lost.  What does the "FM" knob do?  Is it some combination of amplitude and frequency ratio?  Is there a way to adjust amplitude and frequency ratio separately?

I was able to get some really interesting sounds out of the FM portion, and there are certainly things that can be done on the Pro 2 that cannot be done of the DX.  However, I want to figure out how the FM controls work so I can get the most out of the Pro 2.  Thank you in advance for any help!

[BobTheDog]

Hi,

The P2 has exponential FM not Linear FM, is that the difference you are hearing?

They did update the P12 to support Linear FM and there is dome hope this will happen to the P2 in the future as well.

[jdt9517]

While I'm not that familiar with exponential, I think what I am hearing is a different beast.  I'll explain what my concern is and maybe you can advise me of whether this is a linear vs. exponential question.

The way I learned digital FM is that it normally has two basic controls - frequency ratio between carrier and the modulator, and amplitude of the modulator into the carrier.   The frequency ratio acts sort of like the waveform selection of an analog oscillator, e.g., sawtooth, square wave, etc., but much more complex waveforms can be created.  Amplitude acts like a VCF on an analog (although it is more directly related as a volume control of the modulator).

So, analogizing it to an analog synth, I set the frequency ratio to get the complex waveform, then I adjust amplitude (modulator volume control) like I would the VCF to determine how many of the partials I want to keep.  Having the amplitude "volume control" all the way up is like having the VCF wide open so all the partials come in.  Turning down the amplitude is like slowly closing the VCF - fewer and fewer partials go through.  In the digital world, the frequency ratio determines what sidebands can be created around the carrier frequency, including the harmonic or inharmonic nature of the sidebands.  The amplitude controls how many of the possible sidebands are actually created.   

On the Pro 2, it seems like I have one control doing double duty - both setting the ratio and the amplitude.  That's where my confusion is.  Is there truly only one control to set the relationship between the carrier and modulator, or do I have access to both basic controls?  What I see so far is that the FM control deals primarily in the frequency ratio and to a lesser extent the amplitude.  I don't see any separate control for amplitude.  Of course, Dave Smith, the genius that he is, may have devised a different way of controlling these two parameters.  I haven't seen any tutorials of the FM section. 

So, I'm trying to get to the bottom of how to work the controls and whether I can control both.

[BobTheDog]

The "FM AMOUNT" setting is the "Amplitude" or amount of modulation, the tuning of the other oscillator is determining the frequency of the modulator.

The DX-7 used Linear Phase Modulation which is also different to Linear FM or Exponential FM.

So even on the P12 with Linear FM it doesn't act in the same way a DX7 does if you self modulate the osc or use a 1:1 ratio from another osc.

[jdt9517]

Thanks, Bob.  I will be able to play with it later today.

So, on exponential FM, does the ratio vary when I turn up the amplitude ("FM Amount")?  That would explain why I had to change the modulator frequency to get to harmonic sidebands.  Also, do you have any links to working with exponential FM?  Thanks!

[BobTheDog]

I'm not an expert but I think the ratio stays the same, it is defined by the frequencies of the two oscillators.

What does change is the tuning of the end result, so as you increase the modulator effect "FM AMOUNT" the tuning alters, so to keep things in tune you have to change the tuning of the carrier.

So the only way to keep it sounding in tune is to use static FM (no envelopes on "FM Amount" or modulator frequency) and change the frequency of the carrier so the final result is in tune.

Basically for me exponential FM is hopeless, especially as the operator count increases.

I had the same problem as you when I first got the P12, I was thinking what the hell is going on here, as you can see from this thread: http://dsiforum.com/viewtopic.php?t=5738

[jdt9517]

Bob,

Assuming that the problem I am having is exponential FM, I think it still is useful.  It can create sounds that stay in tune over the entire keyboard.  Here's my experiment:

1.  Turn off all ocs, modulators and envelopes

2.  Set VCA envelope to fast attack and full sustain  The pic on the screen should show a rectangle.

3.  Set osc 3 to C2, pulse waveshape with a mod about 35.  Make sure there is no detuning of this osc.

4.  Set osc 1 to C#2, fine tune +10.  In OSC MODS set FM AMOUNT to 60.  Output to 127. 0 osc slop.  The waveshape should be a sine with 0 mod.

5.  Set osc 2 to C2, fine tune to -9, Output to 0, osc slop 0.  The waveshape should be a sine with 0 mod.

6.  Cutoff 164 (full), resonance 0.

PLAY

Osc's 1 and 2 are in tune with osc 3 and play in tune throughout the range of the keyboard.  If you turn off osc 3, the sound of osc's 1 and 2 is roughly a pulse wave. 

From here, turn back on osc 3, and if you detune osc 2, you can start getting beating and phasing.   Change the FM MOD amount and the pitch of the FM sound changes.  You have to go back and retune.  This is where I think the FM MOD does not simply change amplitude.  It seems to affect the freq ratio too. 

An interesting effect on this patch is to have Env 4 be a modulator to the pitch of osc 2.   Set the sustain of env 4 so that the pitch of osc 1/2 will hold as a unison or 5th to osc 3 - then start having fun with the attack and decay.  All kinds of fun sounds come out of it there. 

[jdt9517]

Just saw an ad for the P-12 "now with linear FM".  The more I play with the FM on the Pro 2, the more I agree with Bob that it is an exponential FM issue.  Wonder if the Pro 2 will now get the linear FM since the P-12 now has it.

[BobTheDog]

I think they have said that they will look at getting Linear into the P2, so fingers crossed.

The P2 is overdue an update really, there are a few bugs that need fixing and talk of sequencer updates.

We just have to wait and see.

[jobinho]

I think they have said that they will look at getting Linear into the P2, so fingers crossed.

The P2 is overdue an update really, there are a few bugs that need fixing and talk of sequencer updates.

We just have to wait and see.

Certainly is due an update! Hoping for FM - I heard Carson mention it's dependent on whether the Pro 2 processor can handle it, or something like that... here's hoping.

[nielsd]

YES, this would be cool. Fetched an "old" Ya,aha FS1R yet in between to fit some FM "requirements" and opened this synthesis a bit more to me, but to have at least "basic" (but linear) FM functionality on the Pro2 would be a door opener to a new dimension of new sounds on that nice machine.

Is there any firmware update for the Pro2 uprising anywhere on the horizons?

[ihearanewworld]

jdt9517, thanks for the little FM experiment. I have always used FM on the Pro 2 in an experimental, "let's see what happens when I put FM on this OSC" sort of way, but your posts helped me understand it a bit better, and I can approach it from a more intentional direction.