Please forgive the long message, but I want to lay out in detail why I think there's something amiss with linear FM on the Solaris.

The possibility of using Linear FM on the Solaris was something I was really interested in, but having now tried it, the current implementation of linear FM just doesn't seem right to me. I've spent some time today investigating what might be different about the Solaris version of "LinFM" by comparing to the results of linear & exponential FM on my old Nord Modular and viewing FFTs of each using the Voxengo SPAN VST plug-in.

I set up a simple carrier-modulator pair of sine waves on the Solaris, both oscillators set to the same frequency (A3=220Hz), and did the same on the Nord Modular. Now, when I used exponential FM on both synths ("Pitch" on the Solaris) the results were almost exactly alike, which one would expect. At low modulation depths one sees a characteristic short series of evenly-spaced harmonics, but as the modulation depth is increased that series gets shifted upward in frequency because there's a DC component in the modulation due to the asymetry inherent in the exponential modulation. The other prominent feature is a strong low frequency component that is introduced. Here's two images showing the effect of increased modulation depth between the two images (10% depth -> 16.9% depth):

Now, with Linear FM you shouldn't see the sort of harmonic shifting that occurs in exponential FM. Also, irrespective of the depth of the modulation, partials are only supposed to appear at integer multiples of the base frequency when using a 1:1 ratio of carrier-modulator, although the amplitude of those partials will change as the modulation index increases. That's not what you see when using LinFM on the Solaris, though. First, here's the Nord Modular doing linear FM (oscillators are still 1:1 frequency @ 220Hz).

Note how the partials all occur at integer multiples of the base frequency of 220Hz (i.e. 220, 440, 660, 880, 1.1k etc). Now look at the Solaris version using the same base frequency of 220Hz.

It's wierd to note that the Solaris LinFM actually resembles exponential FM more than it does linear FM. There's both the shifted series due to some DC component (300, 520, 740, 960 etc) and the strong low-frequency component. Most noticably none of the partials appear at integer multiples of the base frequency of 220Hz, they're all over the place!

Due to the above, my conclusion is that LinFM on the Solaris isn't actually linear FM, which is a real shame for me personally because it's the only feature I've been disappointed in so far. There must be something awry here, right? I'd really LOVE to see this be fixed so that we can use true linear FM on the Solaris and I don't believe we need a new oscillator type to fix it: the existing LinFM just needs to be changed to work the way linear FM should. Thoughts?