I think the operation of Joranalogue’s Generate 3 (G3) eurorack module helped me to understand the two different modes of FM (assuming my understanding is correct). Like the Neoni, the module has what Joran calls a “bias switch” to toggle between what the G3 manual calls “normal linear FM” (what most people are familiar with) and “through-zero FM” (the new thing in Neoni, even though it’s not new because, as Jason mentions in his video, these are ideas published decades ago).
G3 has a bipolar attenuator controlling the depth of FM (knob 3 in the manual). But with no modulator input, the knob is normaled to an internal offset voltage. In this mode of operation, by sweeping the knob from its zero point in the middle (0V offset) to its left- or rightmost points (+/-5V offset), we move the oscillator’s frequency from 0 (stalling the oscillator core) to the fundamental frequency. The crucial point here, it seems, is that the fundamental frequency is attained at 5V.
So what happens when we input a modulator? When the bias switch is down, G3 adds an offset of +5V to the modulator, which corresponds to normal linear FM. As I understand it, oscillators typically go +/-5V, so the offset applied by the bias switch means that when the modulator signal is at its center point (which would traditionally be 0V if it weren’t for the +5V offset), the pitch of G3 is at its fundamental frequency. In other words, under normal linear FM, the modulator moves the frequency above up and down, but the center point is the carrier’s fundamental. (The funny thing with this through-zero terminology is that in normal linear FM mode, if the modulator has lower amplitude than -5V, then we would end up modulating through zero, but nonetheless, this is not what Joranalogue and Instruo are calling “through zero” for these modules.)
“Through-zero FM” on the G3 instead corresponds to having the bias switch up, so that no offset is applied to the modulator. But this means that when the modulator is at its center (zero) point, the carrier (G3), is not at its fundamental frequency. Instead, the fundamental is now attained when the modulator reaches its extremes of +/-5V.
The practical implication of all of this, as others have noted above, and as Jason’s Neoni video makes clear, is that TZFM flips the role of the carrier and modulator relative to normal linear FM. By the way, from the discussion above, this doesn’t have anything to do with analog or digital. It’s only about whether the fundamental frequency is attained when the modulator is at its zero point or at its extremes.
The funny thing is I don’t even think Joran knew if TZFM had any melodic application. I recall this modwiggler post from back in May, where Joran says that TZFM mode “is more suited for percussive and noisy sounds.” That’s true if you try to use it like normal linear FM without understand the carrier-modulator role reversal. Amazingly, the post Joran was responding to actually wrote, “It’s almost as if the modulator signal becomes the carrier and the G3 becomes the modulator - does that make sense?” And it turns out they were exactly right, thanks to Jason’s video that finally explained the right musical application of TZFM.