VPM

[tpantano]

I've searched a bit around the web, and read through the R3 Manual, but I'm still having trouble grasping exactly what VPM does. I know it gives a more metallicy sound to a patch, and adds in hints of another harmonic to a sound, but I'm not sure what it's doing specifically to the sound or why to choose a specific harmonic (although I know that thirds fifths and octave sound the most musically correct)

[X-Trade]

VPM is basically like using cross mod, or frequency modulation. It stands for Virtual Phase Modulation. Phase modulation is popular because it is similar to FM but not patented by Yamaha, so cheaper to implement. It also doesn't bend the pitch like FM does, so no additional compensation is needed in the DSP code.

It is creating a second (virtual) sine wave oscillator, and modulating the waveform's phase with the output of that oscillator.

The number you select is actually the harmonic index, not an interval. So 0=unison, 1=octave above(+12), 2=octave & a fifth(+19), etc. hence why you don't hear much difference with really high numbers, because you are getting into divisions of a tone difference.

Naturally, the whole point of 'Virtual Phase Modulation' is that it isn't actually creating a second oscillator, just mimicking the effect somehow. The same as how the unison mode simulates the stacking of five oscillators without actually using that many oscillators or voices - it is virtually simulated within the DSP resources of oscillator 1.


I haven't tried it, I can't remember if KORG's 'cross modulation' is actually phase modulation. If it is, then for example a VPM with an index of 1 should sound the same as using cross mod at the same depth with oscillator 2 set to a sine wave at +12 semitones.


note I did notice that oscillator 2's sine wave isn't actually very pure. It has upper harmonics that aren't present on oscillator 1, I don't know if this would have any noticeable effect on phase modulation.

[Gribs]

Reading here - basically looking for the same info except for the EXB-RADIAS. I am surprised that there is not more in depth information in the Radias or EXB-RADIAS docs concerning how VPM works.

I am relatively new to Korg but not synthesizers or FM. While dinking around with my week-old M3M (+ EXB-RADIAS) tonight I found a program called "Advanced VPM E.P 4" that sounds a lot like an FM-style bell-like piano. It has the detuned metallic sound in the lower octaves that sounds a lot like basic FM-style EP sounds. So I started looking for info on VPM and found zip in the various PDFs.

[X-Trade]

Yes,
AFAIK it is a simple sine-wave phase modulator. the selected waveform is the carrier, the sine wave is the modulator.
Control 1 sets the modulation depth, Control 2 sets the harmonic index integer (frequency multiple). Disappointingly it doesn't do sub-harmonics. You have to use the cross modulation with oscillator 2 for that. Then you can select a different waveform for the modulator too.

I can see VPM can be handy mostly when you still want to use oscillator 2 or if you really need a pure sound that oscillator 2's sine wave can't provide.

You can get some interesting wavesequences too using the modulation sequencer to modulate both control 1 & 2 to get varying depth and harmonic index. Combine this with oscillator 2 on rindmod and maybe the waveshaper tied into an EG or LFO and you're in pure digital FM-like synthesis territory.

By the way, found this in the manual under 'Modulation Type Selection':

VPM
VPM (Variable Phase Modulation) uses a sine wave at a
harmonic (integer) multiple of the oscillator 1 funda-
mental to modulate the phase of oscillator 1, producing
metallic-sounding overtones. In contrast to Cross Mod-
ulation which produces sounds that have a modulating
character in the waveform, VPM uses a sine wave at a
harmonic multiple of oscillator 1 to modulate the phase
of oscillator 1, meaning that the waveform will remain
static while overtones are added. The sine wave used for
modulation is built into oscillator 1, so the waveform of
oscillator 2 will not affect the sound of oscillator 1.
If “Wave” is Saw, Squ, Tri, or Sin, you can adjust the fol-
lowing parameters for the selected waveform.

[slammah2012]

so, in a nutshell, if you are modulating a sin wave with VPM, you will get the effect of pulling out Drawbars on an organ with the sin as the fundemental and the VPM as the harmonic overtone series up untill you create a sawtooth wave...

[X-Trade]

so, in a nutshell, if you are modulating a sin wave with VPM, you will get the effect of pulling out Drawbars on an organ with the sin as the fundemental and the VPM as the harmonic overtone series up untill you create a sawtooth wave...

I would have to check Phase Modulation theory (I have a handbook from the university somewhere on all manner of synthesis techniques), but I'm fairly sure the result is more complex than that. The key word in the description at least is metallic overtones.

It doesn't produce a straight harmonic series. The only thing which is moving in a straight harmonic series is the modulating oscillator, which is a sinewave which isn't heard directly - it is just modulating oscilator 1's phase. Control2 sets this sine wave's integer multiple of the oscillator's fundamental frequency. Hence it sets the harmonic index of the modulator (sine wave) oscillator)

Certainly other synthesis techniques which are referred to as similar - such as Frequency Modulation (FM) and Amplitude Modulation (AM, similar to ring modulation) - create 'sidebands' of harmonic content on either side of the fundamental frequency.