The legend filter key tracking

[paulbinns]

I was following along tonight with a video that was comparing a vintage mini Moog with the behringer equivalent, and comparing it to The Legend.
At one point in the video, the presenter demonstrates the key track self oscillation, and when comparing it with The Legend, it doesn't seem like the key track is 100% when both filter key track switches are on. On higher notes it seems okay but as you get lower on the keyboard the notes get sharper. it's only slight but it's enough to mean that you can't really play melodies (Or harmonies when playing polyphonically) across the keyboard and keep them in tune.
here is the point in the video where it's being demonstrated : https://youtu.be/3pRiUSf_QFw?t=1152
The mini Moog and the behringer both track correctly and in tune.

to replicate:
switch off all oscillators or turn the volume down on each one
set the resonance to maximum
set both key tracking switches on
tune the filter with the cutoff knob
play some notes

It becomes particularly noticeable around C2 to C3

[Mark]

I can't test at the moment, but have you made sure that there aren't any active filter modulations in the mod matrix on the back panel?

[Mark]

You might also want to edit the filter settings on the back panel as well.

[paulbinns]

I have switched off all modulations on the back.
Also the filter settings on the back do have some impact on the pitch, specifically with the symmetry knob, but this does not effect the tuning relationship of the notes. It simply makes all of the notes sharper or flatter equally, as far as I can tell.
Likewise the model revision switch affects the pitch, but again not the pitch relationships.
Just to be sure that my ears weren't tricking me, I loaded up the tuner plugin in logic and I can easily see that the same note at different octaves was shifting its tuning, and not in a consistent way.

[Mark]

I had chance to check last night and you're correct, the key tracking is off. Richard? Marcin?

[Kriminal]

It may be a case of that's how the hardware was that they modelled

[Mark]

True.

[Richard]

It may be a case of that's how the hardware was that they modelled

Yes, correct. In analog synths with no digital parts or any microprocessors controlling stuff, the filter keytrack is almost never precise and can differ substantially between each individual unit. The Model D in the video seems to be fairly precise, but not exact either (to my ears).

On a side note, the Model D is not capable of playing low notes at full resonance, because the resonance vanishes below 100 Hz or thereabouts. So at best this would be useful for melodies somewhere in the middle. Maybe some kind of switch like "accurate filter keytrack" would be useful.

[paulbinns]

A switch to turn on accurate filter key tracking sounds like an excellent idea. This would allow the filter resonance to be used as an extra oscillator against the existing oscillators, as well as allow it to be used musically in conjunction with other instruments.
I'm curious however to understand how changing this might change the behavior of the filter. Is the key tracking behavior an essential part of accurate modelling of the filter?