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I played my friend's Alesis Micron for a while. I had this bright sawtooth bass lead patch and I had one fader set to lower the filter frequency.

At one point, I noticed that if I used the fader to do a manual 'wobble', the pitch of the synth patch would appear to fluctuate ever so slightly in time with my fader movements. If I recall correctly, the pitch would go down as the filter goes down, and vice versa. But once I stop moving the filter, the pitch returns to the patch's 'real' pitch.

What could be the cause of this?

Now, I should emphasise: I DESIGNED THIS PATCH FROM SCRATCH. There was NO pitch modulation of ANY kind, except for the default pitch wheel.

I know analogue filters work on a time constant, so I was thinking: maybe since when you move the cutoff of a real analogue filter, you adjust the time constant, you're inducing a varying delay, thereby causing a pitch shift effect. Any engineers want to pitch in?

I don't know exactly why it happens but it done this with a minimoog when I had access to one - thought it was down to it being old and battered.

Sweet. If it truly is natural behaviour for analogue filters, it would make sense that the Micron's modelled analogue filters would attempt to recreate that behaviour.

Resonance possibly?

jrisreal wrote:Resonance possibly?

yeah if the resonance is turned up high it can sound like the pitch is changing when altering the cutoff frequency

1point5 wrote:

jrisreal wrote:Resonance possibly?

yeah if the resonance is turned up high it can sound like the pitch is changing when altering the cutoff frequency

It's not resonance - the filter on that patch was at maybe 20% resonance, hardly enough to give that 'tcheeeeooooow' filter sweep sound.

I'm talking a much smaller pitch change - like a semitone at most.

i dunno how low you had the filter set in relation to the fundamental of the note you were playing, BUT.. i first noticed this years ago, playing with Rebirth..
if you lower the filter enough (i've not looked up the actual phyics, but i think you'd have to cut below the fundamental), you will perceive a slightly lower note than is actually playing, but i think you need fairly high resonance for this .. that could be it..
or just janky electronics. who knows.

Ohhhhh... That could be it. I mean, I still think my time constant theory is valid, but the thing you're talking about (called Steven's rule, I'm told) could definitely contribute.