Filter sweep causes pitch bend...?

[henrebotha]

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?

[legend4ry]

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.

[henrebotha]

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.

[jrisreal]

Resonance possibly?

[1point5]

Resonance possibly?

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

[henrebotha]

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.

[narcissus]

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.

[henrebotha]

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.