Differing values for LFO depth for Peak and Trough?
February 25, 2011 4:31 PM   Subscribe

Is there a way to change LFO depth so that it isn't one number that applies to both peak and trough, but that you can have the peak be one value and the trough at another?

I was thinking about LFOs the other day and realized that one knob that set a depth value forced one into a limited way of altering the sound. Say I had a depth of 25, that would make both trough and peak 25 difference from baseline. But what if I wanted to make the trough 25, and the peak 10? The basic setup of LFO depth settings don't allow one to do that.

I'm thinking perhaps applying a bandpass filter somehow to it, or an envelope? Or secondary LFO? Is that possible? If so, how would one do it, and if that doesn't do it, why don't we have this capability in modern synths as a default (say one knob for trough depth, and one knob for peak depth).
posted by symbioid to Sports, Hobbies, & Recreation (6 answers total)
A DC bias is what that is called. And it doesn't sound like anything but is bad for your speakers.
posted by aubilenon at 4:42 PM on February 25, 2011

Agree with aubilenon.

Try to picture what the speaker cone moving back and forth would look like if you tried to get it to keep moving in one direction and not coming fully back in the other direction.
posted by Diplodocus at 4:46 PM on February 25, 2011

Wow, that was fast and informative. Thanks!

When you mentioned that it made me realize I'd heard the term w/r/t audio tape? Also, Renoise has a "DC Offset" that I'd seen but never thought to look up for any reason, but it looks like it's all related. Huh.
posted by symbioid at 4:54 PM on February 25, 2011

I guess my glib reply isn't necessarily that apropos. You're probably using the LFO to control stuff not routing it directly to your stereo. Like if you're using it to control the frequency of another oscillator. You can then mix it in with a constant voltage / value to shift it up or down. The oscillator itself though only has a control for how big it oscillates, because that makes more sense for most applications as well as making more sense from an implementation point of view.. If you need this it's not hard to add in a constant offset afterwards, so there's no need to complicate the oscillator with that.

I don't believe the word "bias" referring to magnetic tape means the same thing, but I'm not certain.
posted by aubilenon at 4:58 PM on February 25, 2011

You're basically asking for a LFO with asymmetrical waveforms. Most synths don't offer that - you'd have to settle for 17.5 +/- 7.5.

This would be really easy with a digital modular synth like the Nord or Reaktor, but probably beyond the capabilities of most fixed-architecture synths.
posted by scose at 9:53 PM on February 25, 2011 [2 favorites]

This should be possible in a software DAW with a bit of kludging: just automate the depth of the first LFO with a second LFO set on a square wave. The overall time below and above the set point will still be equal but the first oscillator will travel different lengths, like so. Both LFOs need to have the exact same frequency for this to work.

I'm not an analog expert but I'm guessing that historically, it just hasn't been a priority for most hardware synths to chain oscillators in that particular way.
posted by en forme de poire at 9:50 PM on February 26, 2011 [1 favorite]

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