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Linear FM on VCOs with no linear inputs?
MUFF WIGGLER Forum Index -> Modular Synth General Discussion  
Author Linear FM on VCOs with no linear inputs?
Suineder
Alright, so I have a Behringer Neutron coming in the mail to get me started in my musical explorations and find what I like before trying to expand too much. Anyway, the Neutron's oscillators don't have linear pitch inputs, which got me wondering about how to work around that limitation. My first guess is to route the modulator through a logarithmic VCA before it goes into the exponential pitch input. I imagine some level shifting would be called for since logarithms are undefined for negative inputs.

Is this already a known technique? It wouldn't surprise me if so.
jorg
The log converter won't really give you the same result, due to the negative-log problem you mention. You might be able to tap into a linear node inside their expo converter (most actually have such a node as an incidental part of the circuit).
ranix
the Happy Nerding FM Aid is what you seek, but I've hear they're becoming harder to find
Suineder
jorg wrote:
The log converter won't really give you the same result, due to the negative-log problem you mention. You might be able to tap into a linear node inside their expo converter (most actually have such a node as an incidental part of the circuit).

That's about what I expected. But the opportunity for a different sound than normal is certainly intriguing.

ranix wrote:
the Happy Nerding FM Aid is what you seek, but I've hear they're becoming harder to find

Ah yeah, the FM Aid looks super cool and has had me interested for a while.

Thank you both!
thor.lucas
I was wondering what the use of linear frequency modulation is. Why would this be preferred over V/Oct?
hinterlands303
thor.lucas wrote:
I was wondering what the use of linear frequency modulation is. Why would this be preferred over V/Oct?


This is a completely non-technical explanation, but for audio rate modulation Linear FM gives more subtle results vs. Exponential (v/oct) FM. Typically you would use audio rate linear FM to affect timbre whereas you would send CV to the exponential input to affect pitch. Audio rate exponential FM can still sound really cool but you're going to lose your tracking pretty quickly while doing it whereas with linear FM you can (usually) get your oscillator to track 1v/oct while it is being FM'd.
starthief
- Expo FM sounds nice and strong, but you can't modulate the depth without changing the tuning.

- Linear FM can have the depth changed dynamically and is easier to tune in the first place. But if the modulation is too strong, you still lose pitch tracking (because the oscillator can't go less than 0Hz, but it can still go upward its full range). A lot of oscillators limit the linear FM depth to prevent that.

- Thru-zero linear FM (TZFM) lets you modulate much more strongly and maintain the tuning -- the oscillator can effectively go into "negative frequency," which is just positive frequency with the phase flipped, so the math works out.

- Phase modulation is practically similar to thru-zero linear FM (unless you're going to combine it with expo FM).
Blairio
starthief wrote:
- Expo FM sounds nice and strong, but you can't modulate the depth without changing the tuning.

- Linear FM can have the depth changed dynamically and is easier to tune in the first place. But if the modulation is too strong, you still lose pitch tracking (because the oscillator can't go less than 0Hz, but it can still go upward its full range). A lot of oscillators limit the linear FM depth to prevent that.

- Thru-zero linear FM (TZFM) lets you modulate much more strongly and maintain the tuning -- the oscillator can effectively go into "negative frequency," which is just positive frequency with the phase flipped, so the math works out.

- Phase modulation is practically similar to thru-zero linear FM (unless you're going to combine it with expo FM).


I haven't tried it, but why not run your fm source through a full wave rectifier? Would that prevent the modulated oscillator going into -ve frequency?
starthief
Blairio wrote:
I haven't tried it, but why not run your fm source through a full wave rectifier? Would that prevent the modulated oscillator going into -ve frequency?


Yes, but you still can't use it dynamically since it won't average to 0 -- same problem as exponential FM. And you're adding harmonics so it's going to sound different.
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