Multi-split harmonic tremolo

[Mark Hammer]

I've been recently musing about a sort of uber-harmonic-tremolo with multiple bandpass sections having their amplitude swept by unsynced LFOs, instead of just a simple 2-way lows-vs-highs split.  Let's say, for argument's sake, a 4-way/band split.  Much like the Dimension C chorus, I don't know if having variable modulation speed would really add much, since four unsynced LFOs would not yield any audible periodicity.  My intent/plan was that each LFO would drive an LED/LDR combo to vary the amplitude at a mixer stage.

So, in all, 2 dual op-amps for the quartet of LFOs (I'm going with the single op-amp subcircuit used in the Phase 90), a quad op-amp for the 4 bandpass filter sections, and another dual for the input and output/mixer stages.

If no speed controls are planned for, then what controls could there be?  Well, I suppose Drive and output volume levels are appropriate.  An option might be to have toggles to swap LFOs for a pair of bands.  Again, since the LFOs are unsynced, it's hard to know if such a thing would even be perceptible.  But then I suppose that would depend on how similar or dissimilar the individual LFO rates would be.

Let's say the two lower bands have LFOs sweeping at near identical unsynced rates, and the two upper bands are also similar in rate to each other but a bit slower than the two lower ones.  Swapping the LFOs of the two middle bands might be an audible change in tone and character.

I'm just riffing here.  All ideas welcome.

[Steben]

This might result in a Faux Phaser effect?

[Mark Hammer]

This might result in a Faux Phaser effect?

Well, somewhere in that family.  People often confuse harmonic tremolo and Uni-vibe.  My view is that this particular confusion is because the main signal seems to always be present and perceptible, and one's attention is not being drawn to any particular point in spectrum, the way it is with a phaser.  In the case of harmonic tremolo, what is modulated is the relative contribution of highs and lows, but the sound never seems to "go away and come back" as it does with full-spectrum tremolo.

Harmonic tremolo uses a single LFO, that countersweeps the two bands, such that they are always synchronized.  When I briefly experimented with dual-unsynced LFOs for a stereo Magnavibe, I was impressed with just how rich it sounded and how much less listener-fatigue set in.  The aperiodicity seemed to be key.  So the idea of 4 different unsynced LFOs moving the relative amplitude of multiple bands around seemed attractive; something one could listen to as a set-and-forget kind of effect.

I suppose the extent to which it might mimic a phaser would depend on how widely the each LFO modulated the amplitude of its corresponding section.  I was aiming for something that would undulate somewhat like a harmonic tremolo.  And since the filter sections are likely to overlap with each other.  The sound would never seem to go away too much.  Could I do that with just two LFOs and a traditional high/low split as used in harmonic tremolo?  Possibly, but there WILL come a point where the two unsynced LFOs line up for a moment and the amplitude of the two bands is either reduced OR increased substantially.  And that kind of defeats the purpose.  I'm betting that using a quartet of unsynced LFOs means there will never be a point where all four LFOs are in the same state (high or low) at the same time.

Because the LFOs are triangle-generators, and I'm planning for medium-speed sweep, the result should not sound at all like a sample-and-hold, as it might with square-wave LFOs.

[ElectricDruid]

There's probably (ok, almost certainly..) some weird phase-shifts resulting from the filtering that mean that even if all you did is split the signal and then mix it back together again, it still won't sound the same as the dry signal. After all, if it did, it'd be a speaker crossover, and that's a very specific filter design, especially for four bands.

So I think the thing sounds interesting. It should add a nice flavour - not quite phase shifter, not quite tone control, not quite tremolo, but with some wobbly movement of all three. Could be nice.

Good luck! Let us know how you get on!

[Mark Hammer]

Will do.  I'm splitting my time between that and learning how to operate my newly-acquired Volca FM.