Audio-rate modulation in pedal designs

[ElectricDruid]

Hi all,

Aside from Ring modulators, are there other pedals that use audio rate modulation? Or even just a really fast LFO? How fast?

It comes up a fair bit in the synth world (audio rate modulation of filters is useful, for example) but in a synth you've got the advantage that you can have the modulation tuned to the input, which you can't do in a pedal, unless you go heavily DSP on it.

The reason I ask is I'm working on a new 8-pin LFO chip and I wondered whether there's much love for super-fast LFOs / audio rate oscillators.

Thanks,
Tom

[Kipper4]

Hey Tom
Have you a link to an example of the application in the synth world?
Thanks Rich.

[Mark Hammer]

Many times, side-chain-controlled effects that have too much envelope ripple get described by disgruntled users/builders as being "distorted".  In reality it is essentially audio-range modulation by poorly rectified and smoothed envelopes.  One example that comes to mind is the so-called "soft-distortion" of the old Roland Funny Cat.

The first stage derives a poorly smoothed envelope which is then fed to the gate of a FET that forms the ground leg setting the gain of that first op-amp stage.  Because the gain of that stage fluctuates so much and so fast as a consequence of the poor smoothing, it  sounds like distortion.

Of course, there are two basic aspects that make it sound like distortion rather than modulation:
1) It is fast.  Much faster than what many would consider to be audio-range modulation.
2) It is not steady-state modulation.  I.E., it isn't like modulating an audio signal with a 200hz oscillator whose frequency and amplitude are fixed, leading to predictable sideband products.

But...you asked, and that's my response.

[ElectricDruid]

Hey Tom
Have you a link to an example of the application in the synth world?
Thanks Rich.

I can give you a link to a schematic, if you like. Usually it'd be a modulation path from one of the oscillators to the filter cutoff.

Here's an example from the Sequential Prophet T8 (nuthin' but the best for you, Kipper!):

http://www.synthfool.com/docs/SequentialCircuits/Prophet_Series/Sequential%20Circuits%20Prophet%20T8%20Service%20Manual/22.jpg

At the top of the page is a CEM3340 VCO and the CEM3372 VCF/VCA chip. In between the two of them (and level with the bottom of the CEM3372) is a 4053 switch marked "1/3 U404" and "PM FIL1". "PM" here means "Poly mod"-polyphonic modulation, e.g. per voice. It's fed from the waveform mixer circuit for Oscillator B, via a 3360 VCA providing amount control ("PM OSC 1B").

Tom

[ElectricDruid]

<snip>
it  sounds like distortion.

But...you asked, and that's my response.

Sounds like you're not convinced it's a great idea, Mark. Or does that misrepresent your view?

Tom

[R.G.]

In most cases where I've heard something that I recognized as audio rate modulation, it did come out as distortion, and usually as intermodulation distortion. Useful to the noise-maker side of the distribution, not so much on the tonal side.

Low audio modulation sometimes comes across as a "growling" sound, useful in some situations, I guess,  but that's down in the bass end of things.

If you have a pin or anything that could be perverted that way, use it as a switch to 10x of the LFO. But I wouldn't bet a chip release on it.

[ashcat_lt]

TubeScreamer has audio rate modulation (derived from the input) of gain and filters. 

[Kipper4]

Thanks.
"Nuthin' but the best for you Kipper."
Ok this is above my pay grade still. I'll leave it to those more knowledgable.

Cheers.
Rich

[Mark Hammer]

<snip>
it  sounds like distortion.

But...you asked, and that's my response.

Sounds like you're not convinced it's a great idea, Mark. Or does that misrepresent your view?

Tom

More like I misconveyed my view.  My point was essentially that audio-range modulation already shows up in some things, sometimes intentional and sometimes unintentional, that people either appreciate or "fix".  Search the archives here, and you'll find I've been trumpeting the sonic possibilities of goosing the LFO into the audio range for years now.  I have one of those old blue MXR rack mount digital delays, that was intended to provide everything from flanging clear on out to echoes. And one of its most interesting features was a push-pull switch in the speed pot that goosed the modulation rate x100., yielding a whole other palette of sounds.

I think RG summed it up nicely by alluding to the "noise-maker side" and "the tonal side".  One of the things about audio-range modulation is that sideband products show up, degrading the sense of a note having pitch, because there are sum and difference products.  If that is one's intent for effect, fine.  But if one has intentions to provide content intended to be melodic or provide harmonies, audio-range modulation tends to get in the way.  Knives and forks are great, just don't try eating most soups with them.

However, all of that advice is predicated on doing something very different from the scenario I described with the Funny Cat.  Very slight modulation at random frequencies that are well above the range we normally turn to for audio-range modulation, can be sonically interesting, and the low level of modulation can maintain the perceptible pitch of the note.  It can give it a bit of a Scottish burr.

In the case of the Funny Cat, the content added by the "soft distortion" only serves to give the filter part of it more to filter.  Which in the case of an auto-wah is actually a good thing.

[ElectricDruid]

Thanks for the clarification. I'm also a fan of the "goosed" LFO, if that's what we're calling it I especially like the almost-a-filter sounds you can get from highly resonant phasers and flangers. They *are* filters really, after all, but it's the resonance that gives them "that sound".

As I said at the beginning, this is really a "what to do with the last input pin?" question. I'd been wondering about using it as an LFO/Audio range switch. Other options are a wave distortion like on some of my other LFO chips, or a Tap Tempo multiplier (1/8th notes, quarter notes, 1/2 notes, triplets, etc).

Dunno. I'll think about it some more.

Tom

[bluebunny]

Have you a link to an example of the application in the synth world?

 

[anotherjim]

https://en.wikipedia.org/wiki/Frequency_modulation_synthesis

For synthesizing harmonic sounds, the modulating signal must have a harmonic relationship to the original carrier signal.

Ouch! Pitch tracking required.

[Kipper4]

I had to you tube it.
Basically the one oscillator effecting another oscillator.

[ElectricDruid]

I had to you tube it.
Basically the one oscillator effecting another oscillator.

Exactly that. And as was pointed out, definitely pitch tracking required, and at a high degree of accuracy too. The Yamaha DX7 was a digital synth, and what they did wouldn't be possible with analog. The uncertainties build up too fast and you finish up with a mess for anything but fairly simple FM sounds.

T.

[bluebunny]

You're quite right, Tom, re. the DX7.  But my OCD can't resist being just a little bit facetious occasionally: all those vowels sat in the right order an' all...   

[FUZZZZzzzz]

sorry... I found this intriguing.. don't know if there's an answer to your question.

http://sonicsmith.com

[ElectricDruid]

Very nice, but I'll bet that's a digital chip

DSP deriving pitch and CVs from incoming audio and using it to control subsequent analog processing via CV/Gate/generated ramps/etc. Fair enough. Notice how they focus on "Analog controlled" rather than "Analog"? There's a reason for that...

Tom