CV versus FM

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synthetic
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CV versus FM

Post by synthetic » Tue Dec 04, 2007 11:37 pm

What is the difference between sending an LFO into a 1V/Oct CV input versus sending it into an FM input of an oscillator? DC versus AC input? But what about units with DC FM coupling?

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Post by consumed » Wed Dec 05, 2007 12:36 pm

im not the authority here (im a pretty new guy), but just thinking about it:

- your 1v/oct has no attenuator, your fm input probably does
- your 1v/oct expects 0-10v, your fm input should take +/-5v (or whatever), meaning you can put audio into it.

i think this would be an interesting test to set up.

have you measured the output of your lfo? does it put out +/-5v?

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Post by thermionicjunky » Wed Dec 05, 2007 1:16 pm

I'm sure that there is plenty of variation in nomenclature from one manufacturer to the next. FM could be linear or exponential. Even the v/oct input should respond to bipolar signals. It's probably best to ask about a specific VCO. What they should all have in common is the v/oct keyboard input without attenuator. The Synthesizers.com VCO has DC coupled inputs but can be modified in a few ways for AC coupling.

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Post by synthetic » Wed Dec 05, 2007 3:14 pm

Perhaps one is a log scale and the other a linear scale?

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Post by consumed » Wed Dec 05, 2007 3:45 pm

what lfo and what vco are you using?

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Post by synthetic » Wed Dec 05, 2007 5:03 pm

None yet. :oops: Looking at building a Ken Stone CGS VCO.

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Post by Muff Wiggler » Wed Dec 05, 2007 7:16 pm

tricky question. The v/Oct input is DC coupled, so any negative swing of your LFO will be ignored. The positive swing will tune the VCO, but only in a small range, as it is a linear signal you are sending in there, while it expects an expo one. It'll tune it to low pitches. Probably nothing accurate to get from this, as it is not a scaled voltage.

For the FM input, things are different. The voltage here will change the VCO's frequency from it's 'base' frequency (determined by both the setting of the VCO's tuning knob and whatever voltage is present at the v/Oct input).

How will it change it? Hard to say.

Blacet VCO's have an FM input that is DC coupled ( I believe, I modded all mine). They have a lin/expo switch. So you can choose if you want the change to have a linear or an exponential response.

A DC coupled input will ignore the negative swing of the voltage, and effect the 'base' frequency only on the positive side.

You can very easily (two wires and a switch) add an AC/DC switch to the FM input of the Blacet VCO, so you can 'have it both ways'. I really recommend this.

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Post by felix » Wed Dec 05, 2007 8:48 pm

I have a somewhat relevant question...what is the purpose of having linear vs exponential FM?

Is it simply to match the modulation source (if you have a linear modulation source, use linear FM) or are there "two schools" of FM synthesis.
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Post by Kwote » Wed Dec 05, 2007 8:49 pm

i ran two lfo's mixed together with different rates into the fm, 1v, and oct switch ins last night.

not all at the same time cuz i don't have the right setup for that but maybe i do. lol. i might try that. probably be a disaster :twisted: . but each inputted seperately had some interesting results. directly to the 1v is mostly just extreme high and low sweeps. the oct switch and the fm come off more like a simple sequence not necessarily with good timing depending on the rate settings of the lfo's.

twas fun. thanks for the idears.
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Post by Muff Wiggler » Wed Dec 05, 2007 8:54 pm

felix wrote:I have a somewhat relevant question...what is the purpose of having linear vs exponential FM?

Is it simply to match the modulation source (if you have a linear modulation source, use linear FM) or are there "two schools" of FM synthesis.

nope, it's flexibility.

Sometimes (damn, usually) you have a modulation source with a linear curve. You want the frequency of your VCO to modulate in an EXPONENTIAL fashion to this source. Flip the switch to 'expo' mode.

Other times, you have the same modulation soure with the same linear curve. You want the frequency of the VCO to modulate in a LINEAR fashion to this. Put the switch in 'lin' mode.

The inverse is true. Perhaps you do have an exponential modulation source. Would a sine wave be considered as such? I dunno but I bet 'yes'.

So you have an exponentially rising (or falling voltage). You want direct linear response to this? Put the VCO's FM in 'lin' mode.

Oh no? You want an exponential response to an exponential curve? I bet at this point you know what to do with that little switch ;)

it's all about the options brotha 8)

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Post by Muff Wiggler » Wed Dec 05, 2007 8:56 pm

oh and damn

if you are applying something to the FM input of your VCO, run it through a VCA first.

then you can control the DEPTH of the FM that is being introduced, via the VCA.

really not a complete FM patch without this.

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Post by thermionicjunky » Wed Dec 05, 2007 8:59 pm

I like the sound of audio rate modulation through a linear cv input. It's what I've grown to love from using thyratrons so much. Some people seem to like linear vibrato.

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Post by J.w.M. » Wed Dec 05, 2007 9:24 pm

Muff Wiggler wrote:oh and damn

if you are applying something to the FM input of your VCO, run it through a VCA first.

then you can control the DEPTH of the FM that is being introduced, via the VCA.

really not a complete FM patch without this.
Okay-- really good advice, and the sort of thing that might take me months to figure out on my own (through trial and error). Yet another reason why I like this forum!
Thanks!

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Post by Muff Wiggler » Wed Dec 05, 2007 10:04 pm

no problem!

here's another way to think about it -

because i grew up 'po, I never had access to any synths at all. My first synths were software synths. I learned about synths from them.

One of the things I learned, was that LFO's have three parameters -

Rate, Shape, and Depth.

Pretty self-explanatory. How fast does it go, what shape is it, and how 'high' are the peaks and troughs.

When I got into hardware modulars, and tried to use a VCO as an LFO (and then when I received my first LFO), I couldn't understand why there was no depth control. It seemed like a HUGE omission to me!

It seemed like this: You are either using the full swing of the LFO, which is 10 volts (-5 to +5), or you aren't using the LFO at all. This just went against everything I had learned about synth patches. Where the hell is 'depth'? Why would I use an LFO without depth??

When I tried to use an LFO for vibrato, I was like "wtf? I don't want such a deep vibrato!! Why can't I control the depth???? Arrgh d00d!"

Well, I shot off an email to my buddy The Alison Project, basically bitching about things I didn't understand yet. Main issues were this LFO depth thing, and having to retune the damn VCO all the time. Damn I was a noob :lol:

Anyway, Alison set me straight. The reason there's no depth control, is because it's a MODULAR! I was not yet conceptualizing the purpose of a modular synth properly yet, even though I had a handful of modules and was making patches.

Here's what he told me: Run the LFO through a VCA, and then you get voltage control over the DEPTH of the modulation being provided by the LFO.

Not only can you control the depth of the modulation source (LFO) you can VOLTAGE CONTROL the depth.

(ps: I prefer to think of VCA's as voltage controlled attenuators rather than voltage controlled amplifiers, however of course they are both)

But, Alison, dude, why isn't this VCA/depth functionality built into the LFO?

Because it's an LFO, not a VCA! You should have lots of VCAs available if you have a modular, that's why.

And he was right.

Now, if you don't need VOLTAGE CONTROL of the depth, you can simply use a passive attenuator (like the one built into the Blacet multi) to set the depth. Easy peasy.

-----------------
Sidebar:

This one is cool to 'watch' if you are wrapping your head around this concept, and you have the right modules. Try this:

LFO->Atten->Bar Graph

Keep the atten fully open. You'll see on the bar graph that the LFO's voltage has a full swing. Now slowly lower the attenuator knob. You'll see the swing (or range) of the LFO get smaller and smaller on your bar graph (which is AC coupled {zero is in the middle of the graph}, so you can see the negative and positive swing) both with will reduce at the same rate so you always have a symmetrical waveform, running at the same rate, just at different depths.

Everyone who is even somewhat new at their modular should buy a Blacet Bar Graph. It makes it so much easier to see what is going on.
--------------

Anyway, thusly I learned that any time I send a modulation source to something, to send it through a VCA (or at least an atten) first. This way you can also control the DEPTH of modulation. Not just for FM.

This is/was a very very basic point, and a fundamental, building-block concept of controlling your modular synth. Realizing this was a HUGE eye-opener for me.

-------------------------

Thanks for the nice words. One thing I like about this forum is the chance to share some of the things I've leaned, and "pass on the favour" of people who have taught me things, or the lessons I've learned the long & hard way.

The other thing I like even more, is learning new things. Guys like thermionicjunky and John NonJohn (and Alison) have probably forgotten more than I'll ever know. There's no end to learning, that's one of the reasons that I don't really care for ego and attitude. We are all babes in the woods in a lot of respects.

If this has become a good place to share and to learn, then it's accomplished far more than I could have ever hoped, and we should all be proud. Thanks everyone 8)

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Post by synthetic » Wed Dec 05, 2007 11:30 pm

Can't you just attenuate an LFO through a linear pot? Why do you need to use up a VCA?

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Post by Muff Wiggler » Wed Dec 05, 2007 11:31 pm

synthetic wrote:Can't you just attenuate an LFO through a linear pot? Why do you need to use up a VCA?
Good question!
Muff Wiggler wrote:Now, if you don't need VOLTAGE CONTROL of the depth, you can simply use a passive attenuator (like the one built into the Blacet multi) to set the depth.

there ya go

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Post by felix » Thu Dec 06, 2007 12:10 am

Thanks for that explanation Muff! I had a feeling that was all it meant, but I wasn't sure if there was a "right way"

I totally agree on the VCA with and FM patch. One of initial reasons for starting this joystick controller was for controlling VCAs in that type of application.

Actually one of my favorite sounds is using a ring mod instead of a VCA. The FM modulation source goes into the carrier in and some other mod source (the source that would normally control the VCA) going in the modulator in. I've been doing this with the Plan B M15 and Dalek Modulator, really neat.
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Post by Kwote » Thu Dec 06, 2007 12:18 am

"Not only can you control the depth of the modulation source (LFO) you can VOLTAGE CONTROL the depth."

been using my vca but i didn't think to send something to the cv, was just using it more like a manual atten. i'm gonna go hop on this now.
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Post by synthetic » Thu Dec 06, 2007 12:14 pm

Great stuff, thanks.

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Post by Kwote » Thu Dec 06, 2007 12:39 pm

it was a trip sending an lfo to cv an lfo. lol. but i got some cool results. it didn't work with everything though. i think a gate signal will probably be the most useful, i had some great fun with that as well.

basically i had lfo1 tri :arrow: dual vcaA input :arrow: VCO 1v oct in;
lfo2 sine :arrow: dual vcaB input :arrow: dual vcaA cv in

adjust rates and attenuate vca to taste. some sequence like sounding results.
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Post by thermionicjunky » Thu Dec 06, 2007 2:04 pm

Try envelopes for controlling FM depth. Try anything. Those of you with some type of sequencing or random voltage modules, try it. I like manual control. It works with a lot of the effects that I use FM for. Of course, so many of my control signals end up being recorded by the CV Recorder, so even a knob turn can be stored and manipulated. I only have 3 VCAs that can pass DC, but I have about 10 independent attenuators including 2 in the Modcan X/Y Controller. It usually generates it's voltages through bipolar attenuation of a fixed reference voltage so, of course, it can be switched to attenuate an external input. There's a reason why nobody's modular stays small. There is always something essential that was left out of the original plan. I was lucky to have been studying audio when I got into synthesis. The emphasis on total knowledge of signal path helped me avoid embarrassing noob moments.

Still, nobody should feel embarrassed asking questions on this forum. There is a lot of synthesis knowledge and no apocryphally qualified asshole trolls. We attract visitors ranging from beginners to professionals. Let's keep it that way.

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Post by consumed » Fri Dec 07, 2007 11:33 am

its been a year and a half since i made the modular plunge and im still learning 'the basics'. im at the point where im geeking out trying to measure voltages coming out of modules with my s-l-o-w digital multimeter, to better understand whats happening in my system.


the vca between two oscillators in an FM patch is called a "modulation index"...i just learned about it earlier this year. envelopes are good for modulating the index. another idea is to put some stepped voltage into the index (binary zone, s&h, sequencer, etc) to get varying tones on each keypress or stage. of course, a joystick is also an excellent cv source for this too.

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