How to make a 1V/oct VCO hz/oct

Liquids · Joined: Jun 06, 2011 Posts: 23 Location: CT, USA

While this may sound crazy in general, since linearly controlled VCOs are mostly a nuisance to the predominantly 1V/oct VCOs that are to be found everywhere...right now it's easier for me to work with hz/v linear VCOs.

Certainly an anti-log amp to take a exp 1V/oct CV and make it hz/v can be found in the Korg MS-02 and other sources...

But I am already starting with a tunable hz/oct CV, so I'm looking to make the multitude of VCOs out there available to me to work with.

For example...

Scott Gravenhorst took Rene's simple CMOS VCO which is designed as 1V/oct:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

...and tweaked it so it became a hz/oct VCO:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

I'm not an electronics expert, but have been working with audio electronics for a few years, so I'm not a complete newbie. However, I'm not at all versed in the VCO realm, and I have a lot of holes where typical synth-DIYers might have a lot of understanding...

If anyone can explain some background to me on how to understand and or approach taking a VCO that is designed and anticipating 1V/oct to output audio and work with it to be more friendly to hz/v CV, I'd be more than appreciative!

I think typically people would be asking this question with as specific VCO that they want to control, maybe no more. To be honest, I don't have such a VCO, more just looking for theory, but if it helps, I'll pick Thomas Henry's VCO-1 since I like OTAs:
[img]http://www.birthofasynth.com/Thomas_Henry/pdf/VCO-1/vco1_schem1.pdf[/img]
Partof me wants to think that it might be as simple as eliminating all the circuitry around IC2b and 'jumpering' the connection that would hit the base of Q6 and feed it right to the OTA's control pin via Q6s collector.

Is it that simple here, or not at all? Most VCO schematics have something that seems to take the CV and make it exponential...and yet the input CV is assumed to be 1V/oct which is already exponential, no? This is part of my confusion...

blue hell

Liquids · Joined: Jun 06, 2011 Posts: 23 Location: CT, USA

Unfortuanetly I am still in the dark about how to take something like Ray Wilsons current VCO and make it linear. Even though it looks like he has a 'linear CV' input. Maybe that would work just fine with what I have?...

I should say what I have - I have a CV out that when playing a low E on guitar (82hz) as .5, one octave up is 1v, octave up from that is 2v, next octave is 4v. etc. Highest note is a high D just below the E that would be 8v - that high D is 7v. That's my max voltage output. As you can see, the relation is exponential (since it starts with pitch rather than voltage), when it's tracking correctly...and is only getting better from 'mostly perfect' above the 6th fret and 'sometimes finicky' below.

But I digress...

What's strange is...I have an EXPO CV from my pitch to voltage...
So I need a VCO with a LINEAR input? =^) I'm not inputing a linear CV, I'm inputing a expo CV....
And jacks marked 1V/oct typically mean, input a hz/oct relate voltage that will become 1V/oct aka expo...
But I understand the terminology....

Any further help appreciated....the article was something I'd seen, but I don't really get how to flesh anything out when it comes down to brass tacks.

I apologize that the guitar is my reference- I have a old school CV keyboard and ray's 1V/oct single buss keyboard PCB all rigged too, but I play monophonic guitar melodies better than I do monophonic keyboard melodies...I mean, I have played guitars longer, but I like and use, and compare both.

blue hell · Posted: Thu May 31, 2012 9:37 am Post subject:

1 Volt / Octave is linear per octave, it is the mapping of octaves to frequency that is exponential (and so the mapping from voltage to frequency is expo, but to note names is linear):

so for expo ... say 0 Volt = A = 440 Hz, then 1 V = A1 = 880 Hz, 2 V = A2 = 1760 Hz .. you see the A number linear with the the voltage but the frequency going exponential.

A VCO with linear CV control OTOH works like :

0 Volt = 0 Hz, 1 Volt = 500 Hz, 2 Volt = 1000 Hz, 3 Volt = 1500 Hz (just an example, no idea how an actual scaling would be, just that for every Volt of increase in the CV the same amount of Hz will be added).

Liquids · Joined: Jun 06, 2011 Posts: 23 Location: CT, USA

Thanks for the reply. I do understand the difference between hz/oct CV and 1V/oct....if 100hz is 1V, hz/oct CV will have 300hz at 3V and 400hz at 4V; 1V/Oct would instead have 400hz at 3v and 800hz at 4v.

That said, as to the inner workings of separating a VCO from the expo converter and such, as the vast majority of VCO designs are assuming your average keyboard as a controller, it seems the typically circuit starts by attenuating the voltage, mixing in ofset voltage for tuning, and then, converts that stages output voltage to an exponential current converter (feel free to correct me where I am wrong or I'm using the wrong lingo)...

Exibit 'A' is Ray Wilson's current VCO design, which looks as if it it has a 'linear' input and would accept a voltage source that is already 'expo':

http://www.musicfromouterspace.com/analogsynth_new/VCO20090724REV0/VCO20090724REV0.html

or directly, http://www.musicfromouterspace.com/analogsynth_new/VCO20090724REV0/images/VCO20090724schempg1.gif

But I also suspect that the 'lin' input is there more for something like a pitch wheel than the kind of CV I have, or a MS-20's keyboard controler...not certain either way.

Exibit B is The Korg MS-20 schematic...known to be hz/oct as far as I know....

http://www.korganalogue.net/korgms/images/service/ms20/circ1.gif

I think that the assumption on this VCO is a keyboard that puts out a hz/oct CV.

It might be a stretch to say I have ANY understanding of VCO schematics, but that being said, I still see a transistor pair that looks common to all 1V/oct VCOs in the MS-20 VCO. I thought such a transistor pair was typically not only doing the work of V->I but doing so exponentially...so at that point, I'm lost in how this is a hz/oct VCO or what makes it different.

By comparison, Scott Gravenhorst tweak of Rene's CMOS VCO uses, what I'm told, is a linear V-I converter....it's simple transistor (darlington) in the feedback loop of an op amp, with a diode....

http://home1.gte.net/res0658s/fatman/VCO_pwm_tri_suboctave.html

Of course it's easier for me to look at this one since I can examine the original 1V/oct VCO circuit, and see that some components were removed from the oriinal 1V/oct version (why? I have no idea), as well as utilizing a different V-I converter than the 1V/oct version....

I don't imagine that taking a 1V/oct VCO design and making it hz/oct is always if ever an easy task, but I'd love to begin to learn how to do it, if not merely for the sake of deepening my general understanding of electronics, oscillators, etc, and maybe someday even intelligent implementation based in understanding (dare I be so optomistic).

prgdeltablues · Posted: Fri Jun 01, 2012 2:00 am Post subject:

I *think* the difference is in the topology of the circuit around the first transistor in the pair.

If you look at the V/Oct circuits, the CV input voltage is connected to the base of the NPN. The output current (via the current mirror of the second NPN) is taken from the collector. It's a property of bipolar transistors that the collector current has an log relationship to the base voltage, so this is a log/expo V-I converter.

In the Hz/V circuit, the CV in is connected to the collector of the NPN, effectively becoming the supply voltage. The output current is taken as before from the collector, and is determined not by properties of the transistor but by the external resistance - ie Ohm's law applies, and there is a linear relationship between V and I.