[sdiy] 1v/oct and other scales

[Magnus Danielson]

From: "Toby Paddock" <tpaddock at seanet.com>
Subject: [sdiy] 1v/oct and other scales
Date: Wed, 24 Nov 2004 12:08:52 -0800
Message-ID: <20041124200317.M94223 at seanet.com>

> I have a feeling this isn't going to make sense, but...
> 
> If I want to control a 1v/oct synth to a scale made up of ratios (1/1, 5/4, 
> 3/2,... 2/2 or something like that), is there a simple fromula to help me 
> find what voltage to add?

Yes. Let's say you want to know how much voltage you need to add for the
frequency ratio 5/4, then it is log2(5/4) where log2(x) = log(x)/log(2).
That's all you need to know.

> I know for a regular scale I'd just add 1/12 volt 
> per note. But what about crazy intervals?

Toss it into the formula! Be crazy.

> And the rest is just blabbering...
> 
> I don't have access to any hardware right now or I could just try it and 
> measure. Also, I don't know how a scale based on fractions sounds. Should 
> look good on a scope though.
> 
> Also also, I'm wondering about adding these voltages. For a normal scale, I 
> could add and subtract voltages like 1/12v + 3/12v - 2/12v... and it would 
> jump around on notes and sound OK.
> 
> But if I add voltages needed for ratios like above, would the ratios 
> multiply?

No. It's logarithmic, so it is just different addition terms.

> Would it sound nasty?

Probably not, but then, there are no guarantees. ;O)

> Should look good on a scope though.
> 
> Maybe it's as simple as converting a ratio to octaves, and multiplying that 
> by 1V?
> oct= log(ratio)/log(2)        (either log or ln)
> 
> 1/1 -> 0V
> 2/1 -> 1V
> 3/2 -> log(3/2)/log(2) -> ~0.585V
> 
> Or am I missing something?

No.

Cheers,
Magnus