antilog

[Magnus Danielson]

From: "Paul Maddox" <paul.maddox at ndirect.co.uk>
Subject: antilog
Date: Thu, 10 Aug 2000 11:51:49 +0100

> Dear all,

Hi Paul,

>  Am I correct in assuming that when we use 'log convertors' on VCOs what we
> actually do is use an 'ANTI-LOG' function?

Yes, but you should really call them exponential converters or expo:s for
short. The they preform the "anti-log", i.e. the inverse function of log.
     
                  x
x = ln y <=> y = e

The function that we _really_ want is

       Vi
I = I 2
     0

So that we can use I to control the CCO core (Current Controled Oscillator).
Naturally, the above formula makes Vi to have the 1 V/oct scale.
I0 is the reference current of the converter and thus, a Vi of 0 V will give
the reference current and then the oscillator will oscillate as it base
frequency.

For scaled FM we have the formula modded into:

          Vfm     Vi
I = (I  + ---) * 2
      0    R

Thus, the frequency width of the lineare frequency modulation changes with the
pitch. It is the scaled FM that we tend to see all over in oscillators.

Then we have the unscaled FM, which will take the following formula:

          Vi   Vfm
I = I  * 2   + ---
     0          R

Thus, the frequency deviation does not change with pitch. This is what Juergen
proposed that one should use quite some time ago. It is a simple feat to achive
in a normal sawtooth based oscillator.

>   The reason I am asking is that Im trying to implement an anti-log fcuntion
> in C so that I might add pitch bend modulation to my new version of the OS
> for my MonoWave..

The above formulas should have you up on the right track, don't you think?

>   I've sound some quite heady maths, but was considering 'cheating' by using
> lookup tables and some approximation.
> Heres the page I found , its quite deep.
> http://www.iancgbell.clara.net/clara.net/i/a/n/iancgbell/webspace/maths/logs
> .htm#A0

Hmm... too tired to digest it right now... will look later.

What I *really* would like to see is the expo converter hardware implementation
in the DX7. All digital naturally.

Cheers,
Magnus