square to ramp

WeAreAs1 at aol.com WeAreAs1 at aol.com
Sun Mar 12 22:59:39 CET 2000


Paul Perry, tugging wistfully at his long gray beard, noted:

<<..hmmmmmmm.... a 1v/oct CV is certainly NOT proportional to frequency. >>

...and Moxie, true to his name, and quick as a whip, quipped:

<< No, but to pitch. D-uh! :-)
/Moxie (Pitch is not frequency) >>

Well, it depends on what your definition of the word "is" is...

Yes, it's true that pitch is not directly proportional to a linearly rising 
CV such as 1volt per octave.  And, strictly speaking, it could also be said 
that frequency is proportional to a linear control voltage (at least for a 
linear VCO).  But these definitions are beside the point.  For the purpose of 
our discussion, it's important to note that a 1v/oct CV would be easiest to 
use for the purpose of controlling the amplitude of the ramp wave.  That's 
why I said "a 1v/octave CV will work, if you already have it."

If you use a steady, non-changing CV to control the charge rate of the ramp 
integrator, then as you go higher in frequency with your square wave input, 
the ramp amplitude will get smaller and smaller.  You will also note that the 
ramp's amplitude will not be linearly proportional to the frequency of the 
square input.  In fact, the ramp's amplitude will have an anti-logarithmic 
relationship to the square's frequency (with smaller and smaller amplitude 
changes as the pitch goes higher and higher).  Therefore, to compensate, you 
will need to change the CV input of the ramp integrator with a CV that has a 
logarithmic relationship to the input pitch.  In other words, you need a 
circuit that inputs a voltage proportional to the distance from the end of 
the keyboard, and outputs a voltage that is proportional to the corresponding 
frequency of that particular note.

Since this CV must also take into account not only keyboard pitch, but also 
any additionally summed modulation voltages (LFO's, pitch bend, octave 
ranges, etc.), it's best to get this CV from a trimmable, temp. compensated 
exponential amplifier.  That's why a linearly rising 1v/octave keyboard CV 
would be useful - and probably easier to use for this purpose than, for 
instance, an exponentially rising keyboard CV, such as the one that comes 
from the key CV output of a Korg MS20.

I hope I've explained this well enough to not add further confusion.  If not, 
I refer you again to the Juno 60 service manual.  And I apologize for any 
past and future misappropraitions of the terms "frequency" and "pitch".

BTW, did any of the manufacturers ever get a patent on this particular 
embodiment of the square-to-saw DCO concept?  If so, I'd like to look at that 
patent.  Such a patent might contain a definitive explanation of the concept 
(and would certainly be more clear than my muddled prose).  If anyone knows 
of such a patent, please let me know.

Michael Bacich




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