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.
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