[sdiy] Q: oscillator bank

[Scott Gravenhorst]

"john mahoney" <jmahoney at gate.net> wrote:
>----- Original Message ----- 
>From: "Mike Hall" <m3ha11 at rcn.com>
>
>
>> Just a general question about making an oscillator bank and
>> using only one exponential converter to drive the oscillators:
>> Is it possible?
>>
>> My thoughts on this:
>> - sum the various control voltages: keyboard key, plus modulation,
>plus
>> pitch-bend
>> - apply to the exponential converter
>> - use the converted Hz/V value to drive several oscillators
>> - each oscillator has octave switching via a resistive divider
>> - each oscillator (but one) has scaling to allow relative tuning +/-
>> some interval
>[snip]
>
>I'm glad that you asked, because I don't understand how octave
>switching and relative tuning work with linear oscs.
>
>With 1V/oct expo VCOs, octave switching is done with 1V increments.
>Relative tuning also uses certain voltage increments -- for example, a
>certain voltage will always raise the pitch a fifth. These voltages
>work no matter what the initial pitch of the VCO may be.
>
>With linear oscs, the voltage to increase from frequency X to
>frequency X+N varies depending on the values for both X and N. That's
>why I've never understood how octave switching worked with linear
>VCOs. Am I confused or has this problem been glossed over?
>--
>john (confused one way or the other)


The buffered linear voltage supplies current through the various
scaling pots to summing nodes, one for each linear VCO.  If the
resistance of one VCO is twice that of another VCO, the oscillators
will stay 1 octave apart regardless of the input CV.  For intervals,
all that matters is that the pots represent the ratio of the
frequencies required.  Double the voltage, the current doubles through
each of the input resistors (pots), so the VCOs will track each other
at the interval set by the pots.


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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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