VCO +- output, was [sdiy]linearize OTA for VCF?

[patchell]

TooManySynths wrote:

> Hi All,
>
> > Since my VCO's generate + and - 5 volt waveforms,
>
> Is this a quasi defacto standard.  In barry klein's
> 3340 design he comments that the 3340 outputs 0 to 10v
> and uses opamps to offset the outputs to +-5.  Does
> the 3340 output 0 to 10 because it's easier to design
> the chip that way?  I assume since it was designed to
> be used as part of commercial synthesizers that most
> designs would AC couple the VCO to the VCF and it
> wouldn't be an issue.  On my own 3340 triple VCO
> module I am using opamps to scale to +-5.  I presume
> that this is the "right" thing to do.
>

    The reason you need to level shift the output is so that it will have a "0
volts" dc bias.  If you have any DC bias on your waveform, this will show up as
a thump from your VCA's and VCF's if they are DC coupled.  If all of your
modules are AC coupled, it probably doesn't matter.

    DC coupled PWM is sort of a problem, as the DC offset is 0 only at 50%
pulse width (assuming +/-5 volt swing).

    There are also other reasons why VCO outputs should go between +/-5.  But
my blood sugar level is low right now and I can't remember...

>
> So what about other modulation sources, or using a VCO
> as an LFO?  It seems that other devices, such as most
> EGs typically start at zero, go up, and end at zero.
>

    LFO's are basically like a VCO.  Eneveolope generators are different.  They
don't generate signals you listen to, but rather, control the dynamics of other
modules.

>
> I understand that all can be useful, and that all can
> be changed to one form or another with offset and
> scaling, but, when building modules, what should I
> standardize on?  Should my LFOs also contain the same
> offset scaling of my VCOs?  Should I not offset my
> VCOs?
>

    My personal preference is that all signal generators put out +/- 5 volts.

>
> Thanks
> Daryl
>
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--
 -Jim
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