[sdiy] 1V/OCT

[Scott Gravenhorst]

Oh yeah, I discovered a neat way to LFO modulate a FatMan (555 based) VCO. 
The pin 5 input on the 555 will modulate the VCO with the same results as an
expo VCO (equal modulation regardless of frequency).  But this technique
works only on the 555 or a VCO where the set point for reset is a voltage
input.  This comes at the expense of a slight change in VCO amplitude over
the span of the LFO.  I, however, do not notice this effect as audible at
all.  It is the _one_ thing I do like about the FatMan VCO design.  No VCA
trick required to do equal LFO modulation of the VCO.

Tim Parkhurst <tparkhurst at siliconbandwidth.com> wrote:
>> -----Original Message-----
>> From: Scott Gravenhorst [mailto:music.maker at gte.net]
>> 
>> Hmmm, well I disagree about the transpose point.  It happens in my 3 VCO
>> FatMan with one pot.  It's just an inline rheostat to set the current for
>a
>> given CV.  IMHO, the real drawback of linear is that an LFO (for example)
>> used to modulate pitch has a different effect depending on where the VCO
>> "center" frequency is (center as in center of the effect created by the
>LFO).
>>  For example, if you set the LFO amplitude to give a nice effect at say
>220
>> Hz, it will be practically unnoticable at 3520, only 4 octaves higher.  If
>> you set it for a certain effect at 3520, it will then modulate far more
>> severely at 220.  Expo allows you to just sum the LFO with the pitch CV
>and
>> the modulation effect is the same everywhere in the range of the VCO.  In
>the
>> linear world, there are ways to handle this, however, basically, the LFO
>> voltage is run through a VCA (controlled by the pitch CV) first.  The VCA
>> output is then summed with the pitch CV and applied to the linear VCO.
>The
>> effect will then be the same as expected with an expo VCO.
>> 
>> 
>
>Yeah, I basically meant the same thing (I was assuming the method of
>transposing the VCOs would be via an applied CV), but I like your example
>with the LFO a lot better.
>
>Tim Servo
>
>---
>Outgoing mail is certified Virus Free.
>Checked by AVG anti-virus system (http://www.grisoft.com).
>Version: 6.0.616 / Virus Database: 395 - Release Date: 3/8/2004
> 
>
>
>
>***************************************************************************
>This message is confidential.  The information contained in
>this e-mail message is intended only for confidential use of the
>recipient(s) named above. If the reader of this message is not the intended
>recipient or an agent responsible for delivering it to the intended
>recipient, you are hereby notified that you have received this document in
>error and that any review, dissemination, distribution, or copying of this
>message is strictly prohibited.  If you have received this communication in
>error, please notify us immediately by e-mail, and delete the original
>message.
>***************************************************************************
>
>

=========================================================  
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.