[sdiy] 1V/OCT

[Scott Gravenhorst]

Tim Parkhurst <tparkhurst at siliconbandwidth.com> wrote:
>Hi Guillaume!
>
>Yes, you basically have it right. In a 1V/Octave synth, changing the control
>voltage to the VCO by +1 volt will raise the VCO frequency 1 octave. I don't
>really know if there is an established "middle frequency", although if you
>wanted to cover the entire audio range you might try
>
>0V = 20Hz
>1V = 40
>2V = 80
>3V = 160
>4V = 320
>5V = 640
>6V = 1280
>7V = 2560
>8V = 5120
>9V = 10240
>10V = 20480
>
>Most VCOs use a CV range of 0 to +10 volts, and many will respond to
>negative control voltages by producing even lower frequencies. Another name
>for 1v/oct is "exponential response". This is the standard used by most
>analog synthesizers. The other type of response is Volt/Hertz or "linear
>response". This is the scheme used by some of the earlier Japanese synths
>(early Korg like the MS-10 and MS-20, Roland like the 100 series). With a
>linear VCO, you might get something like 5V = 500Hz, 6V = 1000Hz, 7V =
>1500Hz, 8V = 2000Hz and so on. Linear response was also used in the first
>Moog Taurus pedals and is still used in the PAIA Fatman. Linear VCOs are
>easier to build, but they have several disadvantages - the most obvious
>being that they are not transposed as easily. Think about two or three VCOs
>in a system, each running at a slightly different frequency: with
>exponential VCOs, you can transpose all of them up one octave by simply
>adding 1 volt to the CV; with linear VCOs, each VCO will have to receive a
>different CV to move it up one octave. This is not to say that linear
>response systems are not useful. It's just that exponential response is much
>easier to work with (from the user's point of view), especially in a modular
>system.

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.

>Hope this all makes sense. Like I said, you basically have it right. 
>
>
>Tim (easily transposed) Servo
>
>"Imagination is more important than knowledge." - Albert Einstein
>
>
>
>
>> -----Original Message-----
>> From: Guillaume Fairfield [mailto:gfairfield at rogers.com]
>> Sent: Wednesday, March 10, 2004 11:06 AM
>> To: synth-diy at dropmix.xs4all.nl
>> Subject: [sdiy] 1V/OCT
>> 
>> Hello all,
>> 
>> I'm also new to this whole synth building business and am trying to
>> build my first vco. I think I have a plan but I'm unsure of to the whole
>> 1V/OCT thing.
>> What's the standard? Is it that +1V gives an octave higher or lower? And
>> what's the middle frequency? In other words what frequency represents
>> 0V? Maybe I'm missing something but that's the biggest piece of the
>> puzzle left, apart from all the math.
>> 
>> Thank you for this list...I think I'm gonna learn alot here.
>> 
>> guillaume
>> 
>> 
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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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