[sdiy] VCO and expo converter question

[Harry Bissell Jr]

I still say that the primary FM capability of 
expo controlled oscillators is er... expo.

The expo inputs invariably feature summing amps
allowing multiple CVs... and the linear FM inputs
are usually a single input (without summing)

but I do agree that just about every V/oct VCO
is actually a linear CCO and expo converter... so
the linear input comes for free.. and usually needs
only a single resistor to implement. So especially
in our DIY community... most folks are going for all
the capability they can, expecially for such a low
cost.

In the commercial synth world... often there is NO
linear FM capability. The Prophet V and ProOne do not
use it for sure... I'm not positive about the
Oberheims
but I think they don't either...

This comes from the modulation schemes... in a non
modular archetecture its costs a lot more to route
all the potential sources to the linear input...
whereas
you are sure to have hooked up the expo inputs.

Again... we avoid the question of linear vs expo
sound, because that has been done to death here and
I'm
not sure I'd favor one over another anyway  :^P

H^) harry



--- Scott Bernardi <sbernardi at comcast.net> wrote:
> The CEM and SSM based VCO's I've done (see links
> below) all had linear 
> FM.  I've double checked the datasheets for the
> CEM3340, CEM3374, and 
> SSM2033,  all chips I've built circuits around, and
> they says that the 
> FM inputs are Linear FM inputs. Maybe some
> commercial synths didn't 
> build their FM capability using these inputs.
> I did a short survey of VCO designs (mainly by
> looking at the schematics 
> of DIY sites I had links to) to see whether they
> provided linear and/or 
> exponential FM inputs:
> 
> My og2 CEM3374 based, liner FM  
>
(http://home.comcast.net/~sbernardi/elec/og2/og2_cem3374.html)
> My SSM2033 based, linear FM 
>
(http://home.comcast.net/~sbernardi/elec/og2/og2_ssm2033_vco.html)
> My Lockbox synth VCO, based on Ian Fritz' sawtooth
> VCO, linear FM 
>
(http://home.comcast.net/~sbernardi/elec/lockbox/lockbox_home.html#VCO)
> 
> ASM-1  
>
(http://home.swipnet.se/cfmd/synths/friends/stopp/asm1vco.new.pdf)
> Bergfotron 
> (http://hem.bredband.net/bersyn/VCO/vco_complex.htm)
> Electronics for Music (EFM) vco4d and vco4e  
> (http://www.ele4music.com/kits.html)
> Ken Stone  ASM-1 based (http://www.cgs.synth.net/)
> Jim Patchell's temp compensated VCO  
>
(http://www.oldcrows.net/~patchell/synthmodules/vco.html)
> Roman Sowa's quad VCO has both linear and
> exponential FM inputs  
> (http://www.sowa.synth.net/modular.html)
> RayLand VCO has a linear CV input (ac couple and it
> would be a typical 
> linear FM input)
> (http://atlas.csd.net/~rjwsoft/August2003VCO.html)
> Ian Fritz' sawtooth VCO has both linear and
> exponential FM inputs  
> (http://home.earthlink.net/~ijfritz/sy_cir2.htm)
> Rene Schmitz  555 VCO, both linear and exp FM 
> (http://www.uni-bonn.de/~uzs159/vco2.html) ;  VCO3
> linear FM 
> (http://www.uni-bonn.de/~uzs159/vco3.html), VCO4
> linear FM 
> (http://www.uni-bonn.de/~uzs159/vco4.html)
> Juergen Haible  JH-3 dual VCO both linear and exp FM
> 
>
(http://www.oldcrows.net/~jhaible/tonline_stuff/hj2vco.gif)
> 
> 
> I am not arguing the merits of linear vs.
> exponential FM, just that in 
> circuits I've seen, a linear FM input seems to be
> more common than an 
> exp FM input. Part of that may be just that you can
> use any 1 v/octave 
> exponential input to do exponential FM with, and so
> a dedicated 
> exponential FM input was not created.
> 
> harrybissell wrote:
> 
> >Hi Scott (et al)
> >
> >I was gonna offer to put on my flamesuit, since
> linear / expo FM has
> >been
> >such a ..er... hot topic ??? :^P   on s-diy.
> >
> >Of course almost every expo VCO has the ability for
> linear FM as well...
> >I'm thinking
> >mostly of commecial synths (maybe with SSM or CEM
> chips ?) as being
> >mostly
> >expo FM.  Since you get linear for free... it
> depends on whether the
> >designer brought
> >out the points to allow linear FM or not.  As a
> rule... the expo FM is
> >there (its just the
> >V/oct attenuated, as you said).
> >
> >OK I'll have to hook up my linear input on my old
> Aries VCO. I never
> >used it, since I
> >only had one oscillator at the time and used that
> jack for an octave
> >divider output instead.
> >Now that I have lots of oscillators I could try it
> :^P
> >
> >H^) harry
> >
> >Scott Bernardi wrote:
> >
> >  
> >
> >>Really? Almost every VCO circuit I have seen has a
> linear FM input. I
> >>guess you just would use a regular 1v/octave input
> to do exponential
> >>FM, just attenuate the signal.
> >>
> >>harrybissell wrote:
> >>
> >>    
> >>
> >>>hmmm... I'd have said that expo FM is a lot more
> common than linear
> >>>FM...
> >>>(especially in commercial synths)... but wht not
> have both in this
> >>>case
> >>>?
> >>>There are advantages and disadvantages to both
> linear and expo fm
> >>>:^P
> >>>
> >>>
> >>>The nonlinearity of a 'loaded' pot is often not
> noticible unless you
> >>>have some
> >>>panel graphics that make it obvious...  Making
> the summing resistors
> >>>bigger
> >>>should be OK.
> >>>
> >>>H^) harry
> >>>
> >>>ryan wrote:
> >>>
> >>>
> >>>      
> >>>
> >>>>nice tutorial. thanks!I'm sure glad I asked here
> before getting the
> >>>>PCB made. I thought that Exponential FM would
> have been more
> >>>>typical,
> >>>>thats why I didn't add Linear FM. Yesterday was
> actually the first
> >>>>time I tried putting something into the FM jacks
> on my synth.
> >>>>well,
> >>>>Before yesterday, I had only 1 VCO. the Idea for
> this VCO is to be
> >>>>a
> >>>>simple, fit on a small PCB, and take up little
> panel space. I have
> >>>>Oakley Sound VCOs for my main oscillators, but
> I'd like to get this
> >>>>one to stay in tune with those. for the pots.
> I'm using 100K pots
> >>>>on a
> >>>>100K load. arg. I already spent a bunch of $ on
> those fancy
> >>>>spectrol
> >>>>100k pots. I think it'd be easier to just double
> the values of all
> >>>>the
> >>>>other resistors. I think that would atleast
> reduce the warping
> >>>>enough
> >>>>and the 10K trimmer for V/Oct would be more
> centered around what
> >>>>the
> >>>>value should actually be. I looked at that java
> applet, It looks
> >>>>pretty nasty the way I have the circuit now.
> about the coarse
> >>>>adjustment, I suppose +/-5 octaves would be alot
> more useful. meant
> >>>>to
> >>>>change that. thanks !ryan
> >>>>
> >>>>     I've got a tutorial on pots at
> >>>>    
>
http://home.comcast.net/~sbernardi/elec/og2/partsub_pots.html.
> >>>>
> >>>>     In general to avoid loading effects, you
> want your voltage
> >>>>     divider pots to be 5 to 10 times less than
> the resistance
> >>>>     that loads them. So for a 100K input
> resistor, use a 10K up
> >>>>     to 50K pot. To see the effect of loading, I
> have a link to
> >>>>     Chris List's java applet that plots loading
> effects.
> >>>>     You might want to decrease R2 a bit to give
> yourself more
> >>>>     Coarse freq adj range. As you have it,
> you'll only get 6
> >>>>     octaves. Using a 300K will give you a 10
> octave tuning
> >>>>     range.
> >>>>     The FM input you have is feeding the
> exponential 
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