Gene's DIY OB-8/CS-80 Ribbon Controller

[James: Music Control]

This is all very cool stuff...... but does any one know how to 
repair or re-make a ribbon which is faulty
James walker from Music Control Audio Services ltd .U.K.
Service eng .

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> From: WeAreAs1 at aol.com 
> To: borg0 at jps.net, synth-diy at mailhost.bpa.nl 
> Subject: Gene's DIY OB-8/CS-80 Ribbon Controller 
> Date: Sat, 10 Apr 1999 07:04:54 EDT 
> 
>Regarding his DIY OB-8 ribbon controller, Gene Stopp wrote:
>
><< And the ribbon - I'll try it like this for a while (linear seemed
intuitive
>to me). Does a CS-80 ribbon have a non-linear response, like as you go
>towards the higher frequencies you need more distance travelled for the
same
>musical interval? >>
>
>It's been a while since I last played a CS-80, so I don't remember for sure

>about the linearity of the ribbon's pitch response.  The ribbon is two 
>octaves in physical length, and I'm pretty sure that its pitch pretty
closely 
>matched the pitch of the keyboard notes that were right below it.  That is
to 
>say, you could closely approximate chromatic melodies (Theremin-style), 
>simply by sliding your finger up and down to the location above the
keyboard 
>notes you wanted to play.  Again, it's been a while, so I could be wrong 
>about this.  However, I do have schematics, so here's some tech info on the

>ribbon circuit:
>
>Each CS-80 voice has a crude exponential amplifier (in order to achieve the

>keyboard-position-matching response), since the Yamaha VCO's had linear CV 
>response.  This exponential amp is used to provide the VCO's with the
ribbon 
>CV and octave transpose CV (there's a separate tuning trimmer for each of
the 
>four octave ranges and another trimmer for the ribbon scaling, though, 
>eliminating the need for a real accurate converter).  This CV is fed into
the 
>"FT" input on the VCO IC's, not their Keyboard CV input (which has linear 
>response, and gets its CV directly from the 8-channel D/A output via some 
>S/H's).
>
>Strangely enough, though, the CS-80 ribbon doesn't feed this expo amp 
>directly.  It connects to one of the Yamaha custom IC's, a custom 8-channel

>D/A chip called a YM27600, and sums with the octave transpose info at that 
>point (I think... the documentation is confusing as heck), then it goes on 
>through some S/H's, then to the expo amp/octave select gates, then to the 
>VCO's.
>
>However, the most interesting thing about the CS-80 ribbon is its ability
to 
>make wherever you first place your finger the "zero" pitch point.  For 
>instance, you can put your finger down at the middle of the ribbon, and
then 
>you can bend up/down one octave in either direction sarting from there, or 
>you can put your finger down at the top of the ribbon, and that becomes the

>zero point, from which you can bend DOWN the entire ribbon's two-octave
range 
>(or conversely, UP two octaves, from the bottom of the ribbon).  This also 
>means that you don't ever have to be real careful about exactly where you 
>place your finger when you first touch the ribbon (the pitch won't "jump"
to 
>your finger's absolute position, it just thinks of that position as the new

>relative zero point).  It does this with a clever little pair of simple FET

>S/H circuits and a pair of opamp comparators.  I'd be happy to send you a 
>drawing of this circuit if you're interested in trying it on your homemade 
>ribbon.  It's not very complicated at all, and could probably even have a 
>switch to disable/enable its "finger position memory" function, if you
wanted 
>to read absolute position.  And since your OB-8 VCO's are exponential, you 
>could also have a much larger than two octave range, if you wanted.
>
>Corrections from CS-80 experts encouraged...
>
>Michael Bacich
>