[sdiy] monolithic VCO chips for FM??

[synthplayer88 at spymac.com]

On Fri Aug 20 18:31 , James Patchell <patchell at cox.net> sent:

>A pitch source is something that will produce a musical pitch.  Pitch is 
>what your ears perceive, and is analogous to frequency (believe it or not, 
>they are not the same...).
>
>In order to be used as a pitch source, a VCO must be able to produce the 
>pitches in a predictable manor.  Exponential VCOs, for example, are 
>expected to change their pitch by one octave every time the control voltage 
>changes by 1 volt.  A linear VCO needs to change its pitch by one octave 
>every time the input voltage is doubled...
>
>Now the problem with many of the IC oscillators that were not really 
>designed to be pitch sources (such as the XR2207, XR2206, ICL8038) is that 
>they don't always perform as stated above....all of them are linear 
>oscillators, so when you double the control voltage, the pitch should go up 
>by exactly 1 octave...but you will find that is not the case.  I can't 
>remember, off hand, just how much they can be off, but, we are talking 
>about close to 0.5 to 5%.  Being off by 1% is about 17 cents, which is a 
>lot.  The XR2207 is good to only about 5%, and that makes it completely 
>useless for producing music.  I know.... I built a VCO using this part a 
>long time ago (30 years), and discovered that there was just no way to use 
>it.  The ICL8038 has about 0.5% error, so it will be off about 8 cents, 
>which is better, but you know, when my piano is out by 5 cents, it is time 
>to call the piano tuner.
>
>For a pitch core, you need something that is down around 0.01%.  The ASM-1 
>(Terry Michaels) VCO fits this bill.  With an exponential converter, I can 
>easily get oscilators that tune over 8 octaves with about +/- 1 cent 
>deviation (which is about the limit of what I can measure.
>
>There are VCO chips that you can use, but none of them are any longer in 
>production.  The one I liked best was the CEM3340.  A very nice VCO chip.
>
>Now, if creating musical pitches is not a requirement, then any of the VCO 
>chips listed above will work.
>
>Hope this answers and clarifies...if not, I am willing to try 
>again...:-)....hopefully I can save you from having to relearn everything I 
>did...
>


Hi James,
That was great!!! Thanks for sharing your experience/insight with us! :)

So I guess the exponential VCO doubles the frequency output in 1 Volt input
increment(eg.1v,2v,3v,4v etc..) and linear VCOs works on doubling the input
voltage (eg.1v,2v,4v,8v etc...), therefore it could be a problem if you were to
trigger the VCO with a keyboard with CV out since the frequencies would be out of
place in the next octave.

BTW, are the error consistent in every octave? or is it accumulative on each
octave as you go? 

Nevertheless, I do see ways around that if I was to build a keyboard use these
chips just need to add a trimmer or a potentiometer on each key to null for the
correct frequency but to do that digitally.................would be quite a
challenge! :)

I think all modular control keyboard should have a pot for each key to fine tune
the control output voltage in order to nail the note frequency for precision, may
be this is already available and I am just unaware of this? or may be in midi? 
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