AW: Re: [AH] building an analog keyboard controller for a

[Haible Juergen]

>>>trim pots for tuning each key)
>>
>>Why go to the expense of trimpots in this situation?, if you use carefully
>
>The original Paia keyboards were exponential, for use with their linear
>oscillators. henvce the keyboard had to have precise, non-equal (and
>non-standard) resistor values for each key! The trimpots were essential.
>Jeez, I hate to think how that would sound connected to traditional
>exponential oscillators - hopefully the builder understood this before
>building!


I don't know the Paia circuit the original poster was refering to, so let me
answer in a more generalized way (and when I'm raving against trimpots
later on it's not against Paia. (;->) )

You do not need a bunch of trimpots, neither for a V/Oct nor for a V/Hz
keyboard. You need one or two for setting the scale and offset of the
whole keyboard, but surely not one for each note.

The V/Oct case is easy - all resistor values are equal.

The V/Hz case is increadibly tricky at first glance - until someone found
an increadibly brilliant solution. "Someone" in that case were the Korg
engineers. It's patented und thus disclosed (search for Korg or Keio on the
ibm patent server). Or just get a copy of the Korg MS-20 service manual.
Forgot to mention, you don't get away with one resistor value. 3 different
values are required if memory serves.

This thread brings up unpleasant memories. The first "synthesizer" circuit
I've seen in my life was the "Plastik Sound" article in a (probably long
defunct) German electronics magazine called "ELO".
What did they do ? Used a cheap and dirty CD4046 PLL chip for the
VCO, which is "almost" linear (V/Hz), but just almost. Then they built
a keyboard with a series string of trimpots for tuning each note
individually.
To make things worse, tuning one note would change the tuning of all
notes above (or below?) this one as well. As if their design goals were
"make it as bad as possible" and "show the newbie how complicated
a synthesizer design really is". The latter was exactly the effect it 
had on me. Instead of writing pages and pages about how demanding
their diy project was, and how far it went beyond the normal scope of the
magazine, they should have spent their energy on digging up a circuit
that had better features in the first place.
Why do I sound so angry ? Because it set me back to "admiring
mode" rather than "building mode" for quite some time. 
And the prejudice against V/Hz systems lingered in my head for
two decades.

JH.