VCDO / RC4151

[Debby and Gene Stopp]

Somewhere in the archives is the story of my 4151 ordeals with the Chroma
circuit.

I did end up making a dual Chroma-clone VCO with a 3046 plus two tempco's,
and it ended up taking up some blank panel space in my old Moog system.
These are fine for music VCO's - they track perfectly well over the audio
range. A couple of caveats - first,  the sawtooth has a flat spot on the
bottom of the waveform, a space between the ramps. This changes over the
frequency range, but I don't remember exactly in which direction. I suspect
the ramps get closer as the period decreases. The flat spot is a
considerable fraction of the waveform, like 40%. Also, the low end is
limited to a few hertz, rather than .001 Hz or something like a good VCO
should have (VCO snobbery in action here). Timbrally this is not the same as
a perfect sawtooth, but probably good enough when hidden behind a VCF and
blended in with other voices to boot. Heck, I'm sure it has a timbral
identity of its own which endears Chroma fans.

Note that neither of these issues are a bad thing when it comes to driving a
waveform ROM. But those who are looking to the Chroma VCO as a modular
system building block, beware - this one is fine for pitched synth tones,
but won't do for wide-range applications.

Which leads me to some comments on the current VCO thread - I don't have any
strong opinion here, I'll just throw out some ideas...

The ASM-1 VCO (which actually shouldn't be called "Gene's VCO", it's really
"Terry's VCO"!) has a really kick-ass perfect sawtooth from sub-sub audio to
well beyond 20K. This is why I picked it for the ASM-1. It's just so sharp
and clean.

However, I think that if I were to offer a VCO as a PCB, I'd probably go
with the EN Tri-Square using 3080's. The odd-harmonic waveforms are easier
to obtain (tri/sine/square) and it's important that they be symmetric (at
least in my mind) which may not be as easy using a sawtooth-based core.

- Gene



-----Original Message-----
From: WeAreAs1 at aol.com <WeAreAs1 at aol.com>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Cc: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>;
harrybissell at netscape.net <harrybissell at netscape.net>;
104065.2340 at compuserve.com <104065.2340 at compuserve.com>;
Dave.Peachey at natwest.com <Dave.Peachey at natwest.com>; zumchak at cerg.com
<zumchak at cerg.com>
Date: Thursday, April 08, 1999 7:45 PM
Subject: Re: VCDO / RC4151


>As Terry Michaels pointed out, the RC4151 is second-sourced by Exar, as the
>XR4151.  You can pick up a pdf datasheet at:
>http://www.exar.com/products/xr4151.html
>
>It's an 8-pin IC, and described as a voltage-to-frequency or
>frequency-to-voltage converter.
>
>I found it today in a fairly recent Jameco catalog as Jameco part #29372.
>The price is $1.19 each.
>
>It should be noted, however, that the Rhodes Chroma didn't use the 4151 as
>its actual oscillator.  It used it to perform the reset function on a more
>traditional temperature-compensated, expo-driven, opamp-based sawtooth
>integrator (the service manual refers to it as the "retrace" function - is
>this the same thing as reset?).  The purpose for the 4151 was to ensure
that
>the sawtooth would reset with the sharpest possible edge, and have no
>high-frequency pitch droop.  This is what makes me think that it might work
>well as a wide-range voltage controlled high-frequency clock (with the
>addition of a proper saw-to-square comparator, of course)
>
>I'll try to draw up a schematic of this VCO later tonight, and mail it to
>those who would like to see it - or post it to the list, if it's
sufficiently
>small.
>
>Michael Bacich
>
>
>