Tri 2 Sin, plus Hal Chamberlin book plug

[Sean Costello]

Fraser, Colin J wrote:

> Me, too, but I'm kind of wondering why folks needing a pure sine vc
> oscillator wouldn't just use a temp. compensated self oscillating
> filter.

An oscillating filter, as far as I know, doesn't maintain a steady
signal level - the level of the sine depends on the pitch.  Mind you,
this is for a lowpass filter - using a few allpass stages, with clipping
diodes in the feedback stage, a great quadrature oscillator can be
generated.  The IBM patent site has a Harald Bode patent for a
quadrature oscillator using this design, that is intended for frequency
shifters.

Another good reason:  Having a seperate sine wave output in an
oscillator allows you to really beef up the bass of a sound.  Run the
sawtooth output through a filter like usual, then mix the filter output
with the sine output before sending it through a VCA.  The sine wave
adds bass that was taken away from the signal by a highly resonant
filter.
 
> If you needed other frequency and phase matched waveforms, what about
> building a vco around a self-oscillating filter core, derive a square
> wave from the sine and generate a saw wave by using the leading edge of
> the square to reset a cap being charged by another expo current derived
> from the pitch cv.

I think that, because of the erratic levels of the sine wave, the square
wave wouldn't be "pure," which would mess with the sawtooth shape.  On
the other hand, who cares?  It would probably sound great.  Use a
temperature compensated expo current generator that also allows linear
FM, and you've got what would seem to be an all-purpose module, useable
for both filtering and oscillation.  If you needed steady signal levels,
use clipping diodes in the feedback stage.  [Warning:  I may not know
what I am talking about, so anyone that can point out the errors in what
I wrote, feel free to do so.]  It wouldn't be a good source of sawtooth
and square control signals, but it would be good for audio waveforms.

A few questions for all you synth-DIY gurus:  Could you create a lowpass
filter module that does through-zero FM while resonating?  I know that I
read in some Electronotes issue about modifications to the 4-pole
lowpass filter (4 OTA+buffer lowpass stages with feedback around whole
circuit) that allowed the sine wave to be synced; I think it involved
using a FET in conjunction with the capacitor in the last lowpass stage
that would drain the capacitor when triggered.  Now, it seems that
through-zero FM involves a combination of oscillator sync and waveform
reversal.  Anyone have any ideas?  It would be so cool to have a module
that could be a filter, and a through-zero linear FM quadrature
oscillator.

Anyway, back to work.

Sean Costello

P.S.  I just found the 2nd edition of "Musical Applications of
Microprocessors" by Hal Chamberlin down in Portland, Oregon.  What an
amazing book!  It explains analog synth circuitry better than any other
single book I have (with the possible exception of the Electronotes
Musical Engineers Handbook).  Plus, it offers great explanations of the
digital filtering that I am currently trying to learn - no higher level
math, just good basic explanations.  I would HIGHLY recommend that
anyone on this list that is interested in analog OR digital DIY seek out
this book.  Hal Chamberlin currently works at Young Chang research;
there is a link to him on the PAiA web site.  I think he sells
photocopies of his book for $30; well worth it, in my opinion.