[sdiy] some design questions from a beginner

Fri Dec 13 21:36:25 CET 2002

Inline...

>ok, now, as i begin to design my hardwired synth, i have module layout questions.
>so here we go...
>is it, in anyone's experience, practical to have a switchable filter system
>that switches between a hi and lo pass in series and a hi and lo pass in
>parallel, effectively switching between variable band and notch pass filter
>systems? i could still have just a hi or lo pass with the twist of a knob,
>the advantage is just the variable pass rate. i know that some commercial
>synth uses it (well, i'm pretty sure someone talked about it on the list)

I did...

>but is this pragmatic? does this sound good?

I was less than thrilled with the actual filters I used (an MS-20
variant) but the series / parallel arrangement is really easy to
do. I have a CV input to drive both center frequencies in the same direction (sweep) and one to drive in opposite directions (bandwidth)...

 that's my real question.
>in regards to frequency modulation...i understand this can be done with a
>VCO. does this mean that i should have two VCOs in parallel and then one
>VCO modulating both of them? 

This is most common...

should i just have several VCOs in series? aren't
>they typically in series anyways? and, if so, doesn't this mean that frequency
>modulation is merely an unavoidable (but not negative) consequence of multiple
>VCO synthesis? or should my 'program' VCO be at a differnt frequency range
>than my 'carrier(s)'? i am seized with confusion, as you can probably tell.

Series connection is uncommon (but possible).  FM synthesis with analog
oscillators can be tough because the stability is usually not good enough.

>and, finally, do special effects (ring modulation, distortion, AM) go between
>the VCF and VCA typically? what are the effects of placing them in other
>places? 

They could go either place.... Usually before the filter but sometimes
after.

i can't imagine that they'd go between the ADSR and VCA on the gate/trigger
>signal, but who knows. 

Think  

Sources (VCO, NOISE...) connected to Modifiers (VCF VCA RINGMOD)
controlled by Controllers (ADSR, LFO. Keyboard) and you will get 
most of the common sounds

>phew! slowly i will learn (but that makes it that much more satisfying, doesn't
>it?) 

Build some modules and experiment. Some folks break all the rules
anyway...

H^) harry

sorry for the post length.
>-bc