[sdiy] asm1-user's guide

[MED]

On Monday 10 November 2003 22:02, Luciano Hormazabal wrote:
I'll take a few of these.
Comments inline.

> VCO
>
> What are all the inputs vc1, vc2...6if its a summing amplifier why not use
> only one resistor??: the synth is monophonic anyway.
If you want to (for instance) use a LFO for vibrato on top of the actual pitch 
you are playing (generated NOT straight from the keyboard but through a 
sample and hold circuit -- I'll get to that in a bit :)
The various jacks let you do that without having to use a multiple or other 
"unsavory" means.

> well what are the other inputs anyway?? what could i ever want to connect
> in sync??
LFO for Vibrato, other VCO for FM, ADSR (attenuated) for "slight addition of 
brightness"/pitch increase as part of cycle (it happens on plucked string 
instruments)

> PWM: that's for symmetry isn't it??
PWM decides the pulse width of the waveform (square wave).
Basically, the percentage of the wave that's spent at 0v and the time that's 
spent at output-voltage. 50% duty cycle (50% of the time, the wave is high) 
is a perfect square wave. 100% duty cycle is a simple 5-V (assuming 5V-out 
oscillators :) voltage. Areas in between sound different. If you modulate the 
pulse width, it sounds kind of like phasing.
B
> can you use the sawtooth and the pulse output at the same time??
(i haven't looked at the ASM-1 in particular)
Yes, most likely. They'll be going at the same frequency.

> what is the deal if its linear or exponential??? i mean you just change the
> values of the resistors to trigger the module, isn't it?? or is another
> module recieving just exp voltaje, so if i use a linear one things would
> get ugly??
Exponential versus linear... *sigh* that's almost flamewar material here :)
Linear adjustment is slightly easier to do. To go from 440hz(A4) to 880hz(A5) 
you add x volts, but to go from 880hz to 1760hz (A5 to A6) is 2x volts.
It's okay, but it's a pain -- you cna't transpose things by adding an offset 
voltage. Even harmonizing an octave down is a pain because you need to know 
what frequency the oscillator goes at at 0V and the scale.

With exponential, (the standard we use is 1v/octave) things like offsets for 
transposition or harmonization (i.e. h.aving a filter track an oscillator, 
but an octave higher, to emphasize the harmonic) is much easier, with the 
cost of needing an exponential converter that requires compensation or else 
it drifts.

> VCA
>
> CV1, CV2, Cv3... okay what do i plug in here, the square wave from the
> VCO?? wait: there's an input below....AAAA!!

CV is for whatever voltage output(s) you want to adjust the amplitude.
Typically this is an ADSR circuit -- the ADSR fires from your keyboard's gate 
signal, and the amplitude responds.
But you don't just need to run audio to audio and cv to cv. That's the point 
of a modular -- flexibility.
An interesting thing you could do is have the squarewave output in the CV out, 
but run the noise into the input -- it still keeps the general "square" shape 
and essentially the same frequency, but instead of the top being flat, it's 
noise.

> what is the max voltaje that the VCA can recieve without clipping?? what
> happens if i connect the output of the VCA to the input of the other VCA??
This varies from VCA to VCA -- what also varies is how it clips. Experiment 
and see :)

> Is the output of the VCA what we could call the "regular" audio output??
> what happens  if i connect the VCO directly to the amp?? what voltaje is
> the audio amplifier expecting?? (i know this one depends on the amp, but
> generally speaking)

CAREFUL!!!
VCO output, and a full-on VCA output is MUCH hotter than your amp will be 
expecting.
Sometimes guys make a patchable output that attenuates it down to line-level 
or thereabouts (someone i'm sure will chime in with the actual amplitude)

> ADSR
> what voltaje is the gate expecting?? how can i feed it using the same input
> the VCO gets?? or is there a more efficient way??
gates are typically +5V for as long as the key is held down.
Your keyboard should generate one for you.

> envelope out... how can you shape the waveform with this??? where does it
> go into?? (assume you want to shape the waveform, not the lfo or anything)

You can't "shape the waveform" per se -- but you *can* use it to vary PWM as 
you press the keys, for instance, or use it to change the resonance or cutoff 
freq of the filter by patching it as one of the CV ins to them.

> VCF
>
> here i don't have a clue what the outputs and inputs are. if you just want
> to use the "cutoff and resonance" the way you do in a regular keyboard what
> do you do???

Typical synth signal chain is
VCO -> VCF -> VCA.
put VCO out into the VCF in, and VCF out inot VCA.

> first of all i want to thank you for all your support for everyone in the
> list. it means a lot for someone like me when i ask a question and in ten
> minutes it is already answered!!! thats really something, guys. keep it up.

It really *is* a great group here :)

> now these questions must be fairly obvious for all of you guys, but believe
> me, i am lost in some things!! i am an electronics student so i am becoming
> familiar with the filters and what they do, but you must understand that i
> am new to the world of synth diy. when i can finally build the asm-1 i will
> make a webpage called asm-1 for dummies.. lol. anyway i certainly hope the
> day will come soon when i can stop asking and start answering.
>
> thanks for your help, guys. sincerely.
>
HTH.
-MED