[sdiy] VCO idea

[Ian Fritz]

Charlie --

Sure, that'll work. And, as you guessed, it's been done.

A simple way is to use the LM3914 LED bargraph chip. It has the comparitors 
built in, and it can be wired to just fire one output at a time.

I built a 16-stage sequencer once upon a time using a pair of cascaded LM3914s.

For a current example, which is actually closer to what you are asking 
about, I have a circuit that uses a LM3914 to fire a bank of S/H's to form 
a delay unit.  It's part of my analog physical modeling experiment.
http://home.earthlink.net/~ijfritz/pm_cir3.htm

I've always thought the LM3914 has many interesting musical applications, 
but not all that many have been developed.  With some imagination and an 
accurate VCO (watch that flyback) you can implement  all kind of clever 
devices.

Let us know if you work on this further or have further questions on the 
LM3914.

   Ian


At 05:46 AM 1/13/2003, you wrote:
I woke up this morning with what I thought was an interesting idea for a
VCO...that has probably already been done by someone else.....

--Start with a sawtooth "core" VCO.  The ASM-1 or equiv; any simple
accurate 1V/oct VCO, you don't need any pulse stuff, just the sawtooth.

--The sawtooth output should run from peak (say, 10V) to 0V DC during
each cycle.

--The output of the core VCO goes to a series of 8-10 resistors (accurate
ones, say 100K 1%) arranged in a giant voltage divider ladder.

--The output of each junction of the ladder goes to a comparator (LM311?
LM6511?) and some sort of AND gate subcircuit that traps when there is a
"match" between the sawtooth's output voltage and the particular stage of
the voltage divider the LM311 is tied to.  So you are building a VU
basically, but unlike a VU meter, only a single "light" is shown at any
given time, that lights up to show the current DC voltage of the sawtooth.

--Now, the output of each "stage" of the VU-like device hits a sample and
hold.

--The "hold" on each S/H is re-triggered by the previous or next stage of
the VU ladder.  For added fun, make this adjustable; each S/H's "hold" is
triggered by whatever rung in the ladder you want.


--As far as what to "sample" for each S/H, you could use DC voltages,
giving you (hopefully) analog-sounding waveforms.  For instance, I see a
group of 8-12 sliders.  Each slider is hooked to the input of a single
S/H.  Each slider is a voltage divider.  In practice: The first 3 sliders
set all the up and the others all the down would give you a narrow pulse
wave. A sine-wave looking knob setup gives you a close to sine wave
output.

-- Or you could use AC stuff, other outputs of VCO's, noise, whatever,
feeding the different inputs of each S/H giving you (I don't know I would
guess) ring-modulator sounding stuff.  You could use slowly sweeping DC's,
giving you (I have no idea what it'd sound like).

--Here is what I'd hope: the output frequency of what's above is
determined by the frequency of the sawtooth.  In other words, the whole
thing should stay in pitch with the original frequency of the waveform
generated by the core VCO.

--Now, you'd need some way to "demux" the outputs each S/H.  Into a single
audio signal.  That's your audio output.  Haven't figured this one out
yet, but I have a feeling it's doable.

Which leads me to questions for the synth-diy menches:

a) Does this make any sense?

b) Will it "work"?  Am I wasting my time because the above scenario is
massively flawed from the standpoint of what can and can't be done
electronically?

b') Assuming I can make the above work, can anyone speculate if it will it
be "musical" in a traditional sense?  IE: will the above scenario never
stay in tune, making it useful for only atonal, avante-garde (as opposed
to western sounding, 12 semitones an octave) stuff?

c) Assuming it can be done, someone must have already thought of this, and
someone must be already doing this.  To me it seems like a simple idea.
If so, I'd like to see the schematic to see how he or she has pulled it
off.  Maybe someone can suggest a link....I figure I could learn a lot
studying that schematic and/or circuit design, and may want to incorporate
it into the synthesizer I am building now.  EG: I see an analog shift
register on Ken Stone's site that has the same sort of S/H thinking of the
above, but is clock-controlled, not VCO sawtooth controlled, and has less
stages.....http://www.cgs.synth.net/

Sorry about the length of this post....hope I have made myself clear.
Thanks in advance for all the help....