[sdiy] VCO idea

[charlie lamm]

How exactly to make the "VU" in question does seem to be a problem....
Yes, as I see it, I need only one "LED" active at a time for this to work.

Hmm, I wonder if this is a job for a CPU....with an A/D convertor or
onboard A/D sampling the amplitude of the incoming signal and switching
its line low and high accordingly.

Might be easier than trying to do it with a lot of components, but maybe
not cheaper....

Anyway, thanks for all the input.  I have to finish my ASM PCB and then
that may be the next thing I work on.  Whatever I come up with I will
share w/ the group....


On Mon, 13 Jan 2003, Theo wrote:

> Problem with the LM3914 it that there are TWO leds/outputs active when
> switching to the next stage. This may or may not be a problem with your
> application.
> For building a quantizer I tried adding positive feedback to pull the input
> voltage out of the "two led" range. For my quantizer this was not a good
> solution because it only worked ok with a rising input voltage. A falling
> input voltage still made two outputs to be active at the transition points.
>
> Cheers,
> Theo
>
> From: Ian Fritz <ijfritz at earthlink.net>
>
> > 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....
> >
>