[sdiy] VCO idea

[Theo]

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....
>