[sdiy] Thru-Zero, was as3340 PW/tuning issue

[Andrew Simper]

The good thing about linear FM is it keeps the perceived pitch constant,
and having a through zero VCO increase the range this can happen on, but
typically there will still be an upper limit on the modulated pitch, which
will again change the perceived pitch much like clipping at 0 Hz. If you're
doing log pitch modulation then you will never reach negative frequencies
anyway, so you're all good with a regular wide range VCO.

Cheers,

Andy

On Mon, 6 Jan 2020 at 03:06, Quincas Moreira <quincas at gmail.com> wrote:

> Here’s my video on the Generate3 by Joranalogue. It does through zero
> EVERYTHING, and may be useful to understand the concepts
>
> https://youtu.be/ymEGfpxk8CA
>
> Quincas
>
> Sent from my iPad
>
> On 5 Jan 2020, at 12:09, David G Dixon <dixon at mail.ubc.ca> wrote:
>
> 
>
> Adam,
>
>
>
> To answer your question, let’s talk about triangle-core VCOs for a moment,
> since this is what I have the most direct experience with.
>
>
>
> The typical tricore VCO is a “tri-square” core – a comparator compares the
> sum of the triangle wave and a voltage-limited (i.e., with back-to-back
> zeners) square wave (generated from the comparator) with ground.  When this
> signal passes through ground, the comparator and the square wave flip
> states.  This changes the direction of current to the triangle integrator,
> and defines the voltage of the triangle apices.  That’s how a basic
> tri-square oscillator works, such as you would find in a Dixie VCO.
>
>
>
> To get Linear FM on this beast, a linear VCA is inserted in the loop
> between the square-wave generator and the triangle integrator.  If the gain
> of this VCA is taken to zero, then the oscillator stops.  If the gain of
> this VCA is taken negative, what happens?  The triangle and the square wave
> are now not in the correct phase relationship, so that their sum will move
> away from ground, and will get stuck at one of the voltage rails.  Again,
> the VCO will stop because the comparator will never flip.  In order for the
> VCO to continue oscillating under this condition, the triangle signal being
> summed at the comparator input must be inverted.  This requires a bit of
> logic to read the polarity of the Linear FM VCA’s gain (or some other
> convenient indicator of the triangle current’s direction) to drive an
> optional inverter.
>
>
>
> When this is all done correctly, the VCO will continue to oscillate
> normally even though the Linear FM gain has gone negative (i.e., has passed
> “through zero”).  If the TZFM circuit has been designed well, then there
> will be no perceptible “glitch” when the FM passes through zero, and the
> resulting waveform will pass smoothly and symmetrically through a slope of
> zero.  This is what is achieved with, for example, a Rubicon VCO.
>
>
>
> If Linear FM gain is broad and passes through zero this gives a much
> deeper and richer-sounding FM.  It is the basis of bells, mallots, and all
> sorts of other strange and wonderful effects.  You generally do not find
> TZFM on Roland, Korg or Sequential synths.  There was (is) nothing
> fundamental preventing these makers from incorporating analog TZFM – they
> just chose not to.  When I finally get around to building my own dream
> monosynth, it will definitely have TZFM built in (it’s actually pretty
> easy, and getting easier – I’ve got it down to almost no additional
> circuitry beyond the conventional VCO core – maybe two or three additional
> opamps and a JFET – thanks to my beloved 2164).  Hopefully, that will
> happen sometime later this year.
>
>
>
> Dave
>
>
> ------------------------------
>
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org] *On Behalf Of *Adam
> Inglis
> *Sent:* Saturday, January 04, 2020 6:25 PM
> *To:* ColinMuirDorward; Synth DIY
> *Subject:* [sdiy] Thru-Zero, was as3340 PW/tuning issue
>
>
>
>
>
> Thanks Colin
>
> Someone in that thread says they would love a "through-zero version”.
>
>
>
> Seems to be a lot of talk about TZ all over these types of forums - why?
> What’s the big deal? How are these different from the VCOs in my Roland,
> Korg and Sequential synths?
>
>
>
> Sure I could google but I know I’d get better education here ;-)
>
>
>
> Adam
>
>
>
>
>
>
>
> On 5 Jan 2020, at 6:26 am, ColinMuirDorward <colindorward at gmail.com>
> wrote:
>
>
>
> Just a note to say thanks to everyone contributing to this conversation.
> I've applied lots of what I learned here to my design, and I've got the
> PWM/pitch interaction down to what I think is an acceptable minimum.
> Anyways, it's nothing special, but I've started a build thread over at
> muffs <https://www.muffwiggler.com/forum/viewtopic.php?f=17&t=225670>,
> for anyone who needs yet another low cost 3340 vco. I need several, so
> thought I may as well go public in case anyone else does, too. I'll be
> using JLCPCB assembly for all the resistors and caps, and the rest will be
> through hole at home. I have lots of as3340s, so can include with the PCB
> if anyone needs. 3340 is running on +10/-5, so the circuit should be at
> home in 12v or 15v setups. I'm in Yukon, Canada.
>
> Thanks,
>
> Colin
>
>
>
> On Fri, Jan 3, 2020 at 12:39 PM David G Dixon <dixon at mail.ubc.ca> wrote:
>
> The Intellijel Rubicon (and Rubicon-2) gives the option of generating
> pulse waves from Triangle, Saw, or Double-Saw (twice the frequency).  It’s
> not a whole synth, just a TZFM VCO.
>
>
> ------------------------------
>
>
>
>
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