[sdiy] Really Low, LFO

[Kyle Stephens]

Cheater,

Yeah, I figured an integrator-based solution with say an electrolytic cap would be leaky alright (an electro would be there was a cheap way to get large capacitances - polys in those ranges are $10 or more easy). I ought add I'm trying to do this relatively cheaply, but we'll see I suppose.

DSP would be the defacto way to go, but like I said it's a can of worms (not to say I don't like getting into those, just when I can wrangle them!)


_Kyle

--- On Fri, 4/2/10, cheater cheater <cheater00 at gmail.com> wrote:

> From: cheater cheater <cheater00 at gmail.com>
> Subject: Re: [sdiy] Really Low, LFO
> To: "Kyle Stephens" <lightburnx at yahoo.com>
> Cc: synth-diy at dropmix.xs4all.nl
> Date: Friday, April 2, 2010, 2:15 PM
> Leakage current of your capacitor and
> off-resistance of the transistor
> used for resetting the core are going to be the issues, I
> think.
> 
> If you want an infrasonic LFO I suggest doing it in DSP,
> there's no
> advantage to using analog at those frequencies that I can
> think of.
> 
> 
> On Fri, Apr 2, 2010 at 22:51, Kyle Stephens <lightburnx at yahoo.com>
> wrote:
> > Can anyone point me to some reading material on
> infrasonic oscillator design? I want to build a really low,
> low LFO, with a period of around half an hour (yes, _that_
> slow!).
> >
> > As HF designs have their share of issues that get
> worse in that spectrum, infrasonic has its problems too I
> would imagine - I just don't know what exactly they might
> be.
> >
> > For my app, accuracy isn't as important as
> precision/repeatability, though if both are possible then
> both are welcome.
> >
> > Closest I've found so far is this:
> >
> > http://tinyurl.com/yf9wzp3
> >
> > I reckon I can tailor it if I make the integrator cap
> several decades larger, to shift that 0.1Hz to 100KHz range
> several decimal places downward.
> 
> Eh, not really about the size of the cap, it's more about
> being able
> to control the current accurately enough.
> 
> > I've a copy of AD's Nonlinear Circuits Handbook, which
> has some material on the topic, though nothing too specific
> on freqs beneath 1 Hz (half an hour is ~ 0.0001 Hz).
> >
> > I spoke to Prof. Lanterman about this a while back,
> and he recommended a microcontroller based solution,
> 
> Yep, I second that. DSP options (and for an lfo you don't
> even need a
> dedicated dsp uC such as a 56k) give you accuracy AND
> repeatability;
> but they're easy  to badly implement on their own
> (aliasing, ...)
> although, comparing the typical sampling frequency you'd
> run this at
> to the frequency you expect, the error in pitch coming from
> aliasing
> on the discontinuity (which gives you only an error of +-1
> sample,
> maybe 2) will be minuscule. Your biggest problem will be
> implementing
> an accumulator register big enough, I think.
> 
> D.
> 
> > and while that's something I want to break into, it's
> a whole 'nother can of worms too. If this can be done in
> analog, it'd be a cool achievement and an enrichment
> exercise if nothing else... :]
> >
> >
> >
> > _Kyle (so slow it Herz?)
> >
> >
> >
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