[sdiy] Really Low, LFO

[jays at aracnet.com]

Kyle,

I good place to start with digital stuff is get something like the Arduino boards and work with those. You can get a CPU board fairly cheap, open source development system and prototyping shields/daughter boards for your own stuff. Check out www.sparkfun.com. It's yet another place to spend lots of money.

Jay S.

Kyle Stephens wrote:

> Jay - good point on the precision op amps, figured that would come into play.
> 
> Cheater, I got the term from my dad, an EE, has often thrown around "second order interference" in regards to stuff I've built, usually in the context of "How do you know what you're hearing isn't second order interference?" said with a rolling of the eyes. He also has asked how do I know if a given schematic wasn't benevolently designed to blow up in my face, to which I roll my own eyes -_-.
> 
> First order effects are what a given part is ideally "supposed to do". Real world problems, like noise and thermal drift etc. are second order. Erm, or so I would figure at least - those who know better correct me if otherwise.
> 
> I'll get a crack at the digital some day, and recon that's how a weather station would do it now these days anyway? I otherwise just have visions of Richter-like readout graphs being slowly printed...
> 
> 
> _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, 3:22 PM
> > Hi Kyle,
> > 
> > On Fri, Apr 2, 2010 at 23:35, Kyle Stephens <lightburnx at yahoo.com>
> > wrote:
> > > Accuracy means getting, in this case, a specific
> > frequency. Precision means it may be wrong (not
> > necessarily), but at least it's consistently 'wrong'.
> > >
> > > My ears are liable to pick out what "sounds good,"
> > independent of any specific value like 5.000Hz, and I'd like
> > to be able to pick more than less the same point again and
> > again when I set the pot to a given position, relatively
> > free of stuff like temp drift or other second order
> > interference.
> > 
> > What's second order interference? I tried finding the
> > definition of
> > that but was unable to. I've seen this mentioned on the
> > list every now
> > and then but can't figure out what this is.
> > 
> > > I brought it up as I figured most material on function
> > gens that slow would be scientific equipment where both
> > those factors are important concerns.
> > 
> > You can try looking in weather station designs if you want
> > to. They
> > have to work on very long cycles of statistic series, high
> > chance
> > they'll have very slow lfos, plus they have to withstand
> > the weather.
> > 
> > On Fri, Apr 2, 2010 at 23:45,  <jays at aracnet.com>
> > wrote:
> > > I think I'd agree here, I'd go digital. I do think a
> > DSP would be overkill. Especially with what you get in low
> > end uCs like PICs now days.
> > 
> > Don't take me wrong - I was talking about DSP as in
> > 'digital signal
> > processing' not 'dsp microcontroller' :-) Definitely
> > something that
> > can be done on a bread and butter uC.
> > 
> > > When you have timing this long the quality of parts
> > becomes critical. Things like op-amp offsets start to affect
> > symmetry. I now use LT1013s in some of my LFOs because of
> > this. As mentioned issues with caps become critical and so
> > on.....
> > 
> > Interesting tip, will be checking it out!
> > 
> > On Fri, Apr 2, 2010 at 23:49, Tim Parkhurst <tim.parkhurst at gmail.com>
> > wrote:
> > > On Fri, Apr 2, 2010 at 1:51 PM, 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!).
> > >>
> > >
> > > Hey Kyle,
> > >
> > > How about dividing a square wave waaaaaay down? You
> > can use a variety
> > > of oscillator designs, of course, and you could use an
> > integrator
> > > (with a big honking cap) to smooth the square out.
> > Alternately, you
> > > could possibly use an R/2R converter and sum several
> > divided down
> > > squares (higher freq's get attenuated more) to get a
> > pretty fair
> > > approximation of a slow triangle wave (still might
> > require a smoothing
> > > cap).
> > 
> > Tim,
> > you might be looking at problems such as accumulative error
> > from the
> > frequency of the squarewave, and any fluctuations in pitch
> > will
> > transfer to the divided-down output. Generally - and this
> > is not a
> > fully scientific expression - oscillators seem to be
> > specified more
> > for 'local' pitch than 'global'. For example if you're
> > looking at
> > local pitch (e.g. the pitch over the last 1 second) and the
> > phase
> > jumps every now and then it's statistically insignificant
> > in this
> > scheme so the pitch is still 'within parameters', but the
> > global pitch
> > (i.e. number of cycles / time elapsed) is going to be off.
> > This 'phase
> > jump' might happen because of aliasing. Since the pitch of
> > a
> > divided-down oscillator will depend on more cycles of the
> > original
> > undivided oscillator, it's statistically more prone to this
> > kind of
> > problem. But I don't understand the measurement error
> > science there
> > enough to say for sure if it can be significant enough to
> > be an issue
> > - by intuition, I think so.
> > 
> > Kyle,
> > if you're OK with doing this in digital, then there are
> > most probably
> > single-chip signal generators that will work well for you;
> > give it a
> > search.
> > 
> > Cheers
> > D.
> > 
> 
> 
>