[sdiy] Square to Sine

[Elain Klopke]

Well, there's always tying a string to a post in a speaker cone with a
weight on the other end of the string and then some sort of optical
sensor....

On Tue, Apr 4, 2017 at 8:11 AM, Tom Wiltshire <tom at electricdruid.net> wrote:

> From the document:
>
> "Present implementations limit magic sinewaves to power line frequencies,
> possibly up to 400 Hertz."
>
> and
>
> "Unusual programming techniques are required as each and every
> microprocessor clock cycle is critical."
>
> Sounds like exactly the sort of thing the SDIY list would get into!
>
> and
>
> "As many as 44,000 or more microprocessor instructions may be needed per
> power line cycle."
>
> So it looks like full audio might be pretty demanding. Not impossible,
> perhaps, but at least demanding. For generating sines for a Hammond organ
> (just to take totally random example…) we'd only need up to 6KHz. Assuming
> the 44K-instructions-per-cycle guideline from above, that's a 264MHz
> processor!
> Still, like you, I can't help thinking there must be a (simpler) way…
>
> Tom
>
> On 4 Apr 2017, at 13:26, rburnett at richieburnett.co.uk wrote:
>
> > I have always wondered about magic sinewaves for something like this.
> For those not familiar they are sinewaves generated digitally using PWM
> techniques but the transitions in the PWM pulse-train are carefully chosen
> using some clever mathematics to force most of the low-order harmonics to
> zero.  This *greatly* reduces how much you have to filter the signal in
> order to get a decent sinewave with nice low THD.
> >
> > http://www.tinaja.com/glib/msinexec.pdf
> >
> > Might not be appropriate here?  ...but it's a neat trick nonetheless.
> It's used in power electronic "inverters" for driving motors, but can't
> help thinking it might have some uses in audio synthesis too?
> >
> > -Richie,
> >
> > On 2017-04-04 12:37, Tom Wiltshire wrote:
> >> On 4 Apr 2017, at 11:32, Elaine Klopke <functionofform at gmail.com>
> >> wrote:
> >>> Does this actually work?
> >> http://www.learningaboutelectronics.com/Articles/Square-to-sine-wave-
> converter-circuit.php
> >>> I would assume that being 3 RC networks in series means quite the
> >>> drop in volume.
> >> Yes, it works, and yes, it'll kill the volume. It's a three-pole
> >> passive filter. The recommendation (which they've ignored in this
> >> example) is for each resistor to be ten times the previous one to help
> >> avoid one stage loading the next. So it could be improved. There's an
> >> example in my LoopEnv datasheet, pg.6:
> >> http://www.electricdruid.net/datasheets/LOOPENV1Datasheet.pdf
> >> It'll only work properly at one particular frequency, as you've
> >> realised. Changing the input frequency will change both the level and
> >> the harmonic content of the output (although perhaps not enough to
> >> matter for small changes in input frequency - e.g. vibrato?)
> >>> Also, in a related article they show two networks in series
> >>> converting the square wave to a triangle.
> >> Probably running the square into an integrator. That gives you a nice
> >> triangle, but the same volume problem - the volume halves for each
> >> octave you go up. Have a look at the stuff online about DCOs for more
> >> about this, since this is the problem they had to overcome.
> >>> All of the converters being dependent on the frequency of the input
> >>> wave being roughly the same as what the RC networks are tuned to.
> >> Exactly.
> >> Tom
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