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[dsp] PWM/phase relationship

David Willmore willmore at optonline.net
Wed Jan 25 22:31:50 UTC 2006

> Scott Miller wrote:
> The limitation is really how fast the processor can get data ready.  It's
> got 192 bytes of RAM, which isn't enough to buffer a whole message.  It
> winds up doing HDLC bit stuffing and FCS calculation on the fly.  And
> really, it seems to be more than adequate.  I've never had any trouble with
> proper demodulation.

That's the real test. :)  As long as it works.

> > The idea is to send PWM encoded voice--so as not to have to 
> > modulate the
> > intensity of the laser--which takes some clever work and 
> > doesn't buy us
> > much.  On and off are about all I can trust it to do safely. 
> > :)  I don't
> I haven't messed with laser communications since I was a kid, and we didn't
> have diodes, just He-Ne tubes.  Which I suppose were easy enough to do AM
> with, but I don't think you'd want to pulse them at any significant rate.
> For long range it was easier just to shine it through a fan to modulate it.

You're quite correct.  He-Ne can be AM modulated a bit--not 100% modulation,
of course, but enough.  Laser diodes *can* be modulated to your hearts
content, but the electronics to do it are beyond my interest level.  I
also would like this design to be easily reporoducable by people without
special equipment, so just modulating the input power to a stock dollar
store laser pointer is about as far as I want to go.  Anyone who wants
to get fancier than that is going to have to do the rest themselves as
well. :)

I had once looked at using an LCD shutter to modulate a He-Ne laser,
but they don't generally have a very fast switching rate, so I gave
up on that idea.  Now laser diodes are cheap, so I'm giving them a

> > Hmm, that may work as you'll have plenty of gain from 
> > directivity--I assume
> > you'll keep a telescope on it as the 'receive antenna'.  You 
> Yep, that's the idea.

Then I think you'll have pretty good luck.  That front edge of
the output of a strobe is quite easily controlable--the tail
edge is a bit harder.

> > a yellow filter over the lens to act as a 'blue block' to 
> > filter out any
> > sky noise--which should mostly be blue. 
> Or just fly it at night.  =]

Okay, fine. :)  As long as you're happy with that limitation.  The laser
guys have done the same thing--either come inside and make the contact
or do it at night outside.  I'm hoping to be able to get around some
of those limitations.

> > 'face to face', so it's not really an issue.  Expecially since they're
> Warning: Do not look into laser with remaining eye.   =]

*laugh*  yeah, that's important.  It makies aiming a little delicate--you
can only aim one side at a time safely.  Maybe some kind of dichroic
splitter could solve that problem?  Use a blue LED as a beacon for aiming
and red for the communication?  I'll burn that bridge when I cross it. :)


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