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Re: Re: PSK, what is BPSK? Fourier escapes me at the moment.



Hi Laura

OK, you've got me interested...

For Linux I guess there's GNU source code for stuff like the G3RUH modem. Is
this part of one of the standard Redhat or Slackware releases, or is it
available elsewhere?

Do you have a good DSP for Dummies (like me!) type reference you could
recommend?

Even better would be some sort of Micro$oft-esque example so I don't have to
dust off my Linux partition.

I'm interested in embedded DSP you mentioned too. What's the development
languages and environment like? Will I have to brush up my assembler or is
it all C these days? Either way I'd like to at least give it a go. The last
bit of assembler I did was over five years ago, which was an IEEE floating
point package for an embedded Z80 based duplex radio telemetry unit less
than the size of a cigarette pack. The modulation for that was simple FSK.
So I'm not too frightened to try. Those were still just in the days when we
still cared about how many instruction cycles it took to execute a routine!

Perhaps my thoughts on software are based to much on closed systems like
CISCO and Microsoft. If you thought Microsoft was a closed environment, you
should try CISCO.

Maybe you'll change my opinion on software after all and I can put my scope
back in the cupboard.

73 Howard G6LVB

----- Original Message -----
From: "laura halliday" <marsgal42@hotmail.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, September 11, 2000 5:28 PM
Subject: [amsat-bb] Re: PSK, what is BPSK? Fourier escapes me at the moment.


> Christopher Cox <cobox@urec.net> wrote:
>
> >Well Laura, no reason to waist a few transistors where a
> >few thousand could do the job....:-)
>
> No TNC has a "few" transistors. They have many thousands.
> There is also the issue of component availability: they
> just don't make the parts for G3RUH modems any more. So
> you either do it in DSP, or you don't do it at all.
>
> >But seriously, being self taught, I understand comb filters
> >and delta filters, but Fourier escapes me at the moment.
> >
> >You mentioned that BPSK may be difficult to track. I thought
> >my DSP-2232 would...
>
> The catch is that you have to be already fairly close in
> your tuning. One rule of thumb is 10% of the data rate,
> which is +/- 120 Hz on the 1200 baud BPSK satellites.
>
> The reason is that DSP demodulation measures the signal
> phase by comparing it to two orthogonal references. The
> phase at that instant is then arctan(q/i), and by watching
> the phase change, you can recover the modulation.
>
> If the tuning isn't exactly right, that angle will change
> within each bit. If it changes enough, you can't tell the
> bits apart. Is it modulation? Or is it frequency drift?
>
> If the drift is modest you can correct. If it isn't, you
> can't. Have a look at the source code for any PSK31 imple-
> mentation, since they do this too.
>
> >What is the difference between BPSK and PSK another 90
> >degree signal?
>
> With phase modulation you can modulate by any angle you
> want. Many systems modulate the phase in 4 increments of
> 90 degrees each, hence QPSK (Quaternary Phase Shift Keying).
> Depending on how accurately you think you can measure
> phase angles in the presence of noise, you can use more
> phase increments - there are systems that have used 256-ary
> phase shift keying. But the simplest phase shift to generate,
> and the most robust to detect is +/- 180 degrees, Binary
> Phase Shift Keying. BPSK. PSK is the general term. BPSK
> is one kind of PSK.
>
> DSP hardware is cheap: DSP eval boards offer serious MIPS
> per dollar - I'm partial to the Analog Devices ADSP-2181
> board, because it's the cheapest of the lot (<$US100).
> Contrary to some postings on this list, the software is
> easy to change. During development, you'll download code
> on the fly. Once you have something you like, you'll burn
> a $5 EPROM, plug it in and use it. This is difficult? This
> is expensive?
>
> The cheapest PC sound card will record all there is to
> record from a communications-grade audio channel. The rest
> of the demodulation is MIPS, which are plentiful with modern
> processors. Linux (needless to say :-) has support for
> various protocols out the sound card, up to G3RUH-compa-
> tible 9600 baud MSK. Winmodems do 56k with similar
> technology.
>
> I drool over the IF DSP stuff my employers do, and hope,
> some day, to be able to afford to do it myself. For
> now, my sound card and eval board will keep me amused.
>
> Laura Halliday VE7LDH     "Que les nuages soient notre
> Grid: CN89mg               pied a terre..."
>                                    - Hospital/Shafte
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