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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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