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Ham-Digital Digest Mon, 13 Dec 99 Volume 99 : Issue 315
Today's Topics:
APRS PROTOCOL SPECIFICATION
German packet radio
Icom IC-PCR1000 RS-232 control protocol for sale
KAM/Hostmaster/Pactor Mode
RS-232 HELP for internal RTTY modem in DSP-599zx
Used HAM Software?
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Loop-Detect: Ham-Digital:99/315
----------------------------------------------------------------------
Date: Mon, 6 Dec 1999 06:31:19 -0600
From: "Steve Sampson" <ssampson@usa-site.net>
Subject: APRS PROTOCOL SPECIFICATION
No one is getting rich off of APRS. The man who started the protocol
owns the copyright. You can't call it APRS unless it is compatible.
With a written spec, you can now do that. So what's the real problem
here? This isn't a socialist country, so you're going to have to
come to grips with everything not being provided by the government.
Here's a true example. My two older brothers designed a license plate
for their hot rod club. They drew it up on the kitchen table, and had 5
aluminum plates made. These were hand carved into wood, and then
cast. One of the club members wanted more plates so he made a
copy and had some more cast. Then he decided that he'd make a
bunch of ski-jackets with the logo. Too make a long story short, the
club lost their ownership of the logo, as it wasn't copyright. One
person makes a few bucks by stealing the logo, but the real problem
is that the joy went out of an otherwise simple experience.
Rich wrote in message ...
>On Sun, 05 Dec 1999 10:33:18 GMT, Robin Gilks <g8ecj@abmdata.demon.co.uk>
wrote:
>>As a matter of interest - is it true that the term APRS is Copyright and
>>cannot be included in the name of a software or hardware product without the
>>payment of a license fee?
>>
>>If this is the case I can't see much point in publishing a spec if all its
>>going to do is line someones pocket...
>
> You can always say that the product is "APRS Compatible".
>
>- Rich
>.
------------------------------
Date: Mon, 13 Dec 1999 02:03:31 +0100
From: Nico Palermo <nicopal@microtelecom.it>
Subject: German packet radio
Sounds really interesting.
Is the DFE used after a "whitening" filter? Either *it* performs the
task?
Besides removing ISI effects in multilevel GFSK, does it works on binary
ISI-free FSK?
Nico Palermo, IV3NWV
________________________
Alexander Kurpiers wrote:
>
> Nico Palermo wrote:
>
> > MSK performance is worse than BPSK only if demodulated by a coherent
> > detector which takes symbol-by-symbol decisions (3 dB penalty), either
> > by an incoherent one (6 dB penalty), or by a frequency discriminator. In
> > the latter case the optimum modulation index is 1.25 (not 0.5) and there
> > is a ~7 dB penalty over Viterbi decoding.
>
> You can get back a lot if you use a DFE after the frequency
> discriminator. This is because the PSD of the noise is not white after
> the non-linear demodulator. The DFE can be used to compensate for the
> noise correlation. This is not as complicated as viterbi decoding and of
> course not as demanding in terms of computational power. The viterbi
> decoding becomes only feasable for simple QPSK-type receivers eg. used
> for GSM where h=0.5 (MSK). If the modulation index is arbitrarily
> chosen, the receiver is much more complicated (a bank of correlators in
> front of the viterbi). Note that this has absolutely nothing to do with
> coding!!! We are talking about the modulation having memory. No coding
> used.
>
> I«ve done some simulations for 4-FSK using a Gaussian filtered FSK with
> BT=0.4 and h=0.2. The receiver was a frequency discriminator followed by
> a T/2-DFE. I gained about 6dB in S/N compared to a "normal" receiver
> without DFE and with ISI-free modulation ala G3RUH (BTW: when CPFSK
> people talk about RC (raised cosine), then they mean raised cosine in
> the TIME domain, so 2-RC is a raised cosine that spannes over 2 symbols.
> The G3RUH baseband modem also features a raised cosine impulse, but in
> the FREQUENCY domain...) .
>
> The later realized "real-world" modem was unfortunatly worse than the
> simulation (of course, this is what you expect the "real-world" to be),
> but it still was quite good compared to the "simple" case with only a
> linear post-detection filter (G3RUH/DF9IC type modems). There is still
> room for improvement for this DSP based modem, but I have no time in the
> moment...
>
> 73« Alexander
>
>
>
> --
> *----------------------------------+---------------------------------------*
> | Alexander F. Kurpiers | Voice:
To be continued in digest: hd_99_315B
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