[aprssig] 9600 Packet was RE: EB-432/PCSAT2/APRS Question

[Dave Baxter]

Hi...

You may find that though the audio transformers will appear to screw up
stuff like 9k6 packet, it's not the frequency response as such that's
the problem, but the phase shift or group delay vs frequency that they
and associated passives cause.  After all, the apparent bandwidth even
of a good transformer isolation system should be well in excess of
what's needed even for 9k6 packet.

However, from what I've seen so far, unless the radio has a dedicated
9k6 RX audio outlet, you need to tap off direct from the discriminator
(or whatever FM/PM detector it uses) to go to the 9k6 modem etc.

If you know the parts involved in the "isolator", it may be possible to
model the problem, and design an equalisation network so you can keep
the passive isolation, and keep the phase distortion to a minimum within
the needed bandwidth.  Look at the waveform, and "eye" pattern
with/without the transformer isolation, and you'll see what's happening.

This was a very common problem in the days of magnetic tape storage for
data, using audio cassettes, some makes of player would work, some would
not, but they all "sounded" the same.  It was only when you examined the
waveform in detail you saw the problem, and that was at only 2400bd.  I
can see how 9k6 packet could suffer similar problems...

I can also see how software decoding can accomodate more distortion than
many hardware modems.  After all, a dial up land line modem can link to
another similar unit over the vagarities of the analog PSTN, and (on a
good day!) give you a 56k link.  They go through all sorts of negotiaton
and equalisation within their DSP software, keeping the "link" within
the 300Hz to 3300Hz band.

It may be worth contacting G3LIV and ask the question about 9k6 and his
isolators..

Cheers.  Hope you can work without isolation, or it's yet more
hassle.....

Dave G0WBX.


> -----Original Message-----
> From: Ralph Milnes [mailto:ralphmilnes at patmedia.net] 
> Sent: Saturday, May 27, 2006 5:50 PM
> To: aprssig at lists.tapr.org
> Subject: [aprssig] 9600 Packet was RE: EB-432/PCSAT2/APRS Question
> 
> John wrote:
> 
> >One of the reasons I am using the Soundmodem is that it will decode 
> >1200 and 9600 baud packets simultaneously.
> 
> >I also have a 9600 baud modem that I will try tonight instead of the 
> >soundmodem.  It may just be that the software demodulator needs
> >tweaking somehow (audio levels/filter response maybe).   The 
> hardware  
> >modem should (in theory, I think) be better able to cope with 
> >variations.  And at least it will rule that out.
> 
> >Regards
> >de John
> >EI7IG