[OZAPRS] Decoding HF APRS- Robust Packet -SCS DSP-tnc

Fri Jul 7 16:21:45 EST 2006

actually there seems to be a much better solution: ROBUST PACKET 300/600
from the german company SCS (wellknown for Pactor-III Controllers).
They have now introduced a new DSP-TNCand APRS Tracker that supports
conventional Packet
in 300baud/1200Baud/9600/19200 and RobustPR in 300/600Baud as well as
stand
anlone APRS Tracker with NMEAin/NMEAout and smart beaconing etc.
In 300Bd HF-AX25 the DSP-TNC can receive multiple packets in spectrum
+-400Hz around center frequency (1500Hz) simultaneously.
The special ROBUST Packet Mode is much better suited for HF-channels and
specified as follows:
Bandwith 500 Hz at-30dB
Modulation: pulse shaped OFDM (BPSK,QPSK) similar to PACTOR-III
Speed: 200 or 600 bits/sec netto
Cestfactor: 3,0 or 4,2dB
Delay spread: +-8msec
Code: full-frame-interleaved rate/2 or rate 3/4
the costs are around 250 Euro,which is more than a Tinytrak, but a fully
featured KISS-TNC with Robust-PR and stand alone APRS-Tracking features.
see www.scs-ptc.com/datasheets/scs_tracker_dsp_tnc_en_v5.pdf

73 de Mike oe3mzc/VK3FPF/6
----- Original Message ----- 
From: "Darryl Smith" <Darryl at Radio-active.net.au>
To: "'VK / ZL APRS Users'" <ozaprs at aprs.net.au>
Sent: Friday, July 07, 2006 6:00 AM
Subject: Re: [OZAPRS] Decoding HF APRS


> Actually, lack of forward error correcting on AX-25 is not really all
that
> much of an issue, at least from my point of view. I am happy to be
> contradicted, but if someone placed an FEC'ed payload inside an AX-25
> packet, and if there was a single bit error, it is unlikely that the FEC
> would be able to recover the payload, even if it were able to recover
100
> bad bits.
>
> The reason for this is that FEC requires a physical layer where a single
bit
> error will not corrupt the bits around it by the mere fact that it
happened.
> The AX25 physical layer will not allow this - with single bit errors
> corrupting all the bits following!
>
> Eh? Why? Well, the physical layer uses NRZI coding, or Non Return To
Zero
> Inverted. Basically what this means is that if a single bit is of the
wrong
> state, all the following bits will be inverted. Well, that is, all the
> following bits until we get another bad bit. All the data between the
two
> bad bits will be inverted, but the rest will be good. But the odds of
having
> only two bad bits and having them close together is not good.
>
> Oh, but FEC will detect that first error, and correct it, will it not?
Well,
> no. FEC will not detect so many errors. It is very hard to work out
which
> the first error is. VERY HARD!
>
> What this means is that AX25 is bad on air. NRZI is really really bad on
> air. If we add FEC to an AX25 packet things will improve, but only if we
> remove NRZI coding.
>
> ---------
> Darryl Smith, VK2TDS POBox 169 Ingleburn NSW 2565 Australia
> Mobile Number 0412 929 634 [+61 4 12 929 634 Int] - 02 9618 6459
> www.radio-active.net.au/blog/ - www.radio-active.net.au/web/tracking/
>
>
> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
> Behalf Of Robert Thirkettle
> Sent: Friday, 7 July 2006 1:41 PM
> To: VK / ZL APRS Users
> Subject: Re: [OZAPRS] Decoding HF APRS
>
> Hi All
> Another problem is that with AX-25 there is no forward error
> correction just a check sum. If 1 bit is corrupted then whole packet is
> discarded. AX 25 is not really suited to HF with its atmospheric
variations
> that can cause phase distortion and other problems to the data. These
types
> of problems are not obvious to the ear but the TNC sure sees them.
>
> Regards Rob Thirkettle  ZL3RX
> Dept  Physics & Astronomy
> University of Canterbury
> Private Bag 4800,
> Christchurch 8020,
> New Zealand
> Tel: +64 3 364 2510
> Fax: +64 3 364 2469
> mailto:rob.thirkettle at canterbury.ac.nz
>
>
> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
> Behalf Of G T & C A Beagley
> Sent: Friday, 7 July 2006 2:47 p.m.
> To: VK / ZL APRS Users
> Subject: Re: [OZAPRS] Decoding HF APRS
>
> I have similar problems. I regularly decode VK3MY and a couple of other
> VKs plus a W1 on my AEA DSP-2232 and although I can hear a lot more
> stations in there they rarely seem to decode even when signals are
> respectable. I have found tuning on HF to be extremely critical. About
> 20hz either way can make a big difference.
>
> Graham
>
> ZL1GTB
>
> Rob wrote:
>
> >Not sure if anyone can help me
> >I'm Trying to decode APRS on HF
> >I have a Dual Port Kantronics KAM XL
> >I find it excellent on VHF but when i tune it on HF i can decode VK3MY
> >every time ok but all the other stations i seem to be having trouble
> >I'm using a Codan 7727 B tune to 10.147.10 USB
> >I can audible here other traffic ok anyone got any Ideas
> >
> >Cheers
> >de VK4HAB
> >
> >_______________________________________________
> >Ozaprs mailing list
> >Ozaprs at aprs.net.au
> >http://aprs.net.au/mailman/listinfo/ozaprs
> >
> >
> >
>
>
>
>
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