[OZAPRS] Decoding HF APRS

[Darryl Smith]

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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