[aprssig] ARPS physical layer

Tapio Sokura oh2kku at iki.fi
Thu Sep 16 06:58:45 EDT 2010


On 16.9.2010 11:44, Derek Love wrote:
> So far as I can see, there is no mention at all of a preamble and the only
> sync sequence I can see is the 7E start flag, which seems to be a very poor
> choice?

My understanding is that most implementations don't make a difference 
between preamble and sync sequences, but just send zeroes, i.e. 
alternating tones, for the period the user has configured as tx delay. 
These are often pretty long, 200 milliseconds is typical and some use a 
tx delay up to 500 ms like the old Kenwood APRS radios. A lot less would 
often be enough. At the end of the "sync" sequence a single HDLC flag is 
sent and then the packet, finished with the CRC checksum and another 
HDLC flag. Another packet may immediately follow the HDLC flag ending 
the previous packet.

> I can't see any specification for which state the NRZI coding process should
> be started with either......

You mean whether to begin at a high or low tone? It doesn't matter, pick 

> I have also found a reference to a data scrambler somewhere, but no
> indication if it's part of the standard or not.

The references to a data scrambler you found are probably for the 9600 
bps G3RUH variant. The 1200 bps "regular" packet doesn't use a 
scrambler, if you don't count the bit stuffing required to keep HDLC 
flags unambiguous as a scrambler.


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