[aprssig] ARPS physical layer

Derek Love DLove at app-tech.co.uk
Thu Sep 16 04:44:16 EDT 2010

Thanks for the info guys, maybe I should be a bit more specific.

I have the APRS 2.0 spec, I know about HDLC, NRZI, byte reversal, bit
stuffing, CRC/FCS generation and stop/start (7E) flags.

However, I come from a PMR / Data radio environment, so my first questions
What's the preamble sequence and length?
What's the sync sequence and length?

The preamble is usually a 1100 sequence of 16 or 24 bits to allow the RF
sections to settle down and the receiving modem to set its levels up. The
Sync sequence is usually 16 or 24 bits carefully chosen so they don't
correlate well with Gaussian noise so that false detects are minimised.

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

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

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

Derek Love

-----Original Message-----
From: Curt, WE7U [mailto:curt.we7u at gmail.com]
Sent: 15 September 2010 22:48
To: TAPR APRS Mailing List
Cc: Derek Love
Subject: Re: [aprssig] ARPS physical layer

There's a byte-wise CRC16 algorithm that was published in an IEEE
magazine in the late 70's or early 80's that I used when I wrote
assembly for decoding packets in '84 or '85.  Worth checking out if
you're writing code.

Curt, WE7U.                         <http://www.eskimo.com/~archer>
    APRS:  Where it's at!                    <http://www.xastir.org>
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