[aprssig] ARPS physical layer

Scott Miller scott at opentrac.org
Thu Sep 16 16:41:07 EDT 2010

I discovered early in the development of the OpenTracker that a long 
string of 7E flags just doesn't do a good job of syncing the receiver. 
My decode rate drastically improved when I switched to 0's.

There are some other audible differences, too.  Some of the TNCs use 
V.23 instead of Bell 202 which has the tones 100 Hz closer to the 
center, I think.  Also, I think there's a difference between the 
digitally synthesized tones (specifically in the transition between 
tones) used by things like the OpenTracker and TinyTrak and purely 
hardware-based modems like in the KPC-3.

(Another trip to the desert and another missed rendezvous, Phil - maybe 
*next* summer we'll have a chance to sit down and have a beer.  =])


On 9/16/2010 7:26 AM, Phil N6TCT wrote:
> Nice!  I've always wondered why, by ear, the preambles of packets from
> some TNCs sound different than those from other TNCs!
> Phil
> John Hansen wrote:
>>  [...] The bytes you have before the address are the 7E flags.  In
>>  addition, you'll find that many TNC's idle during the TXDelay period
>>  using 00's instead of 7E's.
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