[aprssig] 9600 APRS

Stephen H. Smith wa8lmf2 at aol.com
Tue Mar 1 23:38:11 EST 2011

On 3/1/2011 5:55 PM, Wayne Sanderson wrote:
> I regularly used a wireless PDA years ago that ran on a dedicated
> packet data network. It used an interesting transmit/receive scheme.
> The upload side of the device sent data up to the network at 4800, but
> the downstream packets from the network came down at 19200. The idea
> was that the end user in everyday use generally sent very little up
> compared with receiving quite a bit down, which was how they got away
> with the speed difference. It made sense too, considering that the end
> user only sees the speed of the data he receives, and doesn't care
> what speed the stuff he sends goes out, as long as it goes.

This is exactly how standard residential DSL Internet service works.   The 
service which is technically ADSL (Asymmetric Digital Subscriber Line) normally 
has 3 to 5 times the speed on the downlink side.    Since the use normally only 
sends a few mouse clicks or typed characters upstream to unleash a 
multi-megabyte flood down stream it works very well  ---- UNTIL you want to do 
FTP   UPLOADS   of big files, or send (rather than receive) email with big 

> I wonder whether a dedicated offset freq for digi access set to a
> lower speed with network reception on the mobiles set at the higher
> speed would ever be possible. I imagine one would either need 2 TNCs,
> each set to a different speed, or a TNC set to 9600 or whatever speed
> one wanted to run on the main channel and a straight one way transmit
> only modem on the uplink channel set at a lower speed.
> I wonder whether a skillfully written software modem might be able to
> handle all speeds on the same channel and be able to send/receive them
> all.

There are already at least two apps that can do more or less what you want:


The AGW Packet Engine soundcard softmodem can do this now.   If you have a 
soundcard with STEREO inputs, AGW can implement a separate TNC on the left and 
right channels.  They do not need to be running at the same speed.    By 
connecting the 300 baud side's input and the 9600 baud side's output to the 
radio, you could do exactly what you are proposing.

Note that MIC inputs on computer sound systems are almost universally single 
channel, while LINE level inputs are always stereo.   The only device I am 
aware of that provides stereo low-level (i..e. mic level) inputs is the Griffen 
Electronics iMic USB-connected external sound system.  Review here on my web site:



There is also already a ham "soundcard" app that autoselects multiple baud 
rates and even modes on receive.    G4HYG's  APRS Messenger HF APRS application 
can transmit on PSK63, PSK250,  QPSK63, QPSK125, QPSK250, GMSK125 and GMSK250 
and automatically receives ANY of these modes without switching!   This 
application is based on the mmVari dll that is the heart of mmSSTV and mmRTTY. 
      Details here:




Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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"APRS 101"  Explanation of APRS Path Selection & Digipeating

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