[aprssig] AGWPE dual speeds ?

Stephen H. Smith wa8lmf2 at aol.com
Sat Dec 8 14:53:51 EST 2007

Andrew Rich wrote:
> Does the sound card modem agwpe happily decode 9k6 and 1k2 on two 
> different channels ?
> At the moment I keep changing from 9k6 to 1k2 for sats.
> I can wire the 9k6 and 1k2 out of the ICOM 706 on two different ccts.
AGW will operate on 2 channels at once, simulating a Kantronics KAM or 
9612 dual port TNC.  You can use any combination of 300, 1200 or 9600 
baud on the two ports.  

However, you must have a STEREO audio input to your sound card to 
implement the dual-port mode.   This means your sound system MUST have a 
LINE level input in addition to the MIC input, since virtually all MIC 
level inputs are monophonic. (The second contact of the 3.5mm stereo 
MIC-in mini-jack carries DC power for electret mics; not a second audio 

You will probably have to throw together a simple op-amp audio pre-amp 
with perhaps 10-20x gain for the 9600 baud side.  The so-called 9600 
baud output of the transceiver is actually just the raw output of the FM 
discriminator before de-emphasis or squelch. Normally this is a very low 
level signal, typically 50-100 millivolts.  The LINE-IN of the sound 
system will require a signal more like 500-1000 mV.  

This audio amp should have very good low frequency response (large 
coupling caps or even direct DC-coupling) since the 9600 baud signal is 
actually a square-wave logic-level waveform (not audio tones). The 
preservation of the SHAPE of the wave form (i.e. no rounding of the 
corners or level shifts of the horizontal parts of the square wave) is 
important at 9600.   This is by contrast with 1200 where only the 
FREQUENCY of the wave is important. [This is why transformer-coupling of 
9600 amost never works while it works perfectly at 1200.)

Details on the 6-pin mini-DIN DATA/PACKET connector on the 706 and many 
other radios is here on my website:

     <http://wa8lmf.net/miscinfo>        Look for the file   
*MiniDIN6-Packet.pdf*    on this page.


Stephen H. Smith    wa8lmf (at) aol.com
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