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[aprssig] "Best" packet decoder solution

Stephen H. Smith wa8lmf2 at aol.com
Mon Dec 27 18:25:38 UTC 2010


On 12/27/2010 8:44 AM, Lynn W. Deffenbaugh (Mr) wrote:
> Note that the one that is missing there is the KPC-3+ which, according to 
> what I've read, fails miserably if the signal is flat on the air (not 
> pre-emphasized) but de-emphasized on the receiving end before being fed to 
> the KPC-3.  The KPC-3 apparently doesn't like the low tone being louder than 
> the high tone resulting in a lack of decoding.
>
> Some have recommended that the audio interface for the KPC-3 be set up so the 
> transmit audio (KPC to radio) goes through the pre-emphasis circuit, but the 
> received audio (radio to KPC) be pulled directly from the discriminator and 
> before the de-emphasis (meaning, don't pull it from the speaker output).  
> This would mean that "correct" on-the-air packets (pre-emphasized, with highs 
> louder than lows) would be decoded ok as would non-emphasized (both tones 
> equal) packets.


The KPC3/3+ have an internal jumper to enable / disenable RX de-emphasis as 
required to accommodate connections to either the  flat direct discriminator 
connection (6-pin mini-DIN connector on many transceivers) or de-emphasized 
speaker audio.     In addition, I have always added a little "re-emphasis" to 
KPC3 hookups by connecting whatever RX audio I am using to the KPC through a 
smallish value series capacitor (typically .01  to .05 uF) to attenuate the 
lower tones a bit.    The RX audio input sensitivity of the KPC3 is high (it 
will work reliably down to the 10s of millivolts) so the added loss of the cap 
on a 100-500 millivolt discr or speaker connection is negligible.   A second 
internal jumper can change the audio input impedance of the KPC3 from low 
(around 500 ohms) to high (around 10K).    Varying the value of the series 
coupling capacitor along with the two values of input Z gives you a lot of 
options to skew the input frequency balance.


>
> I have a KPC-3+ here, but I haven't yet hooked it up to anything for 
> comparison to the TNCs listed in my previous e-mail.  I haven't made time for 
> this because apparently the KPC-3+ also has trouble when operated long term 
> in KISS mode which is my preferred TNC interface.

Where have you encountered KISS issues with the KPC3?   I have a KAM (VHF side 
is essentially identical to a KPC3 while the HF side uses two independent 
switched-capacitor tone detector channels like classic RTTY "TU"s) -and- a 
KPC3+ in my ham "super server"  VHF/HF  igate-UIwebserver-APRN-Echolink  
setup.   Picture and details here:

<http://wa8lmf.net/EPIAserver>

Both run in KISS mode and have run for months at a time without any 
problems.    The KAM in KISS mode provides two logical KISS data streams (one 
for HF, one for VHF)  on a single physical serial port.  They are then 
separated by AGWpe into separate AGW logical ports which are then used by 
different copies of UIview.

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Finally, you say you found AGWpe Pro to be the best performer.  Have you 
compared it to the freeware version?    I had assumed the main difference in 
the "Pro" version was the fancier GUI.   Is there an actual difference in the 
decode performance?

I have been considering buying the Pro version, but mainly because it's HF mode 
uses the standard 1600/1800 Hz  KAM/TNC2  HF tones instead of the 2100/2300 
tones of the freeware version.  (The higher tones can present problems with the 
limited passband of many HF SSB transceivers.)





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