[aprssig] Dirty signals vs. hardware TNCs

Stephen H. Smith wa8lmf2 at aol.com
Thu May 12 00:32:40 CDT 2016

On 5/11/2016 5:47 PM, Bill Vodall via aprssig wrote:

> Isn't there some audio capture and playback tool, maybe Audacity, that
> could be used to capture off the air audio, visually check for issues,
> and playback to see how it decodes.
> We had hardware for that 25 years ago in the X1J nodes - today we
> should be able to do it in software.
> Bill

Audacity works perfectly for this kind of stuff.  Even better, connect it's 
record input to the "9600 baud" pin of the 6-pin mini-DIN "data port" on a 
radio.   The so-called 9600 baud pin is actually raw non-de-emphasized 
discriminator output, so you get an accurate deviation measurement at any 
modulating frequency, unaffected by RX de-emphasis that you get at a speaker jack.

Two software tools for working with these kinds of tests and measurements are:

V-B Cable     This is a freeware tool similar to the pay-for "Virtual Audio 
Cable".   It is a virtual audio patch cord that lets you connect the output of 
one soundcard application to the input of another.  For example, take the 
playback output of Audacity and route it into the input of a software TNC for 
testing.   I use it frequently to play back off-the-air recordings of PSK31, 
MFSK16, Olivia, RTTY, MT63, etc into soundcard digimodes software at radio club 
presentations about digi modes.


Visual Analyzer   A sound card app that emulates an entire rack of HP audio 
test gear.  It features an audio spectrum analyzer, dual-trace storage scope, 
audio level meter, audio tone generator including sweep generator & 
sine/square/pulse function generator.  It even has a distortion/SINAD meter 
when used with a high-quality sound card.  And it's  FREE !

Connected to the output of the mini-DIN discriminator port, you can see very 
precisely the relative levels of audio on incoming signals and whether they are 
clean sine waves or bizarrely distorted.


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

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