[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.
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
EchoLink: Node # 14400 [Think bottom of the 2-meter band]
Home Page: http://wa8lmf.net
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