[aprssig] Soundcard Packet Analyzer

Stephen H. Smith wa8lmf2 at aol.com
Sat Oct 22 21:44:59 CDT 2011

On 10/22/2011 7:20 PM, Bob Bruninga wrote:
> What we really need is a PACKET ANALYZER software!
> Mike Chesena, KA2ZEV suggested something like this.
> I think it is a GREAT idea!
> Do we have any DSP jockies that can write this code?  Or probably most of it alreayd exists somewhere.  But the self-calibration will be important so we are all comparing to the same orange.
> Bob, WB4APR
> _______________________________________________

You can achieve something close to this with three off-the-shelf Windows 
applications together NOW, with no DSP code needing to be written.

1)     The well-known AGW Packet Engine is a 300/1200/9600 baud KISS TNC 
entirely in software that will run on most sound systems.    AGWpe has an 
internal "tuning aid" display that can create an audio spectrum analyzer 
display, a waterfall (time domain spectrum analyzer) display similar to most 
PSK programs, and perhaps most importantly, a simple oscilloscope display that 
can clearly display the relative level of the the 1200 and 2200 Hz tones (and 
the relative level of different users' bursts).

2)     The companion freeware program "AGW Monitor"  automatically connects to 
AGWpe's TCP/IP output when started, with absolutely no configuring or tweaking. 
It creates an on-screen display similar to the MONITOR mode of most classic 
TNCs with the header information (callsign,  path, destination, etc on one line 
and the packet payload on a second line.   (Or alternatively, you can point any 
web browser to localhost:8000 and view the same info via AGWpe's built-in 
internal web server output, along with useful stats such as a MHEARD list of 
stations that shows how many times each one has been heard.)

3)     The third application (and second sound card app) is the absolutely 
amazing Visual Analyser 2011.    This *FREEWARE*  program, written by an 
Italian electrical engineering PhD candidate at the University of Rome, 
emulates a whole stack of HP audio test instruments in a single 5 MB exe file.
.     From the sound card record input(s), Visual Analyzer creates a dual-trace 
audio scope with adjustable triggering, a dual-trace audio spectrum analyzer, 
an audio frequency counter, an audio voltmeter that can display in either 
millivolts, or dB similar to a "VU" meter, a phase meter (if you have the same 
tone freq in both channels) and a harmonic distortion meter.  All these 
parameters can display in separate windows at the same time from an audio input.
.     At the same time, it can create a dual-tone audio GENERATOR, an audio 
sweep generator, a function generator (sine, square, triangle,etc) or a pulse 
generator with fully configurable pulse duration and repetition rate on the 
PLAYBACK (output side) of the sound system, assuming that the sound card is 
capable of full duplex operation.    Finally with a few simple external circuit 
components, the program can become a ZRLC bridge!

The spectrum analyzer is capable of linear or log display on either the X-axis 
(audio frequency) or Y-axis (amplitude). In the default, mode, the Y-display is 
log (expressed in dB) and is limited only by the dynamic range and noise floor 
of your sound card.  Even a crap bottom-of-the-line sound system should be able 
to display a 50 dB dynamic range, and a GOOD one 90-110 dB.  A zoom slider lets 
you expand the display to a much smaller dB range on the Y-axis, to the point 
where you can easily see tone skewing of only a dB or two, or accurately 
measure the noise floor of a radio 20-30 dB below peak deviation.  By default, 
the spectrum display X-axis is a 3-decade 20 Hz to 20,000 Hz log presentation, 
but it can be switched to linear and over a specified min and max range.  For 
normal radio response testing, I set the display to 200Hz-5000Hz linear.  The 
scope/analyzer has switchable custom high-pass and low-pass filters that can 
keep low frequency components like PL or DCS from producing "ripple" on the 
display of higher frequency tones, or to remove hiss-induced "grass" from 
off-the-air monitoring of weak signals.

"Out of the box", the program displays relative values referenced to the 
minimum and maximum counts of the sound card A-D converter.  The program has 
provisions for being calibrated against a known external audio source with the 
results made permanent in a .ini file.  The Visual Analyzer scope and analyzer 
can then display absolute audio millivolt and dBm values.

I hacked a Bearcat 760 scanner and tapped the output of it's discriminator, 
wiring it to the existing "Tape Recording output mini-jack on the back panel . 
In turn, I attached the audio input of my iMic external USB sound card  (Click 
Here for review of this device on my web site <http://wa8lmf.net/ham/imic.htm>) 
to the Bearcat's discriminator, and calibrated the pair with my IFR-1500 
service monitor.     Using the external sound system lets me move the assembly 
from one PC to another without having to re-calibrate it each time.   [Note 
that the 6-pin mini-DIN "DATA" or "PACKET" jack on many FM-capable radios will 
provide the same kind of raw non-de-emphasized discriminator output for these 
kinds of measurements.]     The Bearcat/iMic pair now serves as a quite usable 
deviation meter and signal monitor.

Looping the sound card audio generator output back into one channel of the 
sound card scope input lets you do very precise lissajous-figure frequency 
measurements on received PL tones, DTMF tones, TNC mark/space tones in CAL 
mode, etc fed into the other sound card input channel.

It is not too well known that since Windows XP, it is possible to have more 
than one program accessing the sound system simultaneously.    I routinely run 
mmSSTV (analog SSTV), EasyPal ("digital" SSTV) and AGW Packet Engine off the 
same sound card at the same time in my APRN (combined SSTV and APRS) setup.     
    In a similar manner, I run Visual Analyser (for a closeup view of the wave 
forms) and AGWpe (to display the packet information) simultaneously.   About 
the only thing I can't measure exactly is the TXD, but as you have pointed out, 
this can be easily guessed at by ear.

Download URLs:

AGW Packet Engine (Free Version)

AGW Monitor Companion to AGWpe

Visual Analyzer 2011 Information

Visual Analyzer 2011 Download



Stephen H. Smith    wa8lmf (at) aol.com
=== Now relocated from Pasadena, CA back to 8-land (East Lansing, MI) ===
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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