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[aprssig] *** APRS TNC Test CD Now Available ***

Stephen H. Smith wa8lmf2 at aol.com
Mon Oct 30 07:18:13 UTC 2006

***   APRS TNC Test CD Now Available   ***

I have produced a CD containing signals for testing and comparing the 
performance of different packet TNCs, and radios that might be used with 
these TNCs.  The recordings of live off-the-air APRS activity are also 
useful for demonstrations in locations where live signals aren't 
available, such as a basement conference room.

This disk is a "CD-Plus" combination disk that contains CD-ROM data 
files viewable on a computer, and standard CD audio files playable on 
any normal home or car audio CD player, boombox or DiskMan.

The audio files could also be played on an old CD-ROM drive standalone, 
without a computer, by connecting the TNC under test to the drive's 
front panel earphone jack, or to the 4-pin analog audio jack on the 
drive's rear panel. (A CD-ROM drive normally doesn't need computer 
support to play audio CDs. A drive with a front panel volume control and 
track-select button is preferred.)

Playing back standard CD audio rather than CD-ROM .wav or .aiff audio 
data files avoids the timing errors and incorrect playback sample rates 
that often accompany low-cost software-based  PC sound systems (i.e. 
motherboard AC97-based systems).

Details on the disk, links for downloading the 530MB image file for 
creating this CD, and a link for purchasing a ready-made CD, are located 
on my website at:


Scott, N1VG, of OpenTrac fame ( http://opentrac.org ) is handling the 
sale of recorded disks on his Argentdata e-commerce site, and has also 
graciously volunteered to host the large downloadable CD image files on 
his server.  

About The Tracks

---- Track 1 is an off-the-air recording of 40 minutes of activity on 
144.39 MHz in Los Angeles, California, during the afternoon rush hour at 
about 5:00 PM when the channel is totally saturated to several hundred 
percent of the Aloha threshold. It contains a variety of over- and 
under- deviated signals, packet collisions, rapid-fire packets with 
practically no pause between them, raw NMEA string trackers, TinyTraks, 
clueless idiots using CW ID on packet, etc. All periods of dead air over 
about 1 second have been edited out so that 40 minutes of real-life 
activity have been compressed to 25 minutes on the CD.

This track is intended to be played back directly into TNCs to compare 
the performance of various TNCs "under fire" in the real world . The 
rapid pace of the packets should be a good test of the ability of 
buffers in TNCs and associated applications to hand a rapid flow of data 
without overrunning.

---- Track 2 is identical to Track 3 except that a precise 
6db-per-octave/20db-per-decade de-emphasis between 300 and 3000 Hz has 
been applied to simulate the typical de-emphasis found at the volume 
control or speaker output of the typical land-mobile receiver. This can 
used to evaluate the relative performance of TNCs in handling raw 
non-deemphasized discriminator audio vs de-emphasized speaker audio. The 
effectiveness of jumper-selected equalization networks available inside 
some TNCs can also be tested. This track was created by applying the 
graphic equalizer filter in Adobe Audition set to simulate the standard 
EIA land-mobile radio de-emphasis curve.

---- Track 3 is a recording of a Kenwood D700 transceiver sending a 
Mic-E position report, taken from the wideband low-distortion 
discriminator output of an IFR1500 service monitor.  This recording of a 
single packet was then copied and pasted repeatedly in the Adobe 
Audition audio editor. The result is 20 bursts each minute, for 5 
minutes, for a total of 100 absolutely identical packets.

This track is intended to modulate an RF signal generator. Connect the 
CD player to the external modulation input of a generator, and set the 
playback level so that the data bursts deviate the generator about 3.5 
KHz. Connect a radio's receive audio output to the TNC under test. 
Connect the RF generator to the radio's antenna input. While the 
recording is playing, gradually decrease the generator RF output level 
until the TNC starts failing to decode. Since the recording contains 
exactly 100 bursts, counting the number of successful decodes can 
directly indicate the percent success rate at various RF carrier levels.

Possible uses of this track:
1)     Compare different TNCs connected to the same radio.

2)     Connect the same TNC to different radios to compare the 
weak-signal data performance of various receivers.

3)     Evaluate the relative receive performance of audio taken from the 
radio's speaker versus audio taken from dedicated "data" or "packet" 
jacks or outputs. (De-emphasis and audio bandwidth may be different at 
these two sources.)

--- Track 4 is an off-air recording of 25 minutes of a single mobile 
D700 beaconing every 12 seconds (maximum beacon rate) on a quiet channel 
(no other stations) while driving around the San Gabriel Valley area of 
Los Angeles. The signals were monitored in Pasadena, California. Most of 
the drive test course was 8-10 miles from the fixed station, far enough 
away from the monitoring fixed station to create some mobile flutter, 
multipath and loss of quieting. In several stretches, several successive 
packets are audible on the recording but failed to decode with the AGW 
packet engine.

Again, this recording was taken from the non-de-emphasized direct 
discriminator output of a Yaesu FT-1500b via the 6-pin mini-din data 

This track is intended to be played back directly into TNCs under test. 
An APRS application running on a computer attached to the TNC should 
create a realistic moving object.

The three following tracks are for TNC alignment rather than testing. 
They are intended to evaluate TNC demodulator tolerance to tone "skew" 
(unequal levels of the 1200 and 2200 Hz tones).

---- Track 5 is a direct recording of one minute of a KPC3+ TNC in the 
CAL mode sending the alternating 1/0 test pattern (i.e. alternating 
between 1200 and 2200 Hz tones) with both tones at the same amplitude.

---- Track 6 is the same recording with a precise 6 dB/octave 20 
dB/decade DE-emphasis applied as in Track 3 above.

---- Track 7 is the same recording with a mirror-image precise 6 
dB/octave 20 dB/decade PRE-emphasis applied.


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


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