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<font face="Arial">I am in the process of producing an audio CD
recording of various types of APRS packet signals. I intend for it to
be a source of exactly repeatable noisy and distorted signals for
comparing the performance of different TNCs under various
less-than-ideal conditions. The idea is to be able to use any audio CD
player or old CD-ROM drive to play back test signals into each TNC in
turn and compare the results. I have been using an audio editor
(Adobe Audition - formerly known as Cool Edit) to record various types
of packet bursts off-the-air. These recordings are being made using the
extremely low distortion wide-band demodulator of my IFR-1500 service
monitor (instead of the discriminator of an ordinary radio) in order to
get the highest-possible-quality reference signal unbiased by the IF
and audio response characteristics of any particular radio. <br>
<br>
It's generally accepted that 1200 baud packet is
transmitted through the MIC jack of a radio, and therefore should
acquire the same
audio preemphasis as a voice signal (i.e. the 2200 Hz tone of standard
1200-baud should have
almost TWICE the deviation as the 1200 Hz tone). For the best TNC
receive performance, corresponding DE-emphasis needs to be used at the
receiver, either by taking the RX audio off the speaker, or by
de-emphasis networks inside the TNC. (H</font><font face="Arial">igher-speed
formats are normally transmitted "flat" due to direct coupling to the
transmitter modulator.) </font><font face="Arial"> <br>
<br>
In the course of capturing signals with the recording setup, I was
shocked to discover that the Kenwood D700 transmits 1200 baud "flat"
with no pre-emphasis at all!<br>
<br>
Here are two GIF images that are screen captures of the Cool Edit
screen showing the envelope of the AFSK 1200 baud packet burst as
captured off the IFR's discriminator:<br>
</font><font face="Arial"><br>
This is a display of the waveform over an entire
1/2-second Mic-E burst from the D700. You can see the sync tones at
the beginning of the burst - note the long delay before the data
starts. The very tall "grass" at the extreme left and right of the
picture is unsquelched white noise before and after the burst. If you
look closely at the right side, you can see a millisecond or so of dead
carrier between the end of the packet tones and the radio unkey. </font><br>
<font face="Arial">
<a class="moz-txt-link-freetext" href="http://webs.lanset.com/wa8lmf/1-Mic-E-Burst.gif">http://webs.lanset.com/wa8lmf/1-Mic-E-Burst.gif</a>
<br>
<br>
<br>
The Cool Edit display lets you zoom into the display until you can see
the individual cycles of the audio wave form. Here is about a 700%
enlargement zoomed in at the end of the sync and the beginning of the
data. Note that the low tone (fewer cycles per unit distance) is
actually HIGHER in level than the high tone!<br>
</font><font face="Arial"><a class="moz-txt-link-freetext" href="http://webs.lanset.com/wa8lmf/Mic-E-Burst-Extreme-Closeup.gif">http://webs.lanset.com/wa8lmf/Mic-E-Burst-Extreme-Closeup.gif</a>
</font><br>
<font face="Arial"> <br>
<br>
The waveform
display clearly shows that the D700 is transmitting 1200 baud WITHOUT
the customary preemphasis. This would seem to suggest
that D700s </font><font face="Arial">(At least at 1200 baud.) </font><font face="Arial">are at a real disadvantage compared to other APRS devices
when transmitting to digipeaters and other users employing standard
receive de-emphasis. De-emphasis applied to a signal already
transmitted
"flat" will result in the high tone being 6 dB LOWER in level than the
low tone. This is especially bad news for TNC2s (i.e. UI-Digi
installations) whose modems have a well-known intolerance for high
tone being at a lower level than the low tone. <br>
<br>
<br>
<br>
<br>
Stephen H. Smith wa8lmf (at) aol.com <br>
<br>
Home Page: <a class="moz-txt-link-freetext" href="http://wa8lmf.com">http://wa8lmf.com</a><br>
<br>
New/Updated "Rev G" APRS <a class="moz-txt-link-freetext" href="http://webs.lanset.com/wa8lmf/aprs">http://webs.lanset.com/wa8lmf/aprs</a><br>
Symbols Set for UI-View, <br>
UIpoint and APRSplus:<br>
</font>
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