[aprssig] Kenwood D700 TNC Transmit Equalization Screwed Up!

[Stephen H. Smith]

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. 

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.  (Higher-speed formats are 
normally  transmitted "flat" due to direct coupling to the transmitter 
modulator.)  

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!

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:

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.
         http://webs.lanset.com/wa8lmf/1-Mic-E-Burst.gif    


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!
         
http://webs.lanset.com/wa8lmf/Mic-E-Burst-Extreme-Closeup.gif      
     

The waveform display clearly shows that the D700 is transmitting 1200 
baud WITHOUT the customary preemphasis.  This would seem to suggest that 
D700s (At least at 1200 baud.) 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.  




Stephen H. Smith             wa8lmf (at) aol.com
                                                    
Home Page:                   http://wa8lmf.com

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