[OZAPRS] Kenwood D700 TNC Transmit Equalization

Richard Hoskin (VK3JFK) vk3jfk at amsat.org
Wed Apr 27 22:01:27 EST 2005



Digipeater operators may find the emails below of interest.


In VK3 about half of the digipeaters have reduced or no equalization of FM
in them.


Visit http://datagrp.vic.wicen.org.au/xswmods/mfj1270b.htm for information
on the MFJ1270B equalization Gotchas.






Message: 16

Date: Mon, 25 Apr 2005 02:05:22 -0700

From: "Stephen H. Smith" <wa8lmf2 at aol.com>

Subject: [aprssig] Kenwood D700 TNC Transmit Equalization Screwed Up!


To: TAPR APRS Mailing List <aprssig at lists.tapr.org>

Message-ID: <426CB2D2.8000504 at aol.com>

Content-Type: text/plain; charset="iso-8859-1"


  I previously posted this about a month ago, but it got lost in a flood 

of 40-50 posts/day about two hot-button issues raging at the time.  Now 

that things have quieted down on the list, I am reposting this 

interesting discovery. 


================   PASTE  REPOST =========================


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 



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.




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!





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 <http://wa8lmf.com/> 


New APRS Symbols Reference Chart      



New/Updated "Rev G" APRS     http://webs.lanset.com/wa8lmf/aprs

Symbols Set for UI-View,


Message: 4

Date: Mon, 25 Apr 2005 13:54:26 -0400

From: "Robert Bruninga" <bruninga at usna.edu>

Subject: Re: [aprssig] Kenwood D700 TNC Transmit Equalization Screwed

      Up! (Repost)

To: <wa8lmf2 at aol.com>, <aprssig at lists.tapr.org>

Message-ID: <s26cf72d.019 at FSGWHUB.usna.edu>

Content-Type: text/plain; charset=US-ASCII


Yes, the TNC is used at both 9600 and 1200 baud and the 9600 baud has to
be flat to work well.  Apparently 1200 baud is also transmitted and
received flat.  This is why it is important to make sure in all
digipeaters that there is no audio roll-off any worse than the usual
de-emphasis. or even better, that the TNC is connected to the


A good compromise is to set about 3 dB of de-emphasis at the digis to
account for both types of users.




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