[aprssig] Some followup comments on the TNC Test CD

Stephen H. Smith wa8lmf2 at aol.com
Mon Oct 30 16:14:11 CST 2006

   I received a question about the use of Tracks 1 and 2 on the TNC Test CD

> Stephen,
> Is track 2 the best way to test a TNC for typical audio, or would data 
> recorded from an actual radio speaker jack be a better?  I'm 
> interested in learning more about the audio that most users would 
> present to a TNC, which would not be from the discriminator.
> Thanks,

The majority of users take de-emphasized audio from the speaker of a 
radio for use with their TNCs because it's the easy thing to do, but 
this has many dis-advantages.

1)   The level varies, obviously,  depending on the volume control setting.

2)   The audio de-emphasis curve in many radios is implemented 
incorrectly and  varies wildly from the EIA standard.  (Some radio  
designs apply excessive amounts of audio high-frequency rolloff 
(de-emphasis) in the RX audio power amp in an effort to attenuate 
popping noise and make the radio sound "quieter".)  A TNC connected to 
such "mangled" audio will perform less than ideally.

3)   The speaker audio is usually squelched which can add many 
milliseconds of delay to the start of decoding, forcing other users to 
increase the TXD time to compensate.

4)    A de-emphasized speaker-audio-type  connection absolutely WON'T 
WORK for 9600 baud operation.  

Users have been inhibited from using the direct discriminator connection 
due to the need to make connections inside the radio.  However, many 
modern radios now have the 6-pin mini-DIN "data" or "packet" connector 
on their rear panels which provides easy access to the raw discriminator 
audio.     In this case, the discriminator connection will overcome the 
problems above.   

Details on the signals available at the 6-pin mini-DIN connector are in 
this PDF on my website:



    * Traditionally, 1200 baud packet has been assumed to have  TX
      pre-emphasis due to users inserting their TX audio tones into the
      mic jack, and de-emphasis at the other end due to being taken off
      the speaker,   In other words the 2200 Hz high tone has nearly
      twice the deviation on-the-air than the 1200 Hz low tone, due to
      the transmitter mic audio pre-emphasis. 

      (You can see this by putting a TT or KPC3 in the CAL mode and
      selecting one tone at a time while observing the resulting
      deviation on a service monitor. Typically the high tone will be
      set to yield about 3.5 to 4.0 KHz dev. The low tone will then
      produce about 2.5 to  3.0 Khz or so.). 

    * 9600 baud operation is intrinsically transmitted "flat" since it
      uses direct FSK of the carrier (rather than audio tones), 
      achieved by applying DC-coupled logic-level signals directly to
      the transmitter modulator, bypassing the mic amp and pre-emphasis

    * The Kantronics TNCs provide jumper-selectable equalization
      (de-emphasis) inside the box.  You can disable the de-emphasis if
      you use already-demphasized audio from the speaker, or enable
      de-emphasis if you use raw discriminator audio. 

    * The Kenwood D700 and TH-D7 transmit the two tones at EXACTLY THE
      SAME deviation. (Their internal TNCs are connected DIRECTLY to
      their TX modulator and RX discriminator, bypassing the TX mic
      channel with it's pre-emph and RX audio channel with it's

    * PLL-based decoders (such as the TNC2 and clones)  are very
      intolerant of the low tone being higher level than the high tone. 
      They must see the high tone at a level equal to or higher than the
      low tone.  When you have reverse "twist", i.e. low tone at higher
      level than the high tone, they often fail to lock up and
      decode.     Tuned-filter and zero-crossing-detect  TNC
      demodulators are much less affected by this de-emphasis issue than
      PLL types. However, the de-emphasis (or lack therof) does also
      create phase shift that will cause the zero-crossings to be
      slightly advanced or retarded from their correct point in time.

    * If the RX de-emphasis curve on another radio is a little bit
      excessive,  packets received from Kenwoods will arrive at the
      attached TNC with the low tone far LOUDER than the high tone. This
      can cause the attached TNC to fail to decode bursts from Kenwoods. 

The bottom line of this intractable mess is that some users (mostly 
Kenwood owners)  transmit "FLAT" while others transmit "PRE-emphasized". 
Ideally the receiving TNC has to somehow accommodate both.   The best 
compromise is to split the difference. Take the receive audio from the 
discriminator,  and apply just a very slight amount of  de-emphasis at 
the TNC input.   

Or provide for a slight amount of optional (jumper-selectable) 
high-frequency BOOST at the TNC input to accommodate users that connect 
to the radio speaker output.

On my CD, track one emulates receive via raw discriminator, while track 
two emulates de-emphasized receive via speaker or earphone jack.   **TNC 
developers are encourage to use both tracks alternately to optimize 
their TNC performance to tolerate BOTH flat AND de-emphasized audio. **

To make the other tracks (recorded "flat" straight off the 
discriminator) simulate speaker audio,  you will have to connect a 
de-emphasis network between the CD player and the TNC. 

Assuming the CD player has a low output impedance  ( 1K or less, which 
is typical of consumer audio devices) and the TNC under test  has a high 
input impedance (disable any jumpers that select a 600 ohm input Z), 
then a simple RC network can do the job.  Connect a 100K resistor in 
series between the CD player out and the TNC input.  Shunt the TNC input 
to ground with a  750 pF capacitor.   Or use a 47K resistor and a .0015 
uF capacitor.  Approximate this value by paralleling a .001 and a .0005 
uF cap.)  Either of these two networks will approximate the standard 75 
microsecond de-emphasis network.  


Stephen H. Smith    wa8lmf (at) aol.com
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