[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
> 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.
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
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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