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📶 RadioMail 1.4 - The "Audio Frequency Shift Happens" Release
Introducing a Software
Modem for Packet
To get emails on the air
with RadioMail, you can
use the AX.25 protocol,
commonly known as packet
radio. Although it's an
older technology with
modest throughput, it
remains widespread and
is still used in many
areas. Currently, there
are over 1,200 Winlink
stations accessible via
packet.
Previously, using the
packet protocol with
RadioMail required a
TNC, specifically a KISS
TNC. This type of TNC
follows the KISS
protocol, enabling host
applications like
RadioMail to communicate
with it. A KISS TNC
typically combines a
1200 baud AFSK (Audio
Frequency Shift Key)
modem, which encodes and
decodes data into audio
symbols, and a sound
card that converts
digital sound into
analog. Additionally, a
TNC controls the radio's
PTT (Push-To-Talk),
which is crucial for
connection-oriented
sessions, such as those
used by Winlink.
KISS TNCs are available
in radios like the
Kenwood TH-D74/75 and
the PicoAPRS. They can
also be standalone
devices, such as the
Mobilinkd TNC, which can
connect to any existing
radios.
The specification for a
1200 baud AFSK modem is
well-documented and
manageable by most
modern computing
platforms. So, why
hasn't RadioMail
included a built-in
software modem? The main
challenge is that
RadioMail must not only
be able to send analog
audio to the radio but
also control the PTT.
Until recently, most
sound interfaces for
amateur radio required
serial or USB HID
commands to trigger PTT.
However, Apple's
restrictions on USB
peripheral access in iOS
made this approach
infeasible.
Fortunately, Denis K0TX
has addressed this issue
by introducing two new
sound interfaces: the DigiRig
Lite with a
USB-C connector and the
DigiRig
VOX cable, which
includes a passive PTT
circuit on a TRRS audio
cable.
These devices allow the
PTT to be triggered by
sending a tone on the
right audio channel. The
tone is intercepted by a
circuit and isn't
transmitted to the
radio. This enables
RadioMail to control the
PTT and ensures a fast
switch between sending
and receiving.
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🤔 Which One Should I
Get?
If your device has a
USB-C connector, choose
the DigiRig Lite. This
ensures the PTT control
uses active circuitry,
as it can be powered by
USB.
For older devices with a
lightning connector, use
the DigiRig VOX PTT
cable. You’ll also need
a lightning
to 3.5mm headphone
jack adapter
from Apple.
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Creating a High
Performance Modem
With the hardware side
solved, the next
challenge was developing
a software modem. What
exactly does a packet
modem need to do? For
AX.25, the basic
modulation uses the Bell
202 standard, originally
for old telephone
acoustic couplers. It
performs well with
limited bandwidth, like
phone systems. Sounds
simple, right?
Not quite. Typically,
the sound card connects
to the microphone and
headphone jack of the
radio, a channel
designed for voice. The
radio applies signal
filtering to enhance
voice quality, mainly
through emphasis. This
process boosts higher
frequencies, with the
receiving radio later
reducing them via a
low-pass filter.
However, this symmetry
is not always perfect.
Sometimes, the sending
radio may or may not
apply emphasis, and the
receiving radio might
not compensate
correctly. For digital
signals, this can result
in the higher tone being
either attenuated or
amplified by several dB
compared to the lower
tone, a phenomenon known
as “twist.” The Bell 202
standard requires a 1dB
flatness, but over an
amateur radio voice
channel, twist can reach
+/-9dB.
What does this mean for
the demodulator? It
needs to manage
different levels of
attenuation and adjust
accordingly. My goal was
to deliver a seamless
user experience so users
wouldn’t need to worry
about the type of radio
they use. The solution
chosen is to run nine
decoders on the received
signal simultaneously,
aggregate the results,
and retain only the
error-free frames. This
approach ensures
excellent decoding
performance regardless
of the radio used.
I'm pleased to report
that with this approach,
the RadioMail modem
achieves about 93%
successful decoding of
thousands of off-the-air
transmissions recorded
in Los Angeles on a busy
afternoon. Those
recording are available
on the TNC
Test CD, a
benchmark for TNCs.
If you're interested in
learning more about
modem functionality, I
highly recommend
exploring the
educational material on
the theory of AFSK
demodulators.
Special thanks to Rob
WX9O from Mobilinkd for
providing this valuable
information.
Simple Setup
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Connect the DigiRig
to your device and
radio.
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In RadioMail
Settings, select the
Packet Modem option.
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Use the audio
monitor and
diagnostic tool to
ensure that the
radio volume is set
correctly for
optimal decoding,
and the device
output audio volume
is high enough to
properly trigger the
PTT, without
saturating the radio
signal.
Once configured, you're
ready to connect to a
nearby packet station.
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🪲 Bug Fixes &
Improvements
This release includes
several bug fixes and
improvements. Notably,
it enhances support for
9600 baud packet mode
when using the B.B. Link
adapter with the Kenwood
TH-D74/75. Previously,
RadioMail did not set
the baud rate correctly
when connecting to a
station with 9600 baud
support.
With this update,
RadioMail will
automatically switch the
baud rate to 9600 when
setting the radio in
KISS mode.
Please note, this
requires B.B. Link
firmware version 0.7.8
or later. To upgrade
your firmware, open the
B.B. Link Configuration
app, select your
adapter, and tap “Update
Firmware.”
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