November 23, 2024

RadioMail v1.4.0 aka the “Audio Frequency Shift Happens” Released

MacHamRadio iOS, iPad, RadioMail, Winlink 0 Comments

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RadioMail provides access to the Winlink amateur radio email system. Compose, read, and reply to messages from anywhere.

A valid amateur radio license is required to use this app.

NETWORK ACCESS・Send and receive messages via telnet using the main Winlink server or through the Amateur Radio Emergency Data Network (AREDN).

RADIO ACCESS・Pair your device with a VARA or KISS TNC packet modem using WiFi or Bluetooth. Packet mode supports the Mobilinkd TNC, PicoAPRS radio, DireWolf software TNC and the Kenwood TH-D75 radio with the B.B. Link adapter. Configuration of up to two digipeaters is possible for packet or VARA FM mode connections. Note for VARA: An operational version of the VARA modem must be available on a separate computer.

STATION DIRECTORY・Automatic retrieval of station details from Winlink, organized by proximity to your current location. Custom stations can be added for peer-to-peer connections. Note: Peer-to-peer connections are only outbound.

FAVORITES LIST・Save stations to your favorites list for quick access by modes and bands during connection initiation.

PHOTO ATTACHMENTS・Sharing photos is easy with RadioMail. Access your device’s camera directly or attach photos from your library. The app optimizes images to reduce transmission time. Preview and annotate received attachments, save to local folders or iCloud, or share via other apps.

FORMS・Compose and view 100+ standard Winlink forms. Forms are kept current with automatic updates. GPS coordinates are pre-filled according to user location. Received forms can be shared or printed as PDFs. Custom specialized forms can be installed by importing a zip file. Note: Form access is an optional premium feature requiring an in-app subscription.

POSITION REPORT・Utilize your phone’s internal GPS for accurate location data in position reports, which are posted to the Automatic Packet Reporting System (APRS) network.

NOTIFICATIONS・RadioMail fetches new messages automatically and shows notifications while running in the background, eliminating the need to constantly check for new messages. Note: This feature is only available for telnet connections.

ADDRESSES・Send or receive messages with your primary Winlink account, tactical addresses, or secondary accounts. Seamlessly send messages to Winlink and regular email addresses. Look up contacts from the address book and access their email addresses when composing messages.

RadioMail is independently developed and not affiliated with the Amateur Radio Safety Foundation, Inc. (ARSFI) who supports the Winlink Global Radio Email system.

What’s new in Version 1.4.0 aka 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.

🤔 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.

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

Connect the DigiRig to your device and radio.
In RadioMail Settings, select the Packet Modem option.
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.

🪲 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.”

I hope you’ll enjoy this release. If you have questions or feedback, let me know or join the conversations at https://groups.io/g/radiomail

Mahalo,
Georges WH6AZ